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<sect1><title>Acknowledgements</title>
<para>Ori Pomerantz would like to thank Yoav Weiss for many helpful ideas
and discussions, as well as finding mistakes within this document before
its publication. Ori would also like to thank Frodo Looijaard from the
Netherlands, Stephen Judd from New Zealand, Magnus Ahltorp from Sweeden and
Emmanuel Papirakis from Quebec, Canada.</para>
<para>I'd like to thank Ori Pomerantz for authoring this guide in the first
place and then letting me maintain it. It was a tremendous effort on his
part. I hope he likes what I've done with this document.</para>
<para> I would also like to thank Jeff Newmiller and Rhonda Bailey for
teaching me. They've been patient with me and lent me their experience,
regardless of how busy they were. David Porter had the unenviable job of
helping convert the original LaTeX source into docbook. It was a long,
boring and dirty job. But someone had to do it. Thanks, David.</para>
<para> Thanks also goes to the fine people at <ulink
url="www.kernelnewbies.org">www.kernelnewbies.org</ulink>. In particular,
Mark McLoughlin and John Levon who I'm sure have much better things to do
than to hang out on kernelnewbies.org and teach the newbies. If this guide
teaches you anything, they are partially to blame.</para>
<para>Both Ori and I would like to thank Richard M. Stallman and Linus
Torvalds for giving us the opportunity to not only run a high-quality
operating system, but to take a close peek at how it works. I've never met
Linus, and probably never will, but he has made a profound difference in my
life.</para>
</sect1>
<sect1><title>Nota Bene</title>
<para>Ori's original document was good about supporting earlier versions of
Linux, going all the way back to the 2.0 days. I had originally intended
to keep with the program, but after thinking about it, opted out. My main
reason to keep with the compatibility was for Linux distributions like
LEAF, which tended to use older kernels. However, even LEAF uses 2.2 and
2.4 kernels these days.<para>
<para>Both Ori and I use the x86 platform. For the most part, the source
code and discussions should apply to other architectures, but I can't
promise anything. One exception is chapter (fixme), on interrupt handlers,
which should not work on any architecture except for x86.</para>
</sect1>

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<sect1><title>What Is A Kernel Module?</title>
<para>So, you want to write a kernel module. You know C, you've written a few
normal programs to run as processes, and now you want to get to where the real
action is, to where a single wild pointer can wipe out your file system and a core
dump means a reboot.</para>
<para>What exactly is a kernel module? Modules are pieces of code that can be
loaded and unloaded into the kernel upon demand. They extend the functionality of
the kernel without the need to reboot the system. For example, one type of module
is the device driver, which allows the kernel to access hardware connected to the
system. Without modules, we would have to build monolithic kernels and add new
functionality directly into the kernel image. Besides having larger kernels, this
has the disadvantage of requiring us to rebuild and reboot the kernel every time we
want new functionality.</para>
</sect1>
<sect1><title>How Do Modules Get Into The Kernel?</title>
<indexterm><primary>/proc/modules</primary></indexterm>
<indexterm><primary>kmod</primary></indexterm>
<indexterm><primary>kerneld</primary></indexterm>
<indexterm><primary><filename>/etc/modules.conf</filename></primary></indexterm>
<indexterm><primary><filename>/etc/conf.modules</filename></primary></indexterm>
<para>You can see what modules are already loaded into the kernel by running
<command>lsmod</command>, which gets its information by reading the file
<filename>/proc/modules</filename>.</para>
<para>How do these modules find their way into the kernel? When the kernel needs a
feature that is not resident in the kernel, the kernel module daemon
kmod<footnote><para>In earlier versions of linux, this was known as
kerneld.</para></footnote> execs modprobe to load the module in. modprobe is
passed a string in one of two forms:</para>
<itemizedlist>
<listitem><para>A module name like <filename>softdog</filename> or
<filename>ppp</filename>.</listitem>
<listitem><para>A more generic identifier like
<varname>char-major-10-30</varname>.</listitem>
</itemizedlist>
<para>If modprobe is handed a generic identifier, it first looks for that string in
the file <filename>/etc/modules.conf</filename><footnote><para>This file used to be
called <filename>conf.modules</filename> before linux 2.0, but this name is now
deprecated.</para></footnote>. If it finds an alias line like:</para>
<screen>
alias char-major-10-30 softdog
</screen>
<para>it knows that the generic identifier refers to the module
<filename>softdog.o</filename>.</para>
<para>Next, modprobe looks through the file
<filename>/lib/modules/version/modules.dep</filename>, to see if other modules must
be loaded before the requested module may be loaded. This file is created by
<command>depmod -a</command> and contains module dependencies. For example,
<filename>msdos.o</filename> requires the <filename>fat.o</filename> module to be
already loaded into the kernel. The requested module has a dependancy on another
module if the other module defines symbols (variables or functions) that the
requested module uses.</para>
<para>Lastly, modprobe uses insmod to first load any prerequisite modules into the
kernel, and then the requested module. modprobe directs insmod to <filename
role="directory">/lib/modules/version/</filename><footnote><para>If you are
modifying the kernel, to avoid overwriting your existing modules you may want to
use the <varname>EXTRAVERSION</varname> variable in the kernel Makefile to create a
seperate directory.</para></footnote>, the standard directory for modules. insmod
is intended to be fairly dumb about the location of modules, whereas modprobe is
aware of the default location of modules. So for example, if you wanted to load
the msdos module, you'd have to either run:</para>
<screen>
insmod /lib/modules/2.5.1/kernel/fs/fat/fat.o
insmod /lib/modules/2.5.1/kernel/fs/msdos/msdos.o
</screen>
<para>or just run "<command>modprobe -a msdos</command>".</para>
<indexterm><primary>modules.conf</primary><secondary>keep</secondary></indexterm>
<indexterm><primary>modules.conf</primary><secondary>comment</secondary></indexterm>
<indexterm><primary>modules.conf</primary><secondary>alias</secondary></indexterm>
<indexterm><primary>modules.conf</primary><secondary>options</secondary></indexterm>
<indexterm><primary>modules.conf</primary><secondary>path</secondary></indexterm>
<para>Linux distros provide modprobe, insmod and depmod as a package called
modutils or mod-utils.</para>
<para>Before finishing this chapter, let's take a quick look at a piece of
<filename>/etc/modules.conf</filename>:</para>
<screen>
#This file is automatically generated by update-modules
path[misc]=/lib/modules/2.4.?/local
keep
path[net]=~p/mymodules
options mydriver irq=10
alias eth0 eepro
</screen>
<para>Lines beginning with a '#' are comments. Blank lines are ignored.</para>
<para>The <literal>path[misc]</literal> line tells modprobe to replace the search
path for misc modules with the directory <filename
role="directory">/lib/modules/2.4.?/local</filename>. As you can see, shell meta
characters are honored.</para>
<para>The <literal>path[net]</literal> line tells modprobe to look for net modules
in the directory <filename role="directory">~p/mymodules</filename>, however, the
"keep" directive preceding the <literal>path[net]</literal> directive tells modprobe
to add this directory to the standard search path of net modules as opposed to
replacing the standard search path, as we did for the misc modules.</para>
<para>The alias line says to load in <filename>eepro.o</filename> whenever kmod
requests that the generic identifier `eth0' be loaded.</para>
<para>You won't see lines like "alias block-major-2 floppy" in
<filename>/etc/modules.conf</filename> because modprobe already knows about the
standard drivers which will be used on most systems.</para>
<para>Now you know how modules get into the kernel. There's a bit more to the story
if you want to write your own modules which depend on other modules (we calling this
`stacking modules'). But this will have to wait for a future chapter. We have a
lot to cover before addressing this relatively high-level issue.</para>
<sect2><title>Before We Begin</title>
<para>Before we delve into code, there are a few issues we need to cover.
Everyone's system is different and everyone has their own groove. Getting your
first "hello world" program to compile and load correctly can sometimes be a
trick. Rest assured, after you get over the initial hurdle of doing it for the
first time, it will be smooth sailing thereafter.</para>
<sect3><title>Modversioning</title>
<para>A module compiled for one kernel won't load if you boot a different
kernel unless you enable <literal>CONFIG_MODVERSION</literal> in the kernel.
We won't go into module versioning until later in this guide. Until we
cover modversions, the examples in the guide may not work if you're running
a kernel with modversioning turned on. However, most stock Linux distro
kernels come with it turned on. If you're having trouble loading the
modules because of versioning errors, compile a kernel with modversioning
turned off.</para>
</sect3>
<sect3><title>Using X</title>
<para>It is highly recommended that you type in, compile and load all the
examples this guide discusses. It's also highly recommended you do this
from a console. You should not be working on this stuff in X.</para>
<para>Modules can't print to the screen like <function>printf()</function>
can, but they can log information and warnings, which ends up being printed
on your screen, but only on a console. If you insmod a module from an
xterm, the information and warnings will be logged, but only to your log
files. You won't see it unless you look through your log files. To have
immediate access to this information, do all your work from console.</para>
</sect3>
<sect3><title>Compiling Issues and Kernel Version</title>
<para>Very often, Linux distros will distribute kernel source that has been
patched in various non-standard ways, which may cause trouble.</para>
<para>A more common problem is that some Linux distros distribute incomplete
kernel headers. You'll need to compile your code using various header files
from the Linux kernel. Murphy's Law states that the headers that are
missing are exactly the ones that you'll need for your module work.</para>
<para>To avoid these two problems, I highly recommend that you download,
compile and boot into a fresh, stock Linux kernel which can be downloaded
from any of the Linux kernel mirror sites. See the Linux Kernel HOWTO for
more details.</para>
<para>Ironically, this can also cause a problem. By default, gcc on your
system may look for the kernel headers in their default location rather than
where you installed the new copy of the kernel (usually in <filename
role="directory">/usr/src/</filename>. This can be fixed by using gcc's
<literal>-I</literal> switch.</para>
</sect3>
</sect2>
</sect1>
<!--
vim: tw=86
-->

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<sect1><title>Hello, World (part 1): The Simplest Module</title>
<para>When the first caveman programmer chiseled the first program on the walls of
the first cave computer, it was a program to paint the string `Hello, world' in
Antelope pictures. Roman programming textbooks began with the `Salut, Mundi'
program. I don't know what happens to people who break with this tradition, but I
think it's safer not to find out. We'll start with a series of hello world programs
that demonstrate the different aspects of the basics of writing a kernel
module.</para>
<para>Here's the simplest module possible. Don't compile it yet; we'll cover module
compilation in the next section.</para>
<example><title>Hello World (part 1)</title>
<programlisting><![CDATA[
/* hello-1.c - The simplest kernel module.
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_ALERT */
int init_module(void)
{
printk("<1>Hello world 1.\n");
/* A non 0 return means init_module failed; module can't be loaded. */
return 0;
}
void cleanup_module(void)
{
printk(KERN_ALERT "Goodbye world 1.\n");
}
]]></programlisting>
</example>
<indexterm><primary><function>init_module()</function></primary></indexterm>
<indexterm><primary><function>cleanup_module()</function></primary></indexterm>
<indexterm><primary><function>printk()</function></primary></indexterm>
<para>A kernel module must have at least two functions: a "start" (initialization)
function called <function>init_module()</function> which is called when the module is
insmoded into the kernel, and an "end" (cleanup) function called
<function>cleanup_module()</function> which is called just before it is
rmmoded.</para>
<para>Typically, <function>init_module()</function> either registers a handler for
something with the kernel, or it replaces one of the kernel functions with its own
code (usually code to do something and then call the original function). The
<function>cleanup_module()</function> function is supposed to undo whatever
<function>init_module()</function> did, so the module can be unloaded safely.</para>
<para>Despite what you might think, <function>printk()</function> was not meant to
communicate information to the user, even though we use it for exactly this purpose
within this document! It happens to be a logging mechanism for the kernel, and is
used to log information or give warnings. Therefore, each
<function>printk()</function> statement comes with a priority, which is the
<varname>&lt;1&gt;</varname> you see. There are 8 priorities and the kernel has
macros for them, so you don't have to use cryptic numbers. We could've used a macro
instead of the explicit priority level: <function>printk(KERN_ALERT "Hello,
world.");</function> There are 8 priority levels and you can view them (and what
they mean) in the file <filename role="headerfile">linux/kernel.h</filename>. If you
don't specify a priority level, the default priority,
<varname>DEFAULT_MESSAGE_LOGLEVEL</varname>, will be used.</para>
<para>Take time to read through the priority macros. The header file also describes
what each priority means. In practise, don't use number, like
<literal>&lt;4&gt;</literal>. Always use the macro, like
<literal>KERN_WARNING</literal>.</para>
<para>If the priority is less than <varname>int console_loglevel</varname>, the
message is printed on your current terminal. If both <command>syslogd</command> and
<application>klogd</application> are running, then the message will also get appended
to <filename>/var/log/messages</filename>, whether it got printed to the console or
not. We use a high priority, like <literal>KERN_ALERT</literal>, to make sure the
<function>printk()</function> messages get printed to your console rather than just
logged to your logfile. When you write real modules, you'll want to use priorities
that are meaningful for the situation at hand.</para>
<para>There's more I want to show you using "hello world" type programs, but before
we move on, you need to learn how to compile them.</para>
</sect1>
<sect1><title>Compiling Kernel Modules</title>
<indexterm><primary>insmod</primary></indexterm>
<para>A kernel module is not an independant executable, but an object file which will
be linked into the kernel during runtime using insmod. As a result, modules should
be compiled with the <option>-c</option> flag. Also, because the kernel makes
extensive use of inline functions, modules must be compiled with the optimization
flag, <option>-O</option>, although heavy optimization like <option>-O2</option> is
not recommended. Without optimization, some of the assembler macros calls will be
mistaken by the compiler for function calls. This will cause loading the module to
fail, since insmod won't find those functions in the kernel.</para>
<para>Kernel modules also need to be compiled with certain symbols defined. This is
because the kernel header files need to behave differently, depending on whether
we're compiling a kernel module or an executable. You define symbols using gcc's
<option>-D</option> option, and here are a list of symbols that should be defined for
every module you compile:</para>
<indexterm><primary>MODULE</primary></indexterm>
<indexterm><primary>__KERNEL__</primary></indexterm>
<indexterm><primary>__SMP__</primary></indexterm>
<itemizedlist>
<listitem><para><varname>__KERNEL__</varname>: Tells the header files that the code
will be run in kernel mode, not as a user process.</para></listitem>
<listitem><para><varname>MODULE</varname>: Tells the header files to give the
appropriate definitions for a kernel module.</para></listitem>
<listitem><para><varname>__SMP__</varname>: This must be defined if the kernel was
compiled to support symmetrical multiprocessing, even if it's running just on one
CPU. Note how I did this in <filename>hello-1.c</filename>.</para></listitem>
</itemizedlist>
<para> So, let's look at a simple Makefile for compiling a module:</para>
<example><title>Makefile for a basic kernel module</title>
<screen><![CDATA[
# Makefile for a basic kernel module
CC=gcc
CFLAGS := -c -0 -W -Wall -Wstrict-prototypes -Wmissing-prototypes
MODFLAGS := -DMODULE -D__KERNEL__
hello-1.o: hello.c
${CC} ${MODCFLAGS} hello.c
]]></screen>
</example>
<para>Type <filename>hello-1.c</filename> in and compile it. Insert it into the
kernel with <command>insmod ./hello-1.o</command>. Neat, eh? All modules loaded
into the kernel are listed in <filename>/proc/modules</filename>. Go ahead and cat
that file to see that your module is really a part of the kernel. Congratulations,
you are now the author of Linux kernel code! When the novelty wares off, you can
remove your module from the kernel by using <command>rmmod hello-1</command>. Take
a look at <filename>/var/log/messages</filename> just to see that it got logged to
your system logfile.</para>
</sect1>
<sect1><title>Hello World (part 2): The <function>module_init()</function> and
<function>module_exit()</function> Macros</title>
<indexterm><primary>module_init</primary></indexterm>
<indexterm><primary>module_exit</primary></indexterm>
<para>As of Linux 2.4, you can rename the init and cleanup functions of your
modules; they no longer have to be called <function>init_module()</function> and
<function>cleanup_module()</function> respectively. This is done with the
<function>module_init()</function> and <function>module_exit()</function> macros.
These macros are defined in <filename role="header">linux/init.h</filename>. The
only caveat is that your init and cleanup functions must be defined before calling
the macros, otherwise you'll get compilation errors. Here's an example of this
technique:</para>
<example><title>Hello World (part 2)</title>
<programlisting><![CDATA[
/* hello-2.c - Demonstrating the module_init() and module_exit() macros.
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_ALERT */
#include <linux/init.h> /* Needed for the macros */
int my_wonderful_init(void)
{
printk(KERN_ALERT "Hello, world 2\n");
return 0;
}
void my_wonderful_cleanup(void)
{
printk(KERN_ALERT "Goodbye, world 2\n");
}
module_init(my_wonderful_init);
module_exit(my_wonderful_cleanup);
]]></programlisting>
</example>
</sect1>
<sect1><title>Hello World (part 3): The <literal>__init</literal> and
<literal>__exit</literal> Macros</title>
<indexterm><primary><function>__init</function></primary></indexterm>
<indexterm><primary><function>__initdata</function></primary></indexterm>
<indexterm><primary><function>__exit</function></primary></indexterm>
<indexterm><primary><function>__initfunction()</function></primary></indexterm>
<para>This demonstrates a feature of kernel 2.2 and later. Notice the change in the
definitions of the init and cleanup functions. The <function>__init</function> macro
will cause the init function to be discarded and its memory reclaimed for the kernel
once the init function finishes, but only for built-in drivers. It has no effect for
loadable modules.</para>
<para>There is also an <function>__initdata</function> which works similarly to
<function>__init</function> but for init variables rather than functions.</para>
<para>The <function>__exit</function> macro causes the omission of the function when
the module is built into the kernel. This only has an effect for built in modules
since they never exit (and hence don't need a cleanup function). It has no effect on
loadable modules since they need their cleanup function.</para>
<para>These macros are defined in <filename role="headerfile">linux/init.h</filename>
and serve to free up kernel memory. When you boot your kernel and see something like
<literal>Freeing unused kernel memory: 236k freed</literal>, this is precisely what
the kernel is freeing.</para>
<example><title>Hello World (part 3)</title>
<programlisting><![CDATA[
/* hello-3.c - Illustrating the __init, __initdata and __exit macros.
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_ALERT */
#include <linux/init.h> /* Needed for the macros */
static int hello3_data __initdata = 3;
static int __init hello3_init_function(void)
{
printk(KERN_ALERT "Hello, world %d\n", hello3_data);
return 0;
}
static void __exit hello3_cleanup_function(void)
{
printk(KERN_ALERT "Goodbye, world 3\n");
}
module_init(hello3_init_function);
module_exit(hello3_cleanup_function);
]]></programlisting>
</example>
<para>You may see a directive named "<function>__initfunction()</function>" in
drivers written for Linux 2.2 kernels:</para>
<screen><![CDATA[
__initfunction(int init_module(void))
{
printk(KERN_ALERT "Hi there.\n");
return 0;
}
]]></screen>
<para>This macro served the same purpose as <function>__init</function>, but is now
deprecated in favor of <function>__init</function>. Don't use
<function>__initfunction()</function> in your own code.</para>
</sect1>
<sect1><title>Hello World (part 4): Licensing and Module Documentation</title>
<indexterm><primary><literal>MODULE_LICENSE()</literal></primary></indexterm>
<indexterm><primary><literal>MODULE_DESCRIPTION()</literal></primary></indexterm>
<indexterm><primary><literal>MODULE_AUTHOR()</literal></primary></indexterm>
<indexterm><primary><literal>MODULE_SUPPORTED_DEVICE()</literal></primary></indexterm>
<para>If you're running kernel 2.4 or later, you might have noticed the message like:
"<literal>Warning: loading hello-1.o will taint the kernel: no license</literal>"
when you loaded the previous example modules. In 2.4 and later, a mechanism was
devised to identify code licensed under the GPL (and friends) so people can be warned
that the code is non open-source. This is accomplished by the
<literal>MODULE_LICENSE()</literal> macro which is demonstrated in the next piece of
code. By setting the license to GPL, you can keep the warning from being printed.
This mechanism is documented in <filename
role="headerfile">linux/module.h</filename>, and I recommend you read the comments
about this macro in the header file.</para>
<para>A similar mechanism is used to identify the module description
"<literal>MODULE_DESCRIPTION()</literal>", author
"<literal>MODULE_AUTHOR()</literal>" and what device the module supports
"<literal>MODULE_SUPPORTED_DEVICE()</literal>" and is defined in <filename
role="headerfile">linux/module.h</filename>. This info isn't really used by the
kernel itself; it's used as documentation and can be viewed by a tool like
objdump.</para>
<para>We haven't covered devices yet, so the last macro may be a mystery to you, but
keep it in mind. We'll cover char devices shortly.</para>
<example><title>Hello World (part 4)</title>
<programlisting><![CDATA[
/* hello-4.c - Demonstrates tainting messages and documentation.
*/
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#define DRIVER_AUTHOR "Peter Jay Salzman <p@dirac.org>"
#define DRIVER_DESC "A sample driver"
static int __init hello4_init_function(void)
{
printk(KERN_ALERT "Hello, world 4\n");
return 0;
}
static void __exit hello4_cleanup_function(void)
{
printk(KERN_ALERT "Goodbye, world 4\n");
}
module_init(hello4_init_function);
module_exit(hello4_cleanup_function);
/* You can use strings here or a define, as shown. It doesn't matter what you
* actually name the #define's, so "AUTHOR" is as good as "DRIVER_AUTHOR". */
MODULE_AUTHOR(DRIVER_AUTHOR);
MODULE_DESCRIPTION(DRIVER_DESC);
/* This gets rid of the "taint message" by declaring this code as GPL. */
MODULE_LICENSE("GPL");
/* This says that the module uses /dev/testdevice. It might be used in the
* future to help automatic configuration of modules, but is currently unused
* other than documentation purposes. */
MODULE_SUPPORTED_DEVICE("testdevice");
]]></programlisting>
</example>
</sect1>
<sect1><title>Passing Command Line Arguments to a Module</title>
<para>Modules can take command line arguments, but not with the argc/argv you might
be used to.</para>
<para>To allow arguments to be passed to your driver, declare the variables that will
take the values of the command line arguments as global and then use the MODULE_PARM
macro (defined in <filename role="headerfile">linux/module.h</filename>) to set the
mechanism up. At runtime, insmod will fill the variables with any command line
arguments that are given. The variable declarations and macros should be placed at
the beginning of the module for clarity. The example code should clear up my
admittedly lousy explanation.</para>
<para>The <literal>MODULE_PARM</literal> macro takes 2 arguments: the name of the
variable and its type. The supported variable types are "<literal>b</literal>":
single byte, "<literal>h</literal>": short int, "<literal>i</literal>": integer,
"<literal>l</literal>": long int and "<literal>s</literal>": string. Strings should
be declared as "<type>char *</type>" and insmod will allocate memory for them. You
should always try to give the variables an initial default value. This is kernel
code, and you should program defensively. For example:</para>
<screen>
int myint = 3;
char *mystr;
MODULE_PARM (myint, "i");
MODULE_PARM (mystr, "s");
</screen>
<para>Arrays are supported too. An integer value preceding the type in MODULE_PARM
will indicate an array of some maximum length. Two numbers separated by a '-' will
give the minimum and maximum number of values. For example, an array of shorts with
at least 2 and no more than 4 values could be declared as:</para>
<screen>
int myshortArray[4];
MODULE_PARM (myintArray, "2-4i");
</screen>
<para>A good use for this is to have the module variable's default values set, like
which IO port or IO memory to use. If the variables contain the default values, then
perform autodetection (explained elsewhere). Otherwise, keep the current value.
This will be made clear later on. For now, I just want to demonstrate passing
arguments to a module.</para>
<example><title>Hello World (part 5)</title>
<programlisting><![CDATA[
/* hello-5.c - Demonstrates command line argument passing to a module.
*/
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
static int myint = 0;
static char *mystring = "blah";
MODULE_PARM (myint, "i");
MODULE_PARM (mystring, "s");
static int __init hello5_init_function(void)
{
printk(KERN_ALERT "Hello, world 5\n");
printk(KERN_ALERT "integer: %i\n", myint);
printk(KERN_ALERT "string: %s\n", mystring);
return 0;
}
static void __exit hello5_cleanup_function(void)
{
printk(KERN_ALERT "Goodbye, world 5\n");
}
module_init(hello5_init_function);
module_exit(hello5_cleanup_function);
]]></programlisting>
</example>
</sect1>
<sect1><title>Modules Spanning Multiple Files</title>
<indexterm><primary>source files</primary><secondary>multiple</secondary></indexterm>
<indexterm><primary>__NO_VERSION__</primary></indexterm>
<indexterm><primary>module.h</primary></indexterm>
<indexterm><primary>version.h</primary></indexterm>
<indexterm><primary>kernel\_version</primary></indexterm>
<indexterm><primary>ld</primary></indexterm>
<indexterm><primary>elf_i386</primary></indexterm>
<para>Sometimes it makes sense to divide a kernel module between several
source files. In this case, you need to:</para>
<orderedlist>
<listitem><para>In all the source files but one, add the line
<command>#define __NO_VERSION__</command>. This is important because
<filename role="headerfile">module.h</filename> normally includes the
definition of <varname>kernel_version</varname>, a global variable with
the kernel version the module is compiled for. If you need <filename
role="headerfile">version.h</filename>, you need to include it yourself,
because <filename role="headerfile">module.h</filename> won't do it for
you with <varname>__NO_VERSION__</varname>.</para></listitem>
<listitem><para>Compile all the source files as usual.</para></listitem>
<listitem><para>Combine all the object files into a single one. Under x86,
use <command>ld -m elf_i386 -r -o &lt;module name.o&gt; &lt;1st src
file.o&gt; &lt;2nd src file.o&gt;</command>.</para></listitem>
</orderedlist>
<para>Here's an example of such a kernel module.</para>
<example><title>start.c</title>
<programlisting><![CDATA[
/* start.c - Illustration of multi filed modules
*/
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
int init_module(void)
{
printk("Hello, world - this is the kernel speaking\n");
return 0;
}
]]></programlisting>
</example>
<para>The next file:</para>
<example><title>stop.c</title>
<programlisting><![CDATA[
/* stop.c - Illustration of multi filed modules
*/
#if defined(CONFIG_MODVERSIONS) && ! defined(MODVERSIONS)
#include <linux/modversions.h> /* Will be explained later */
#define MODVERSIONS
#endif
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
#define __NO_VERSION__ /* It's not THE file of the kernel module */
#include <linux/version.h> /* Not included by module.h because of
__NO_VERSION__ */
void cleanup_module()
{
printk("<1>Short is the life of a kernel module\n");
}
]]></programlisting>
</example>
<para>And finally, the makefile:</para>
<example><title>Makefile for a multi-filed module</title>
<screen><![CDATA[
CC=gcc
MODCFLAGS := -O -Wall -DMODULE -D__KERNEL__
hello.o: hello2_start.o hello2_stop.o
ld -m elf_i386 -r -o hello2.o hello2_start.o hello2_stop.o
start.o: hello2_start.c
${CC} ${MODCFLAGS} -c hello2_start.c
stop.o: hello2_stop.c
${CC} ${MODCFLAGS} -c hello2_stop.c
]]></screen>
</example>
</sect1>
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<sect1><title>Modules vs Programs</title>
<sect2><title>How modules begin and end</title>
<para>A program usually begins with a <function>main()</function> function, executes a
bunch of instructions and terminates upon completion of those instructions. Kernel
modules work a bit differently. A module always begin with either the
<function>init_module</function> or the function you specify with
<function>module_init</function> call. This is the entry function for modules; it tells
the kernel what functionality the module provides and sets up the kernel to run the
module's functions when they're needed. Once it does this, entry function returns and the
module does nothing until the kernel wants to do something with the code that the module
provides.</para>
<para>All modules end by calling either <function>cleanup_module</function> or the
function you specify with the <function>module_exit</function> call. This is the exit
function for modules; it undoes whatever entry function did. It unregisters the
functionality that the entry function registered.</para>
<para>Every module must have an entry function and an exit function. Since there's more
than one way to specify entry and exit functions, I'll try my best to use the terms `entry
function' and `exit function', but if I slip and simply refer to them as
<function>init_module</function> and <function>cleanup_module</function>, I think you'll
know what I mean.</para>
</sect2>
<sect2><title>Functions available to modules</title>
<indexterm><primary>library function</primary></indexterm>
<indexterm><primary>system call</primary></indexterm>
<indexterm><primary><filename>/proc/ksyms</filename></primary></indexterm>
<para>Programmers use functions they don't define all the time. A prime example of this
is <function>printf()</function>. You use these library functions which are provided by
the standard C library, libc. The definitions for these functions don't actually enter
your program until the linking stage, which insures that the code (for
<function>printf()</function> for example) is available, and fixes the call instruction
to point to that code.</para>
<para>Kernel modules are different here, too. In the hello world example, you might
have noticed that we used a function, <function>printk()</function> but didn't include a
standard I/O library. That's because modules are object files whose symbols get
resolved upon insmod'ing. The definition for the symbols comes from the kernel itself;
the only external functions you can use are the ones provided by the kernel. If you're
curious about what symbols have been exported by your kernel, take a look at
<filename>/proc/ksyms</filename>.</para>
<para>One point to keep in mind is the difference between library functions and system
calls. Library functions are higher level, run completely in user space and provide a
more convenient interface for the programmer to the functions that do the real
work---system calls. System calls run in kernel mode on the user's behalf and are
provided by the kernel itself. The library function <function>printf()</function> may
look like a very general printing function, but all it really does is format the data
into strings and write the string data using the low-level system call
<function>write()</function>, which then sends the data to standard output.</para>
<para> Would you like to see what system calls are made by
<function>printf()</function>? It's easy! Compile the following program: </para>
<screen>
#include &lt;stdio.h&gt;
int main(void)
{ printf("hello"); return 0; }
</screen>
<indexterm><primary>strace</primary></indexterm>
<para>with <command>gcc -Wall -o hello hello.c</command>. Run the exectable with
<command>strace hello</command>. Are you impressed? Every line you see corresponds to
a system call. strace<footnote><para>It's an invaluable tool for figuring out things
like what files a program is trying to access. Ever have a program bail silently
because it couldn't find a file? It's a PITA!</para></footnote> is a handy program that
gives you details about what system calls a program is making, including which call is
made, what its arguments are what it returns. It's an invaluable tool for figuring out
things like what files a program is trying to access. Towards the end, you'll see a
line which looks like <function>write(1, "hello", 5hello)</function>. There it is. The
face behind the <function>printf()</function> mask. You may not be familiar with write,
since most people use library functions for file I/O (like fopen, fputs, fclose). If
that's the case, try looking at <command>man 2 write</command>. The 2nd man section is
devoted to system calls (like <function>kill()</function> and
<function>read()</function>. The 3rd man section is devoted to library calls, which you
would probably be more familiar with (like <function>cosh()</function> and
<function>random()</function>).</para>
<para>You can even write modules to replace the kernel's system calls, which we'll do
shortly. Crackers often make use of this sort of thing for backdoors or trojans, but
you can write your own modules to do more benign things, like have the kernel write
<emphasis>Tee hee, that tickles!</emphasis> everytime someone tries to delete a file on
your system.</para>
</sect2>
<sect2><title>User Space vs Kernel Space</title>
<para>A kernel is all about access to resources, whether the resource in question happens
to be a video card, a hard drive or even memory. Programs often compete for the same
resource. As I just saved this document, updatedb started updating the locate database.
My vim session and updatedb are both using the hard drive concurrently. The kernel needs
to keep things orderly, and not give users access to resources whenever they feel like it.
To this end, a <acronym>CPU</acronym> can run in different modes. Each mode gives a
different level of freedom to do what you want on the system. The Intel 80386
architecture has 4 of these modes, which are called rings. Unix uses only two rings; the
highest ring (ring 0, also known as `supervisor mode' where everything is allowed to
happen) and the lowest ring, which is called `user mode'.</para>
<para>Recall the discussion about library functions vs system calls. Typically, you use a
library function in user mode. The library function calls one or more system calls, and
these system calls execute on the library function's behalf, but do so in supervisor mode
since they are part of the kernel itself. Once the system call completes its task, it
returns and execution gets transfered back to user mode.</para>
</sect2>
<sect2><title>Name Space</title>
<indexterm><primary>symbol table</primary></indexterm>
<indexterm><primary>namespace pollution</primary></indexterm>
<indexterm><primary><filename>/proc/ksyms</filename></primary></indexterm>
<para>When you write a small C program, you use variables which are convenient and make
sense to the reader. If, on the other hand, you're writing routines which will be part
of a bigger problem, any global variables you have are part of a community of other
peoples' global variables; some of the variable names can clash. When a program has
lots of global variables which aren't meaningful enough to be distinguished, you get
<emphasis>namespace pollution</emphasis>. In large projects, effort must be made to
remember reserved names, and to find ways to develop a scheme for naming unique variable
names and symbols.</para>
<para>When writing kernel code, even the smallest module will be linked against the
entire kernel, so this is definitely an issue. The best way to deal with this is to
declare all your variables as <type>static</type> and to use a well-defined prefix for
your symbols. By convention, all kernel prefixes are lowercase. If you don't want to
declare everything as <type>static</type>, another option is to declare a
<varname>symbol table</varname> and register it with a kernel. We'll get to this
later.</para>
<para>The file <filename>/proc/ksyms</filename> holds all the symbols that the kernel
knows about and which are therefore accessible to your modules since they share the
kernel's codespace.</para>
</sect2>
<sect2><title>Code space</title>
<indexterm><primary>code space</primary></indexterm>
<indexterm><primary>monolithic kernel</primary></indexterm>
<indexterm><primary>Hurd</primary></indexterm>
<indexterm><primary>Neutrino</primary></indexterm>
<indexterm><primary>microkernel</primary></indexterm>
<para>Memory management is a very complicated subject---the majority of O'Reilly's
`Understanding The Linux Kernel' is just on memory management! We're not setting out to
be experts in memory managements, but we do need to know a couple of facts to even begin
worrying about writing real modules.</para>
<para>If you haven't thought about what a segfault really means, you may be surprised to
hear that pointers don't actually point to memory locations. Not real ones, anyway.
When a process is created, the kernel sets aside a portion of real physical memory and
hands it to the process to use for its executing code, variables, stack, heap and other
things which a computer scientist would know about<footnote><para>I'm a physicist, not a
computer scientist, Jim!</para></footnote>. This memory begins with $0$ and extends up
to whatever it needs to be. Since the memory space for any two processes don't overlap,
every process that can access a memory address, say <literal>0xbffff978</literal>, would
be accessing a different location in real physical memory! The processes would be
accessing an index named <literal>0xbffff978</literal> which points to some kind of
offset into the region of memory set aside for that particular process. For the most
part, a process like our Hello, World program can't access the space of another process,
although there are ways which we'll talk about later.</para>
<para>The kernel has its own space of memory as well. Since a module is code which can
be dynamically inserted and removed in the kernel (as opposed to a semi-autonomous
object), it shares the kernel's codespace rather than having its own. Therefore, if
your module segfaults, the kernel segfaults. And if you start writing over data because
of an off-by-one error, then you're trampling on kernel code. This is even worse than
it sounds, so try your best to be careful.</para>
<para>By the way, I would like to point out that the above discussion is true for any
operating system which uses a monolithic kernel<footnote><para>This isn't quite the same
thing as `building all your modules into the kernel', although the idea is the
same.</para></footnote>. There are things called microkernels which have modules which
get their own codespace. The GNU Hurd and QNX Neutrino are two examples of a
microkernel.</para>
</sect2>
<sect2><title>Device Drivers</title>
<para>One class of module is the device driver, which provides functionality for
hardware like a TV card or a serial port. On unix, each piece of hardware is
represented by a file located in <filename role=directory>/dev</filename> named a
<filename>device file</filename> which provides the means to communicate with the
hardware. The device driver provides the communication on behalf of a user program. So
the <filename>es1370.o</filename> sound card device driver might connect the <filename
role="devicefile">/dev/sound</filename> device file to the Ensoniq IS1370 sound card. A
userspace program like mp3blaster can use <filename
role="devicefile">/dev/sound</filename> without ever knowing what kind of sound card is
installed.</para>
<sect3><title>Major and Minor Numbers</title>
<indexterm><primary>major number</primary></indexterm>
<indexterm><primary>minor number</primary></indexterm>
<para>Let's look at some device files. Here are device files which represent the
first three partitions on the primary master IDE hard drive:</para>
<screen>
# ls -l /dev/hda[1-3]
brw-rw---- 1 root disk 3, 1 Jul 5 2000 /dev/hda1
brw-rw---- 1 root disk 3, 2 Jul 5 2000 /dev/hda2
brw-rw---- 1 root disk 3, 3 Jul 5 2000 /dev/hda3
</screen>
<para>Notice the column of numbers separated by a comma? The first number is
called the device's major number. The second number is the minor number. The
major number tells you which driver is used to access the hardware. Each driver
is assigned a unique major number; all device files with the same major number are
controlled by the same driver. All the above major numbers are 3, because they're
all controlled by the same driver.</para>
<para>The minor number is used by the driver to distinguish between the various
hardware it controls. Returning to the example above, although all three devices
are handled by the same driver they have unique minor numbers because the driver
sees them as being different pieces of hardware.</para>
<para> Devices are divided into two types: character devices and block devices.
The difference is that block devices have a buffer for requests, so they can
choose the best order in which to respond to the requests. This is important in
the case of storage devices, where it's faster to read or write sectors which are
close to each other, rather than those which are further apart. Another
difference is that block devices can only accept input and return output in blocks
(whose size can vary according to the device), whereas character devices are
allowed to use as many or as few bytes as they like. Most devices in the world
are character, because they don't need this type of buffering, and they don't
operate with a fixed block size. You can tell whether a device file is for a
block device or a character device by looking at the first character in the output
of <command>ls -l</command>. If it's `b' then it's a block device, and if it's `c'
then it's a character device. The devices you see above are block devices. Here
are some character devices (the serial ports):</para>
<screen>
crw-rw---- 1 root dial 4, 64 Feb 18 23:34 /dev/ttyS0
crw-r----- 1 root dial 4, 65 Nov 17 10:26 /dev/ttyS1
crw-rw---- 1 root dial 4, 66 Jul 5 2000 /dev/ttyS2
crw-rw---- 1 root dial 4, 67 Jul 5 2000 /dev/ttyS3
</screen>
<para> If you want to see which major numbers have been assigned, you can look at
<filename>/usr/src/linux/Documentation/devices.txt</filename>. </para>
<indexterm><primary>mknod</primary></indexterm>
<indexterm><primary>coffee</primary></indexterm>
<para>When the system was installed, all of those device files were created by the
<command>mknod</command> command. To create a new char device named `coffee' with
major/minor number <literal>12</literal> and <literal>2</literal>, simply do
<command>mknod /dev/coffee c 12 2</command>. You don't <emphasis>have</emphasis>
to put your device files into <filename role="directory">/dev</filename>, but it's
done by convention. Linus put his device files in <filename> /dev</filename>, and
so should you. However, when creating a device file for testing purposes, it's
probably OK to place it in your working directory where you compile the kernel
module. Just be sure to put it in the right place when you're done writing the
device driver.</para>
<para>I would like to make a few last points which are implicit from the above
discussion, but I'd like to make them explicit just in case. When a device file
is accessed, the kernel uses the major number of the file to determine which
driver should be used to handle the access. This means that the kernel doesn't
really need to use or even know about the minor number. The driver itself is the
only thing that cares about the minor number. It uses the minor number to
distinguish between different pieces of hardware.</para>
<para>By the way, when I say `hardware', I mean something a bit more abstract than
a PCI card that you can hold in your hand. Look at these two device
files:</para>
<screen>
% ls -l /dev/fd0 /dev/fd0u1680
brwxrwxrwx 1 root floppy 2, 0 Jul 5 2000 /dev/fd0
brw-rw---- 1 root floppy 2, 44 Jul 5 2000 /dev/fd0u1680
</screen>
<para>By now you can look at these two device files and know instantly that they
are block devices and are handled by same driver (block major
<literal>2</literal>). You might even be aware that these both represent your
floppy drive, even if you only have one floppy drive. Why two files? One
represents the floppy drive with <literal>1.44</literal> <acronym>MB</acronym> of
storage. The other is the <emphasis>same</emphasis> floppy drive with
<literal>1.68</literal> <acronym>MB</acronym> of storage, and corresponds to what
some people call a `superformatted' disk. One that holds more data than a
standard formatted floppy. So here's a case where two device files with different
minor number actually represent the same piece of physical hardware. So just be
aware that the word `hardware' in our discussion can mean something very
abstract.</para>
</sect3>
</sect2>
</sect1>
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<sect1><title>Character Device Drivers</title>
<indexterm><primary>device file</primary><secondary>character</secondary>
</indexterm>
<sect2><title>The <type>file_operations</type> Structure</title>
<indexterm><primary>file_operations</primary></indexterm>
<para>The <type>file_operations</type> structure is defined in <filename
role="headerfile">linux/fs.h</filename>, and holds pointers to functions
defined by the driver that perform various operations on the device. Each
field of the structure corresponds to the address of some function defined
by the driver to handle a requested operation.</para>
<para> For example, every character driver needs to define a function that
reads from the device. The <type>file_operations</type> structure holds
the address of the module's function that performs that operation. Here is
what the definition looks like for kernel <literal>2.4.2</literal>:</para>
<screen>
struct file_operations {
struct module *owner;
loff_t (*llseek) (struct file *, loff_t, int);
ssize_t (*read) (struct file *, char *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
int (*readdir) (struct file *, void *, filldir_t);
unsigned int (*poll) (struct file *, struct poll_table_struct *);
int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *);
int (*release) (struct inode *, struct file *);
int (*fsync) (struct file *, struct dentry *, int datasync);
int (*fasync) (int, struct file *, int);
int (*lock) (struct file *, int, struct file_lock *);
ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
loff_t *);
ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
loff_t *);
};
</screen>
<para>Some operations are not implemented by a driver. For example, a
driver that handles a video card won't need to read from a directory
structure. The corresponding entries in the <type>file_operations</type>
structure should be set to <varname>NULL</varname>.</para>
<para>There is a gcc extension that makes assigning to this structure more
convenient. You'll see it in modern drivers, and may catch you by
surprise. This is what the new way of assigning to the structure looks
like:</para>
<screen>
struct file_operations fops = {
read: device_read,
write: device_write,
open: device_open,
release: device_release
};
</screen>
<para>However, there's also a C99 way of assigning to elements of a
structure. The version of gcc I'm currently using,
<literal>2.95</literal>, supports the new C99 syntax. You should use this
syntax in case someone wants to port your driver. It will help with
compatibility:</para>
<screen>
struct file_operations fops = {
.read = device_read,
.write = device_write,
.open = device_open,
.release = device_release
};
</screen>
<para>The meaning is clear, and you should be aware that any member of the
structure which you don't explicitly assign will be initialized to
<varname>NULL</varname> by gcc.</para>
<para>A pointer to a <type>struct file_operations</type> is commonly named
<varname>fops</varname>.</para>
</sect2>
<sect2><title>The <type>file</type> structure</title>
<indexterm><primary>file</primary></indexterm>
<indexterm><primary>inode</primary></indexterm>
<para>Each device is represented in the kernel by a <type>file</type>
structure, which is defined in <filename
role="header">linux/fs.h</filename>. Be aware that a <type>file</type> is
a kernel level structure and never appears in a user space program. It's
not the same thing as a <type>FILE</type>, which is defined by glibc and
would never appear in a kernel space function. Also, its name is a bit
misleading; it represents an abstract open `file', not a file on a disk,
which is represented by a structure named <type>inode</type>.</para>
<para>A pointer to a <varname>struct file</varname> is commonly named
<function>filp</function>. You'll also see it refered to as
<varname>struct file file</varname>. Resist the temptation.</para>
<para>Go ahead and look at the definition of <function>file</function>.
Most of the entries you see, like <function>struct dentry</function> aren't
used by device drivers, and you can ignore them. This is because drivers
don't fill <varname>file</varname> directly; they only use structures
contained in <varname>file</varname> which are created elsewhere.</para>
</sect2>
<sect2><title>Registering A Device</title>
<indexterm><primary>register_chrdev</primary></indexterm>
<indexterm><primary>major number</primary>
<secondary>dynamic allocation</secondary></indexterm>
<para>As discussed earlier, char devices are accessed through device files,
usually located in <filename
role="direcotry">/dev</filename><footnote><para>This is by convention.
When writing a driver, it's OK to put the device file in your current
directory. Just make sure you place it in <filename
role="directory">/dev</filename> for a production driver</para></footnote>.
The major number tells you which driver handles which device file. The
minor number is used only by the driver itself to differentiate which
device it's operating on, just in case the driver handles more than one
device.</para>
<para> Adding a driver to your system means registering it with the kernel.
This is synonymous with assigning it a major number during the module's
initialization. You do this by using the
<function>register_chrdev</function> function, defined by <filename
role="headerfile">linux/fs.h</filename>.</para>
<screen>
int register_chrdev(unsigned int major, const char *name,
struct file_operations *fops);
</screen>
<para>where <varname>unsigned int major</varname> is the major number you
want to request, <varname>const char *name</varname> is the name of the
device as it'll appear in <filename>/proc/devices</filename> and
<varname>struct file_operations *fops</varname> is a pointer to the
<varname>file_operations</varname> table for your driver. A negative
return value means the registertration failed. Note that we didn't pass
the minor number to <function>register_chrdev</function>. That's because
the kernel doesn't care about the minor number; only our driver uses it.
</para>
<para>Now the question is, how do you get a major number without hijacking
one that's already in use? The easiest way would be to look through
<filename>Documentation/devices.txt</filename> and pick an unused one.
That's a bad way of doing things because you'll never be sure if the number
you picked will be assigned later. The answer is that you can ask the
kernel to assign you a dynamic major number.</para>
<para> If you pass a major number of 0 to
<function>register_chrdev</function>, the return value will be the
dynamically allocated major number. The downside is that you can't make a
device file in advance, since you don't know what the major number will be.
There are a couple of ways to do this. First, the driver itself can print
the newly assigned number and we can make the device file by hand. Second,
the newly registered device will have an entry in
<filename>/proc/devices</filename>, and we can either make the device file
by hand or write a shell script to read the file in and make the device
file. The third method is we can have our driver make the the device file
using the <function>mknod</function> system call after a successful
registration and rm during the call to
<function>cleanup_module</function>.</para>
</sect2>
<sect2><title>Unregistering A Device</title>
<indexterm><primary>rmmod</primary><secondary>preventing</secondary>
</indexterm>
<para>We can't allow the kernel module to be
<application>rmmod</application>'ed whenever root feels like it. If the
device file is opened by a process and then we remove the kernel module,
using the file would cause a call to the memory location where the
appropriate function (read/write) used to be. If we're lucky, no other
code was loaded there, and we'll get an ugly error message. If we're
unlucky, another kernel module was loaded into the same location, which
means a jump into the middle of another function within the kernel. The
results of this would be impossible to predict, but they can't be very
positive.</para>
<para> Normally, when you don't want to allow something, you return an
error code (a negative number) from the function which is supposed to do
it. With <function>cleanup_module</function> that's impossible because
it's a void function. However, there's a counter which keeps track of how
many processes are using your module. You can see what it's value is by
looking at the 3rd field of <filename>/proc/modules</filename>. If this
number isn't zero, <function>rmmod</function> will fail. Note that you
don't have to check the counter from within
<function>cleanup_module</function> because the check will be performed for
you by the system call <function>sys_delete_module</function>, defined in
<filename>linux/module.c</filename>. You shouldn't use this counter
directly, but there are macros defined in <filename
role="headerfile">linux/modules.h</filename> which let you increase,
decrease and display this counter:</para>
<itemizedlist>
<listitem><para><varname>MOD_INC_USE_COUNT</varname>: Increment the use
count.</para></listitem>
<listitem><para><varname>MOD_DEC_USE_COUNT</varname>: Decrement the use count.
</para></listitem>
<listitem><para><varname>MOD_IN_USE</varname>: Display the use count.
</para></listitem>
</itemizedlist>
<para>It's important to keep the counter accurate; if you ever do lose
track of the correct usage count, you'll never be able to unload the
module; it's now reboot time, boys and girls. This is bound to happen to
you sooner or later during a module's development.</para>
<indexterm><primary>MOD_INC_USE_COUNT</primary></indexterm>
<indexterm><primary>MOD_DEC_USE_COUNT</primary></indexterm>
<indexterm><primary>MOD_IN_USE</primary></indexterm>
</sect2>
<sect2><title>chardev.c</title>
<para>The next code sample creates a char driver named
<filename>chardev</filename>. You can <filename>cat</filename> its device
file (or <filename>open</filename> the file with a program) and the driver
will put the number of times the device file has been read from into the
file. We don't support writing to the file (like <command>echo "hi" >
/dev/hello</command>), but catch these attempts and tell the user that the
operation isn't supported. Don't worry if you don't see what we do with
the data we read into the buffer; we don't do much with it. We simply read
in the data and print a message acknowledging that we received it.
<example><title>chardev.c</title>
<programlisting><![CDATA[
/*
* chardev.c: Creates a read-only char device that says how many times
* you've read from the dev file
*/
#if defined(CONFIG_MODVERSIONS) && ! defined(MODVERSIONS)
#include <linux/modversions.h>
#define MODVERSIONS
#endif
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <asm/uaccess.h> /* for put_user */
/* Prototypes - this would normally go in a .h file
*/
int init_module(void);
void cleanup_module(void);
static int device_open(struct inode *, struct file *);
static int device_release(struct inode *, struct file *);
static ssize_t device_read(struct file *, char *, size_t, loff_t *);
static ssize_t device_write(struct file *, const char *, size_t, loff_t *);
#define SUCCESS 0
#define DEVICE_NAME "chardev" /* Dev name as it appears in /proc/devices */
#define BUF_LEN 80 /* Max length of the message from the device */
/* Global variables are declared as static, so are global within the file. */
static int Major; /* Major number assigned to our device driver */
static int Device_Open = 0; /* Is device open? Used to prevent multiple */
access to the device */
static char msg[BUF_LEN]; /* The msg the device will give when asked */
static char *msg_Ptr;
static struct file_operations fops = {
.read = device_read,
.write = device_write,
.open = device_open,
.release = device_release
};
/* Functions
*/
int init_module(void)
{
Major = register_chrdev(0, DEVICE_NAME, &fops);
if (Major > 0) {
printk ("Registering the character device failed with %d\n", Major);
return Major;
}
printk("<1>I was assigned major number %d. To talk to\n", Major);
printk("<1>the driver, create a dev file with\n");
printk("'mknod /dev/hello c %d 0'.\n", Major);
printk("<1>Try various minor numbers. Try to cat and echo to\n");
printk("the device file.\n");
printk("<1>Remove the device file and module when done.\n");
return 0;
}
void cleanup_module(void)
{
/* Unregister the device */
int ret = unregister_chrdev(Major, DEVICE_NAME);
if (ret < 0) printk("Error in unregister_chrdev: %d\n", ret);
}
/* Methods
*/
/* Called when a process tries to open the device file, like
* "cat /dev/mycharfile"
*/
static int device_open(struct inode *inode, struct file *file)
{
static int counter = 0;
if (Device_Open) return -EBUSY;
Device_Open++;
sprintf(msg,"I already told you %d times Hello world!\n", counter++");
msg_Ptr = msg;
MOD_INC_USE_COUNT;
return SUCCESS;
}
/* Called when a process closes the device file.
*/
static int device_release(struct inode *inode, struct file *file)
{
Device_Open --; /* We're now ready for our next caller */
/* Decrement the usage count, or else once you opened the file, you'll
never get get rid of the module. */
MOD_DEC_USE_COUNT;
return 0;
}
/* Called when a process, which already opened the dev file, attempts to
read from it.
*/
static ssize_t device_read(struct file *filp,
char *buffer, /* The buffer to fill with data */
size_t length, /* The length of the buffer */
loff_t *offset) /* Our offset in the file */
{
/* Number of bytes actually written to the buffer */
int bytes_read = 0;
/* If we're at the end of the message, return 0 signifying end of file */
if (*msg_Ptr == 0) return 0;
/* Actually put the data into the buffer */
while (length && *msg_Ptr) {
/* The buffer is in the user data segment, not the kernel segment;
* assignment won't work. We have to use put_user which copies data from
* the kernel data segment to the user data segment. */
put_user(*(msg_Ptr++), buffer++);
length--;
bytes_read++;
}
/* Most read functions return the number of bytes put into the buffer */
return bytes_read;
}
/* Called when a process writes to dev file: echo "hi" > /dev/hello */
static ssize_t device_write(struct file *filp,
const char *buff,
size_t len,
loff_t *off)
{
printk ("<1>Sorry, this operation isn't supported.\n");
return -EINVAL;
}
]]></programlisting>
</example>
</sect2>
<sect2><title>Writing Modules for Multiple Kernel Versions</title>
<indexterm><primary>kernel versions</primary></indexterm>
<indexterm><primary>LINUX_VERSION_CODE</primary></indexterm>
<indexterm><primary>KERNEL_VERSION</primary></indexterm>
<para>The system calls, which are the major interface the kernel shows to
the processes, generally stay the same across versions. A new system call
may be added, but usually the old ones will behave exactly like they used
to. This is necessary for backward compatibility -- a new kernel version is
not supposed to break regular processes. In most cases, the device files
will also remain the same. On the other hand, the internal interfaces
within the kernel can and do change between versions.</para>
<para>The Linux kernel versions are divided between the stable versions
(n.$&lt;$even number$&gt;$.m) and the development versions (n.$&lt;$odd
number$&gt;$.m). The development versions include all the cool new ideas,
including those which will be considered a mistake, or reimplemented, in
the next version. As a result, you can't trust the interface to remain the
same in those versions (which is why I don't bother to support them in this
book, it's too much work and it would become dated too quickly). In the
stable versions, on the other hand, we can expect the interface to remain
the same regardless of the bug fix version (the m number).</para>
<para>There are differences between different kernel versions, and if you
want to support multiple kernel versions, you'll find yourself having to
code conditional compilation directives. The way to do this to compare the
macro <varname>LINUX_VERSION_CODE</varname> to the macro
<varname>KERNEL_VERSION</varname>. In version <varname>a.b.c</varname> of
the kernel, the value of this macro would be $2^{16}a+2^{8}b+c$. Be aware
that this macro is not defined for kernel 2.0.35 and earlier, so if you
want to write modules that support really old kernels, you'll have to
define it yourself, like:</para>
<example><title>some title</title>
<programlisting>
#if LINUX_KERNEL_VERSION >= KERNEL_VERSION(2,2,0)
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
</programlisting>
</example>
<para>Of course since these are macros, you can also use <command>#ifndef
KERNEL_VERSION</command> to test the existence of the macro, rather than
testing the version of the kernel.</para>
</sect2>
</sect1>

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<sect1><title>The /proc File System</title>
<!-- \index{proc file system} \index{/proc file system}
\index{file system\\/proc} -->
<para>In Linux there is an additional mechanism for the kernel and kernel
modules to send information to processes --- the <filename
role="directory">/proc</filename> file system. Originally designed to allow
easy access to information about processes (hence the name), it is now used
by every bit of the kernel which has something interesting to report, such as
<filename>/proc/modules</filename> which has the list of modules and
<filename>/proc/meminfo</filename> which has memory usage statistics.</para>
<!-- \index{/proc/modules} \index{/proc/meminfo} -->
<para>The method to use the proc file system is very similar to the one used
with device drivers --- you create a structure with all the information
needed for the <filename role="directory">/proc</filename> file, including
pointers to any handler functions (in our case there is only one, the one
called when somebody attempts to read from the <filename
role="directory">/proc</filename> file). Then,
<function>init_module</function> registers the structure with the kernel and
<function>cleanup_module</function> unregisters it.</para>
<para>The reason we use
<function>proc_register_dynamic</function><footnote><para>In version 2.0, in
version 2.2 this is done for us automatically if we set the inode to
zero.</para></footnote> is because we don't want to determine the inode
number used for our file in advance, but to allow the kernel to determine it
to prevent clashes. Normal file systems are located on a disk, rather than
just in memory (which is where <filename role="directory">/proc</filename>
is), and in that case the inode number is a pointer to a
disk location where the file's index-node (inode for short) is located. The
inode contains information about the file, for example the file's
permissions, together with a pointer to the disk location or locations where
the file's data can be found.</para>
<!-- \index{proc\_register\_dynamic} \index{proc\_register} \index{inode} -->
<para>Because we don't get called when the file is opened or closed, there's
no where for us to put <varname>MOD_INC_USE_COUNT</varname> and
<varname>MOD_DEC_USE_COUNT</varname> in this module, and if the file is
opened and then the module is removed, there's no way to avoid the
consequences. In the next chapter we'll see a harder to implement, but more
flexible, way of dealing with <filename role="directory">/proc</filename>
files which will allow us to protect against this problem as well.</para>
<example><title>procfs.c</title>
<programlisting><![CDATA[
/* procfs.c - create a "file" in /proc
* Copyright (C) 2001 by Peter Jay Salzman
*/
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
/* Necessary because we use the proc fs */
#include <linux/proc_fs.h>
/* In 2.2.3 /usr/include/linux/version.h includes a
* macro for this, but 2.0.35 doesn't - so I add it
* here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
/* Put data into the proc fs file.
Arguments
=========
1. The buffer where the data is to be inserted, if
you decide to use it.
2. A pointer to a pointer to characters. This is
useful if you don't want to use the buffer
allocated by the kernel.
3. The current position in the file.
4. The size of the buffer in the first argument.
5. Zero (for future use?).
Usage and Return Value
======================
If you use your own buffer, like I do, put its
location in the second argument and return the
number of bytes used in the buffer.
A return value of zero means you have no further
information at this time (end of file). A negative
return value is an error condition.
For More Information
====================
The way I discovered what to do with this function
wasn't by reading documentation, but by reading the
code which used it. I just looked to see what uses
the get_info field of proc_dir_entry struct (I used a
combination of find and grep, if you're interested),
and I saw that it is used in <kernel source
directory>/fs/proc/array.c.
If something is unknown about the kernel, this is
usually the way to go. In Linux we have the great
advantage of having the kernel source code for
free - use it.
*/
int procfile_read(char *buffer,
char **buffer_location,
off_t offset,
int buffer_length,
int zero)
{
int len; /* The number of bytes actually used */
/* This is static so it will still be in memory
* when we leave this function */
static char my_buffer[80];
static int count = 1;
/* We give all of our information in one go, so if the
* user asks us if we have more information the
* answer should always be no.
*
* This is important because the standard read
* function from the library would continue to issue
* the read system call until the kernel replies
* that it has no more information, or until its
* buffer is filled.
*/
if (offset > 0)
return 0;
/* Fill the buffer and get its length */
len = sprintf(my_buffer,
"For the %d%s time, go away!\n", count,
(count % 100 > 10 && count % 100 < 14) ? "th" :
(count % 10 == 1) ? "st" :
(count % 10 == 2) ? "nd" :
(count % 10 == 3) ? "rd" : "th" );
count++;
/* Tell the function which called us where the
* buffer is */
*buffer_location = my_buffer;
/* Return the length */
return len;
}
struct proc_dir_entry Our_Proc_File =
{
0, /* Inode number - ignore, it will be filled by
* proc_register[_dynamic] */
4, /* Length of the file name */
"test", /* The file name */
S_IFREG | S_IRUGO, /* File mode - this is a regular
* file which can be read by its
* owner, its group, and everybody
* else */
1, /* Number of links (directories where the
* file is referenced) */
0, 0, /* The uid and gid for the file - we give it
* to root */
80, /* The size of the file reported by ls. */
NULL, /* functions which can be done on the inode
* (linking, removing, etc.) - we don't
* support any. */
procfile_read, /* The read function for this file,
* the function called when somebody
* tries to read something from it. */
NULL /* We could have here a function to fill the
* file's inode, to enable us to play with
* permissions, ownership, etc. */
};
/* Initialize the module - register the proc file */
int init_module()
{
/* Success if proc_register[_dynamic] is a success,
* failure otherwise. */
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,2,0)
/* In version 2.2, proc_register assign a dynamic
* inode number automatically if it is zero in the
* structure , so there's no more need for
* proc_register_dynamic
*/
return proc_register(&proc_root, &Our_Proc_File);
#else
return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
/* proc_root is the root directory for the proc
* fs (/proc). This is where we want our file to be
* located.
*/
}
/* Cleanup - unregister our file from /proc */
void cleanup_module()
{
proc_unregister(&proc_root, Our_Proc_File.low_ino);
}
]]></programlisting>
</example>
</sect1>

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<sect1><title>Using /proc For Input</title>
<!-- \label{proc-input
\index{Input\\using /proc for}
\index{/proc\\using for input}
\index{proc\\using for input} -->
<para>So far we have two ways to generate output from kernel modules: we can
register a device driver and <command>mknod</command> a device file, or we
can create a <filename role="directory">/proc</filename> file. This allows
the kernel module to tell us anything it likes. The only problem is that
there is no way for us to talk back. The first way we'll send input to kernel
modules will be by writing back to the <filename
role="directory">/proc</filename> file.</para>
<para>Because the proc filesystem was written mainly to allow the kernel to
report its situation to processes, there are no special provisions for input.
The <varname>struct proc_dir_entry</varname> doesn't include a pointer to an
input function, the way it includes a pointer to an output function. Instead,
to write into a <filename role="directory">/proc</filename> file, we need to
use the standard filesystem mechanism.</para>
<!-- \index{proc\_dir\_entry structure}
\index{struct proc\_dir\_entry} -->
<para><para>In Linux there is a standard mechanism for file system
registration. Since every file system has to have its own functions to handle
inode and file operations<footnote><para>The difference between the two is
that file operations deal with the file itself, and inode operations deal
with ways of referencing the file, such as creating links to
it.</para></footnote>, there is a special structure to hold pointers to all
those functions, <varname>struct inode_operations</varname>, which includes a
pointer to <varname>struct file_operations</varname>. In /proc, whenever we
register a new file, we're allowed to specify which <varname>struct
inode_operations</varname> will be used for access to it. This is the
mechanism we use, a <varname>struct inode_operations</varname> which includes
a pointer to a <varname>struct file_operations</varname> which includes
pointers to our <function>module_input</function> and
<function>module_output</function> functions.</para>
<!-- \index{file system registration}
\index{registration\\file system}
\index{struct inode\_operations}
\index{inode\_operations structure}
\index{struct file\_operations}
\index{file\_operations structure} -->
<para>It's important to note that the standard roles of read and write are
reversed in the kernel. Read functions are used for output, whereas write
functions are used for input. The reason for that is that read and write
refer to the user's point of view --- if a process reads something from the
kernel, then the kernel needs to output it, and if a process writes something
to the kernel, then the kernel receives it as input.</para>
<!-- \index{read\\in the kernel}
\index{write\\in the kernel} -->
<para>Another interesting point here is the
<function>module_permission</function> function. This function is called
whenever a process tries to do something with the <filename
role="directory">/proc</filename> file, and it can decide whether to allow
access or not. Right now it is only based on the operation and the uid of the
current user (as available in <varname>current</varname>, a pointer to a
structure which includes information on the currently running process), but
it could be based on anything we like, such as what other processes are doing
with the same file, the time of day, or the last input we received.</para>
<!-- \index{module\_permissions} \index{permissions} \index{current pointer}
\index{pointer\\current} -->
<para>The reason for <function>put_user</function> and
<function>get_user</function> is that Linux memory (under Intel architecture,
it may be different under some other processors) is segmented. This means
that a pointer, by itself, does not reference a unique location in memory,
only a location in a memory segment, and you need to know which memory
segment it is to be able to use it. There is one memory segment for the
kernel, and one of each of the processes.</para>
<!-- \index{put\_user} \index{get\_user} \index{memory segments}
\index{segment\\memory} -->
<para>The only memory segment accessible to a process is its own, so when
writing regular programs to run as processes, there's no need to worry about
segments. When you write a kernel module, normally you want to access the
kernel memory segment, which is handled automatically by the system. However,
when the content of a memory buffer needs to be passed between the currently
running process and the kernel, the kernel function receives a pointer to the
memory buffer which is in the process segment. The
<function>put_user</function> and <function>get_user</function> macros allow
you to access that memory.</para>
<example><title>procfs.c</title>
<programlisting><![CDATA[
/* procfs.c - create a "file" in /proc, which allows
* both input and output. */
/* Copyright (C) 2001 by Peter Jay Salzman */
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Necessary because we use proc fs */
#include <linux/proc_fs.h>
/* In 2.2.3 /usr/include/linux/version.h includes a
* macro for this, but 2.0.35 doesn't - so I add it
* here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h> /* for get_user and put_user */
#endif
/* The module's file functions ********************** */
/* Here we keep the last message received, to prove
* that we can process our input */
#define MESSAGE_LENGTH 80
static char Message[MESSAGE_LENGTH];
/* Since we use the file operations struct, we can't
* use the special proc output provisions - we have to
* use a standard read function, which is this function */
#if LINUX_VERSION_CODE &gt;= KERNEL_VERSION(2,2,0)
static ssize_t module_output(
struct file *file, /* The file read */
char *buf, /* The buffer to put data to (in the
* user segment) */
size_t len, /* The length of the buffer */
loff_t *offset) /* Offset in the file - ignore */
#else
static int module_output(
struct inode *inode, /* The inode read */
struct file *file, /* The file read */
char *buf, /* The buffer to put data to (in the
* user segment) */
int len) /* The length of the buffer */
#endif
{
static int finished = 0;
int i;
char message[MESSAGE_LENGTH+30];
/* We return 0 to indicate end of file, that we have
* no more information. Otherwise, processes will
* continue to read from us in an endless loop. */
if (finished) {
finished = 0;
return 0;
}
/* We use put_user to copy the string from the kernel's
* memory segment to the memory segment of the process
* that called us. get_user, BTW, is
* used for the reverse. */
sprintf(message, "Last input:%s", Message);
for(i=0; i&lt;len && message[i]; i++)
put_user(message[i], buf+i);
/* Notice, we assume here that the size of the message
* is below len, or it will be received cut. In a real
* life situation, if the size of the message is less
* than len then we'd return len and on the second call
* start filling the buffer with the len+1'th byte of
* the message. */
finished = 1;
return i; /* Return the number of bytes "read" */
}
/* This function receives input from the user when the
* user writes to the /proc file. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t module_input(
struct file *file, /* The file itself */
const char *buf, /* The buffer with input */
size_t length, /* The buffer's length */
loff_t *offset) /* offset to file - ignore */
#else
static int module_input(
struct inode *inode, /* The file's inode */
struct file *file, /* The file itself */
const char *buf, /* The buffer with the input */
int length) /* The buffer's length */
#endif
{
int i;
/* Put the input into Message, where module_output
* will later be able to use it */
for(i=0; i<MESSAGE_LENGTH-1 && i<length; i++)
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
get_user(Message[i], buf+i);
/* In version 2.2 the semantics of get_user changed,
* it not longer returns a character, but expects a
* variable to fill up as its first argument and a
* user segment pointer to fill it from as the its
* second.
*
* The reason for this change is that the version 2.2
* get_user can also read an short or an int. The way
* it knows the type of the variable it should read
* is by using sizeof, and for that it needs the
* variable itself.
*/
#else
Message[i] = get_user(buf+i);
#endif
Message[i] = '\0'; /* we want a standard, zero
* terminated string */
/* We need to return the number of input characters
* used */
return i;
}
/* This function decides whether to allow an operation
* (return zero) or not allow it (return a non-zero
* which indicates why it is not allowed).
*
* The operation can be one of the following values:
* 0 - Execute (run the "file" - meaningless in our case)
* 2 - Write (input to the kernel module)
* 4 - Read (output from the kernel module)
*
* This is the real function that checks file
* permissions. The permissions returned by ls -l are
* for referece only, and can be overridden here.
*/
static int module_permission(struct inode *inode, int op)
{
/* We allow everybody to read from our module, but
* only root (uid 0) may write to it */
if (op == 4 || (op == 2 && current->euid == 0))
return 0;
/* If it's anything else, access is denied */
return -EACCES;
}
/* The file is opened - we don't really care about
* that, but it does mean we need to increment the
* module's reference count. */
int module_open(struct inode *inode, struct file *file)
{
MOD_INC_USE_COUNT;
return 0;
}
/* The file is closed - again, interesting only because
* of the reference count. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
int module_close(struct inode *inode, struct file *file)
#else
void module_close(struct inode *inode, struct file *file)
#endif
{
MOD_DEC_USE_COUNT;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return 0; /* success */
#endif
}
/* Structures to register as the /proc file, with
* pointers to all the relevant functions. ********** */
/* File operations for our proc file. This is where we
* place pointers to all the functions called when
* somebody tries to do something to our file. NULL
* means we don't want to deal with something. */
static struct file_operations File_Ops_4_Our_Proc_File =
{
NULL, /* lseek */
module_output, /* "read" from the file */
module_input, /* "write" to the file */
NULL, /* readdir */
NULL, /* select */
NULL, /* ioctl */
NULL, /* mmap */
module_open, /* Somebody opened the file */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
NULL, /* flush, added here in version 2.2 */
#endif
module_close, /* Somebody closed the file */
/* etc. etc. etc. (they are all given in
* /usr/include/linux/fs.h). Since we don't put
* anything here, the system will keep the default
* data, which in Unix is zeros (NULLs when taken as
* pointers). */
};
/* Inode operations for our proc file. We need it so
* we'll have some place to specify the file operations
* structure we want to use, and the function we use for
* permissions. It's also possible to specify functions
* to be called for anything else which could be done to
* an inode (although we don't bother, we just put
* NULL). */
static struct inode_operations Inode_Ops_4_Our_Proc_File =
{
&File_Ops_4_Our_Proc_File,
NULL, /* create */
NULL, /* lookup */
NULL, /* link */
NULL, /* unlink */
NULL, /* symlink */
NULL, /* mkdir */
NULL, /* rmdir */
NULL, /* mknod */
NULL, /* rename */
NULL, /* readlink */
NULL, /* follow_link */
NULL, /* readpage */
NULL, /* writepage */
NULL, /* bmap */
NULL, /* truncate */
module_permission /* check for permissions */
};
/* Directory entry */
static struct proc_dir_entry Our_Proc_File =
{
0, /* Inode number - ignore, it will be filled by
* proc_register[_dynamic] */
7, /* Length of the file name */
"rw_test", /* The file name */
S_IFREG | S_IRUGO | S_IWUSR,
/* File mode - this is a regular file which
* can be read by its owner, its group, and everybody
* else. Also, its owner can write to it.
*
* Actually, this field is just for reference, it's
* module_permission that does the actual check. It
* could use this field, but in our implementation it
* doesn't, for simplicity. */
1, /* Number of links (directories where the
* file is referenced) */
0, 0, /* The uid and gid for the file -
* we give it to root */
80, /* The size of the file reported by ls. */
&Inode_Ops_4_Our_Proc_File,
/* A pointer to the inode structure for
* the file, if we need it. In our case we
* do, because we need a write function. */
NULL
/* The read function for the file. Irrelevant,
* because we put it in the inode structure above */
};
/* Module initialization and cleanup ******************* */
/* Initialize the module - register the proc file */
int init_module()
{
/* Success if proc_register[_dynamic] is a success,
* failure otherwise */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
/* In version 2.2, proc_register assign a dynamic
* inode number automatically if it is zero in the
* structure , so there's no more need for
* proc_register_dynamic
*/
return proc_register(&proc_root, &Our_Proc_File);
#else
return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
}
/* Cleanup - unregister our file from /proc */
void cleanup_module()
{
proc_unregister(&proc_root, Our_Proc_File.low_ino);
}
]]></programlisting>
</example>
</sect1>

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<sect1><title>Talking to Device Files (writes and IOCTLs)}</title>
<!-- \label{dev-input} \index{device files\\input to}
\index{input to device files} \index{ioctl}
\index{write\\to device files} -->
<para>Device files are supposed to represent physical devices. Most physical
devices are used for output as well as input, so there has to be some
mechanism for device drivers in the kernel to get the output to send to
the device from processes. This is done by opening the device file for
output and writing to it, just like writing to a file. In the following
example, this is implemented by <function>device_write</function>.</para>
<para>This is not always enough. Imagine you had a serial port connected to a
modem (even if you have an internal modem, it is still implemented from the
CPU's perspective as a serial port connected to a modem, so you don't have to
tax your imagination too hard). The natural thing to do would be to use the
device file to write things to the modem (either modem commands or data to be
sent through the phone line) and read things from the modem (either responses
for commands or the data received through the phone line). However, this
leaves open the question of what to do when you need to talk to the serial
port itself, for example to send the rate at which data is sent and
received.</para>
<indexterm><primary>serial port</primary></indexterm>
<indexterm><primary>modem</primary></indexterm>
<para>The answer in Unix is to use a special function called
<function>ioctl</function> (short for Input Output ConTroL). Every device can
have its own <function>ioctl</function> commands, which can be read
<function>ioctl</function>'s (to send information from a process to the
kernel), write <function>ioctl</function>'s (to return information to a
process), <footnote><para>Notice that here the roles of read and write are
reversed <emphasis>again</emphasis>, so in <function>ioctl</function>'s read
is to send information to the kernel and write is to receive information from
the kernel.</para></footnote> both or neither. The
<function>ioctl</function> function is called with three parameters: the file
descriptor of the appropriate device file, the ioctl number, and a parameter,
which is of type long so you can use a cast to use it to pass anything.
<footnote><para>This isn't exact. You won't be able to pass a structure, for
example, through an ioctl --- but you will be able to pass a pointer to the
structure.</para></footnote></para>
<para>The ioctl number encodes the major device number, the type of the
ioctl, the command, and the type of the parameter. This ioctl number is
usually created by a macro call (<varname>_IO</varname>,
<varname>_IOR</varname>, <varname>_IOW</varname> or <varname>_IOWR</varname>
--- depending on the type) in a header file. This header file should then be
included both by the programs which will use
<function>ioctl</function> (so they can generate the appropriate
<function>ioctl</function>'s) and by the kernel module (so it can understand
it). In the example below, the header file is <filename
class="headerfile">chardev.h</filename> and the program which uses it is
<function>ioctl.c</function>.</para>
<!-- \index{\_IO} \index{\_IOR} \index{\_IOW} \index{\_IOWR} -->
<para>If you want to use <function>ioctl</function>s in your own kernel
modules, it is best to receive an official <function>ioctl</function>
assignment, so if you accidentally get somebody else's
<function>ioctl</function>s, or if they get yours, you'll know something is
wrong. For more information, consult the kernel source tree at
<filename>Documentation/ioctl-number.txt</filename>.</para>
<!-- \index{official ioctl assignment} \index{ioctl\\official assignment} -->
<!-- \index{chardev.c, source file}\index{source\\chardev.c} -->
<example><title>chardev.c</title>
<programlisting><![CDATA[
/* chardev.c
*
* Create an input/output character device
*/
/* Copyright (C) 2001 by Peter Jay Salzman */
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
/* For character devices */
/* The character device definitions are here */
#include <linux/fs.h>
/* A wrapper which does next to nothing at
* at present, but may help for compatibility
* with future versions of Linux */
#include <linux/wrapper.h>
/* Our own ioctl numbers */
#include "chardev.h"
/* In 2.2.3 /usr/include/linux/version.h includes a
* macro for this, but 2.0.35 doesn't - so I add it
* here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h> /* for get_user and put_user */
#endif
#define SUCCESS 0
/* Device Declarations ******************************** */
/* The name for our device, as it will appear in
* /proc/devices */
#define DEVICE_NAME "char_dev"
/* The maximum length of the message for the device */
#define BUF_LEN 80
/* Is the device open right now? Used to prevent
* concurent access into the same device */
static int Device_Open = 0;
/* The message the device will give when asked */
static char Message[BUF_LEN];
/* How far did the process reading the message get?
* Useful if the message is larger than the size of the
* buffer we get to fill in device_read. */
static char *Message_Ptr;
/* This function is called whenever a process attempts
* to open the device file */
static int device_open(struct inode *inode,
struct file *file)
{
#ifdef DEBUG
printk ("device_open(%p)\n", file);
#endif
/* We don't want to talk to two processes at the
* same time */
if (Device_Open)
return -EBUSY;
/* If this was a process, we would have had to be
* more careful here, because one process might have
* checked Device_Open right before the other one
* tried to increment it. However, we're in the
* kernel, so we're protected against context switches.
*
* This is NOT the right attitude to take, because we
* might be running on an SMP box, but we'll deal with
* SMP in a later chapter.
*/
Device_Open++;
/* Initialize the message */
Message_Ptr = Message;
MOD_INC_USE_COUNT;
return SUCCESS;
}
/* This function is called when a process closes the
* device file. It doesn't have a return value because
* it cannot fail. Regardless of what else happens, you
* should always be able to close a device (in 2.0, a 2.2
* device file could be impossible to close).
*/
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static int device_release(struct inode *inode,
struct file *file)
#else
static void device_release(struct inode *inode,
struct file *file)
#endif
{
#ifdef DEBUG
printk ("device_release(%p,%p)\n", inode, file);
#endif
/* We're now ready for our next caller */
Device_Open --;
MOD_DEC_USE_COUNT;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return 0;
#endif
}
/* This function is called whenever a process which
* has already opened the device file attempts to
* read from it. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t device_read(
struct file *file,
char *buffer, /* The buffer to fill with the data */
size_t length, /* The length of the buffer */
loff_t *offset) /* offset to the file */
#else
static int device_read(
struct inode *inode,
struct file *file,
char *buffer, /* The buffer to fill with the data */
int length) /* The length of the buffer
* (mustn't write beyond that!) */
#endif
{
/* Number of bytes actually written to the buffer */
int bytes_read = 0;
#ifdef DEBUG
printk("device_read(%p,%p,%d)\n", file, buffer, length);
#endif
/* If we're at the end of the message, return 0
* (which signifies end of file) */
if (*Message_Ptr == 0)
return 0;
/* Actually put the data into the buffer */
while (length && *Message_Ptr) {
/* Because the buffer is in the user data segment,
* not the kernel data segment, assignment wouldn't
* work. Instead, we have to use put_user which
* copies data from the kernel data segment to the
* user data segment. */
put_user(*(Message_Ptr++), buffer++);
length --;
bytes_read ++;
}
#ifdef DEBUG
printk ("Read %d bytes, %d left\n", bytes_read, length);
#endif
/* Read functions are supposed to return the number
* of bytes actually inserted into the buffer */
return bytes_read;
}
/* This function is called when somebody tries to
* write into our device file. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t device_write(struct file *file,
const char *buffer,
size_t length,
loff_t *offset)
#else
static int device_write(struct inode *inode,
struct file *file,
const char *buffer,
int length)
#endif
{
int i;
#ifdef DEBUG
printk ("device_write(%p,%s,%d)",
file, buffer, length);
#endif
for(i=0; i<length && i<BUF_LEN; i++)
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
get_user(Message[i], buffer+i);
#else
Message[i] = get_user(buffer+i);
#endif
Message_Ptr = Message;
/* Again, return the number of input characters used */
return i;
}
/* This function is called whenever a process tries to
* do an ioctl on our device file. We get two extra
* parameters (additional to the inode and file
* structures, which all device functions get): the number
* of the ioctl called and the parameter given to the
* ioctl function.
*
* If the ioctl is write or read/write (meaning output
* is returned to the calling process), the ioctl call
* returns the output of this function.
*/
int device_ioctl(
struct inode *inode,
struct file *file,
unsigned int ioctl_num,/* The number of the ioctl */
unsigned long ioctl_param) /* The parameter to it */
{
int i;
char *temp;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
char ch;
#endif
/* Switch according to the ioctl called */
switch (ioctl_num) {
case IOCTL_SET_MSG:
/* Receive a pointer to a message (in user space)
* and set that to be the device's message. */
/* Get the parameter given to ioctl by the process */
temp = (char *) ioctl_param;
/* Find the length of the message */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
get_user(ch, temp);
for (i=0; ch && i<BUF_LEN; i++, temp++)
get_user(ch, temp);
#else
for (i=0; get_user(temp) && i<BUF_LEN; i++, temp++)
;
#endif
/* Don't reinvent the wheel - call device_write */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
device_write(file, (char *) ioctl_param, i, 0);
#else
device_write(inode, file, (char *) ioctl_param, i);
#endif
break;
case IOCTL_GET_MSG:
/* Give the current message to the calling
* process - the parameter we got is a pointer,
* fill it. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
i = device_read(file, (char *) ioctl_param, 99, 0);
#else
i = device_read(inode, file, (char *) ioctl_param, 99);
#endif
/* Warning - we assume here the buffer length is
* 100. If it's less than that we might overflow
* the buffer, causing the process to core dump.
*
* The reason we only allow up to 99 characters is
* that the NULL which terminates the string also
* needs room. */
/* Put a zero at the end of the buffer, so it
* will be properly terminated */
put_user('\0', (char *) ioctl_param+i);
break;
case IOCTL_GET_NTH_BYTE:
/* This ioctl is both input (ioctl_param) and
* output (the return value of this function) */
return Message[ioctl_param];
break;
}
return SUCCESS;
}
/* Module Declarations *************************** */
/* This structure will hold the functions to be called
* when a process does something to the device we
* created. Since a pointer to this structure is kept in
* the devices table, it can't be local to
* init_module. NULL is for unimplemented functions. */
struct file_operations Fops = {
NULL, /* seek */
device_read,
device_write,
NULL, /* readdir */
NULL, /* select */
device_ioctl, /* ioctl */
NULL, /* mmap */
device_open,
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
NULL, /* flush */
#endif
device_release /* a.k.a. close */
};
/* Initialize the module - Register the character device */
int init_module()
{
int ret_val;
/* Register the character device (atleast try) */
ret_val = module_register_chrdev(MAJOR_NUM,
DEVICE_NAME,
&Fops);
/* Negative values signify an error */
if (ret_val < 0) {
printk ("%s failed with %d\n",
"Sorry, registering the character device ",
ret_val);
return ret_val;
}
printk ("%s The major device number is %d.\n",
"Registeration is a success",
MAJOR_NUM);
printk ("If you want to talk to the device driver,\n");
printk ("you'll have to create a device file. \n");
printk ("We suggest you use:\n");
printk ("mknod %s c %d 0\n", DEVICE_FILE_NAME,
MAJOR_NUM);
printk ("The device file name is important, because\n");
printk ("the ioctl program assumes that's the\n");
printk ("file you'll use.\n");
return 0;
}
/* Cleanup - unregister the appropriate file from /proc */
void cleanup_module()
{
int ret;
/* Unregister the device */
ret = module_unregister_chrdev(MAJOR_NUM, DEVICE_NAME);
/* If there's an error, report it */
if (ret < 0)
printk("Error in module_unregister_chrdev: %d\n", ret);
}
]]></programlisting>
</example>
<!-- \index{chardev.h, source file}\index{source\\chardev.h} -->
<example><title>chardev.h</title>
<programlisting><![CDATA[
\begin{verbatim}
/* chardev.h - the header file with the ioctl definitions.
*
* The declarations here have to be in a header file,
* because they need to be known both to the kernel
* module (in chardev.c) and the process calling ioctl
* (ioctl.c)
*/
#ifndef CHARDEV_H
#define CHARDEV_H
#include <linux/ioctl.h>
/* The major device number. We can't rely on dynamic
* registration any more, because ioctls need to know
* it. */
#define MAJOR_NUM 100
/* Set the message of the device driver */
#define IOCTL_SET_MSG _IOR(MAJOR_NUM, 0, char *)
/* _IOR means that we're creating an ioctl command
* number for passing information from a user process
* to the kernel module.
*
* The first arguments, MAJOR_NUM, is the major device
* number we're using.
*
* The second argument is the number of the command
* (there could be several with different meanings).
*
* The third argument is the type we want to get from
* the process to the kernel.
*/
/* Get the message of the device driver */
#define IOCTL_GET_MSG _IOR(MAJOR_NUM, 1, char *)
/* This IOCTL is used for output, to get the message
* of the device driver. However, we still need the
* buffer to place the message in to be input,
* as it is allocated by the process.
*/
/* Get the n'th byte of the message */
#define IOCTL_GET_NTH_BYTE _IOWR(MAJOR_NUM, 2, int)
/* The IOCTL is used for both input and output. It
* receives from the user a number, n, and returns
* Message[n]. */
/* The name of the device file */
#define DEVICE_FILE_NAME "char_dev"
#endif
]]></programlisting>
</example>
<!-- \index{ioctl\\defining} \index{defining ioctls}
\index{ioctl\\header file for} \index{header file for ioctls} -->
<example><title>ioctl.c</title>
<programlisting><![CDATA[
/* ioctl.c - the process to use ioctl's to control the
* kernel module
*
* Until now we could have used cat for input and
* output. But now we need to do ioctl's, which require
* writing our own process.
*/
/* Copyright (C) 2001 by Peter Jay Salzman */
/* device specifics, such as ioctl numbers and the
* major device file. */
#include "chardev.h"
#include <fcntl.h> /* open */
#include <unistd.h> /* exit */
#include <sys/ioctl.h> /* ioctl */
/* Functions for the ioctl calls */
ioctl_set_msg(int file_desc, char *message)
{
int ret_val;
ret_val = ioctl(file_desc, IOCTL_SET_MSG, message);
if (ret_val < 0) {
printf ("ioctl_set_msg failed:%d\n", ret_val);
exit(-1);
}
}
ioctl_get_msg(int file_desc)
{
int ret_val;
char message[100];
/* Warning - this is dangerous because we don't tell
* the kernel how far it's allowed to write, so it
* might overflow the buffer. In a real production
* program, we would have used two ioctls - one to tell
* the kernel the buffer length and another to give
* it the buffer to fill
*/
ret_val = ioctl(file_desc, IOCTL_GET_MSG, message);
if (ret_val < 0) {
printf ("ioctl_get_msg failed:%d\n", ret_val);
exit(-1);
}
printf("get_msg message:%s\n", message);
}
ioctl_get_nth_byte(int file_desc)
{
int i;
char c;
printf("get_nth_byte message:");
i = 0;
while (c != 0) {
c = ioctl(file_desc, IOCTL_GET_NTH_BYTE, i++);
if (c < 0) {
printf(
"ioctl_get_nth_byte failed at the %d'th byte:\n", i);
exit(-1);
}
putchar(c);
}
putchar('\n');
}
/* Main - Call the ioctl functions */
main()
{
int file_desc, ret_val;
char *msg = "Message passed by ioctl\n";
file_desc = open(DEVICE_FILE_NAME, 0);
if (file_desc < 0) {
printf ("Can't open device file: %s\n",
DEVICE_FILE_NAME);
exit(-1);
}
ioctl_get_nth_byte(file_desc);
ioctl_get_msg(file_desc);
ioctl_set_msg(file_desc, msg);
close(file_desc);
}
]]></programlisting>
</example>
</sect1>

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<sect1><title>System Calls</title>
<!-- \label{sys-call} \index{system calls} \index{calls\\system} -->
<para>So far, the only thing we've done was to use well defined kernel
mechanisms to register <filename role="directory">/proc</filename> files and
device handlers. This is fine if you want to do something the kernel
programmers thought you'd want, such as write a device driver. But what if
you want to do something unusual, to change the behavior of the system in
some way? Then, you're mostly on your own.</para>
<para>This is where kernel programming gets dangerous. While writing the
example below, I killed the <function>open()</function> system call. This
meant I couldn't open any files, I couldn't run any programs, and I couldn't
<command>shutdown</command> the computer. I had to pull the power switch.
Luckily, no files died. To ensure you won't lose any files either, please run
<command>sync</command> right before you do the <command>insmod</command> and
the <command>rmmod</command>.
<!-- \index{sync} \index{insmod} \index{rmmod} \index{shutdown} -->
<para>Forget about <filename role="directory">/proc</filename> files, forget
about device files. They're just minor details. The <emphasis>real</emphasis>
process to kernel communication mechanism, the one used by all processes, is
system calls. When a process requests a service from the kernel (such as
opening a file, forking to a new process, or requesting more memory), this is
the mechanism used. If you want to change the behaviour of the kernel in
interesting ways, this is the place to do it. By the way, if you want to see
which system calls a program uses, run <command>strace
&lt;arguments&gt;</command>.</para> <!-- \index{strace} -->
<para>In general, a process is not supposed to be able to access the kernel. It
can't access kernel memory and it can't call kernel functions. The hardware
of the CPU enforces this (that's the reason why it's called `protected
mode').</para>
<para>System calls are an exception to this general rule. What happens is that
the process fills the registers with the appropriate values and then calls
a special instruction which jumps to a previously defined location in the
kernel (of course, that location is readable by user processes, it is not
writable by them). Under Intel CPUs, this is done by means of interrupt 0x80.
The hardware knows that once you jump to this location, you are no longer
running in restricted user mode, but as the operating system kernel --- and
therefore you're allowed to do whatever you want.</para>
<!-- \index{interrupt 0x80} -->
<para>The location in the kernel a process can jump to is called
<emphasis>system_call</emphasis>. The procedure at that location checks the
system call number, which tells the kernel what service the process
requested. Then, it looks at the table of system calls
(<varname>sys_call_table</varname>) to see the address of the kernel function
to call. Then it calls the function, and after it returns, does a few system
checks and then return back to the process (or to a different process, if the
process time ran out). If you want to read this code, it's at the source file
<filename>arch/$<$architecture$>$/kernel/entry.S</filename>, after the line
<function>ENTRY(system_call)</function>.</para>
<!-- \index{system\_call} \index{ENTRY(system\_call)} \index{sys\_call\_table}
\index{entry.S} -->
<para>So, if we want to change the way a certain system call works, what we
need to do is to write our own function to implement it (usually by adding a
bit of our own code, and then calling the original function) and then change
the pointer at <varname>sys_call_table</varname> to point to our function.
Because we might be removed later and we don't want to leave the system in an
unstable state, it's important for <function>cleanup_module</function> to
restore the table to its original state.</para>
<para>The source code here is an example of such a kernel module. We want to
`spy' on a certain user, and to <function>printk()</function> a message
whenever that user opens a file. Towards this end, we replace the system call
to open a file with our own function, called
<function>our_sys_open</function>. This function checks the uid (user's id)
of the current process, and if it's equal to the uid we spy on, it calls
<function>printk()</function> to display the name of the file to be opened.
Then, either way, it calls the original <function>open()</function> function
with the same parameters, to actually open the file.</para>
<!-- \index{open\\system call} -->
<para>The <function>init_module</function> function replaces the appropriate
location in <varname>sys_call_table</varname> and keeps the original pointer
in a variable. The <function>cleanup_module</function> function uses that
variable to restore everything back to normal. This approach is dangerous,
because of the possibility of two kernel modules changing the same system
call. Imagine we have two kernel modules, A and B. A's open system call will
be A_open and B's will be B_open. Now, when A is inserted into the kernel,
the system call is replaced with A_open, which will call the original
sys_open when it's done. Next, B is inserted into the kernel, which replaces
the system call with B_open, which will call what it thinks is the original
system call, A_open, when it's done.</para>
<para>Now, if B is removed first, everything will be well --- it will simply
restore the system call to A_open, which calls the original. However, if A
is removed and then B is removed, the system will crash. A's removal will
restore the system call to the original, sys_open, cutting B out of the
loop. Then, when B is removed, it will restore the system call to what
<emphasis>it</emphasis> thinks is the original, A_open, which is no longer
in memory. At first glance, it appears we could solve this particular problem
by checking if the system call is equal to our open function and if so not
changing it at all (so that B won't change the system call when it's
removed), but that will cause an even worse problem. When A is removed, it
sees that the system call was changed to B_open so that it is no longer
pointing to A_open, so it won't restore it to sys_open before it is removed
from memory. Unfortunately, B_open will still try to call A_open which is
no longer there, so that even without removing B the system would
crash.</para>
<para>I can think of two ways to prevent this problem. The first is to
restore the call to the original value, sys_open. Unfortunately, sys_open is
not part of the kernel system table in <filename>/proc/ksyms</filename>, so
we can't access it. The other solution is to use the reference count to
prevent root from <command>rmmod</command>'ing the module once it is loaded.
This is good for production modules, but bad for an educational sample ---
which is why I didn't do it here.</para>
<!-- \index{rmmod}\index{MOD\_INC\_USE\_COUNT} \index{sys\_open} -->
<example><title>procfs.c</title>
<programlisting><![CDATA[
/* syscall.c
*
* System call "stealing" sample
*/
/* Copyright (C) 2001 by Peter Jay Salzman */
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
#include <sys/syscall.h> /* The list of system calls */
/* For the current (process) structure, we need
* this to know who the current user is. */
#include <linux/sched.h>
/* In 2.2.3 /usr/include/linux/version.h includes a
* macro for this, but 2.0.35 doesn't - so I add it
* here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h>
#endif
/* The system call table (a table of functions). We
* just define this as external, and the kernel will
* fill it up for us when we are insmod'ed
*/
extern void *sys_call_table[];
/* UID we want to spy on - will be filled from the
* command line */
int uid;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
MODULE_PARM(uid, "i");
#endif
/* A pointer to the original system call. The reason
* we keep this, rather than call the original function
* (sys_open), is because somebody else might have
* replaced the system call before us. Note that this
* is not 100% safe, because if another module
* replaced sys_open before us, then when we're inserted
* we'll call the function in that module - and it
* might be removed before we are.
*
* Another reason for this is that we can't get sys_open.
* It's a static variable, so it is not exported. */
asmlinkage int (*original_call)(const char *, int, int);
/* For some reason, in 2.2.3 current->uid gave me
* zero, not the real user ID. I tried to find what went
* wrong, but I couldn't do it in a short time, and
* I'm lazy - so I'll just use the system call to get the
* uid, the way a process would.
*
* For some reason, after I recompiled the kernel this
* problem went away.
*/
asmlinkage int (*getuid_call)();
/* The function we'll replace sys_open (the function
* called when you call the open system call) with. To
* find the exact prototype, with the number and type
* of arguments, we find the original function first
* (it's at fs/open.c).
*
* In theory, this means that we're tied to the
* current version of the kernel. In practice, the
* system calls almost never change (it would wreck havoc
* and require programs to be recompiled, since the system
* calls are the interface between the kernel and the
* processes).
*/
asmlinkage int our_sys_open(const char *filename,
int flags,
int mode)
{
int i = 0;
char ch;
/* Check if this is the user we're spying on */
if (uid == getuid_call()) {
/* getuid_call is the getuid system call,
* which gives the uid of the user who
* ran the process which called the system
* call we got */
/* Report the file, if relevant */
printk("Opened file by %d: ", uid);
do {
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
get_user(ch, filename+i);
#else
ch = get_user(filename+i);
#endif
i++;
printk("%c", ch);
} while (ch != 0);
printk("\n");
}
/* Call the original sys_open - otherwise, we lose
* the ability to open files */
return original_call(filename, flags, mode);
}
/* Initialize the module - replace the system call */
int init_module()
{
/* Warning - too late for it now, but maybe for
* next time... */
printk("I'm dangerous. I hope you did a ");
printk("sync before you insmod'ed me.\n");
printk("My counterpart, cleanup_module(), is even");
printk("more dangerous. If\n");
printk("you value your file system, it will ");
printk("be \"sync; rmmod\" \n");
printk("when you remove this module.\n");
/* Keep a pointer to the original function in
* original_call, and then replace the system call
* in the system call table with our_sys_open */
original_call = sys_call_table[__NR_open];
sys_call_table[__NR_open] = our_sys_open;
/* To get the address of the function for system
* call foo, go to sys_call_table[__NR_foo]. */
printk("Spying on UID:%d\n", uid);
/* Get the system call for getuid */
getuid_call = sys_call_table[__NR_getuid];
return 0;
}
/* Cleanup - unregister the appropriate file from /proc */
void cleanup_module()
{
/* Return the system call back to normal */
if (sys_call_table[__NR_open] != our_sys_open) {
printk("Somebody else also played with the ");
printk("open system call\n");
printk("The system may be left in ");
printk("an unstable state.\n");
}
sys_call_table[__NR_open] = original_call;
}
]]></programlisting>
</example>
</sect1>

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<sect1><title>Blocking Processes</title>
<indexterm><primary>blocking processes</primary></indexterm>
<indexterm>
<primary>processes</primary>
<secondary>blocking</secondary>
</indexterm>
<sect2><title>Replacing <function>printk</function></title>
<para>
What do you do when somebody asks you for something you can't do right
away? If you're a human being and you're bothered by a human being, the
only thing you can say is: <quote>Not right now, I'm busy. <emphasis>Go
away!</emphasis></quote>. But if you're a kernel module and you're
bothered by a process, you have another possibility. You can put the
process to sleep until you can service it. After all, processes are
being put to sleep by the kernel and woken up all the time (that's the
way multiple processes appear to run on the same time on a single
<acronym>CPU</acronym>).
</para>
<indexterm><primary>multi-tasking</primary></indexterm>
<indexterm><primary>busy</primary></indexterm>
<para>
This kernel module is an example of this. The file (called
<filename>/proc/sleep</filename>) can only be opened by a single process
at a time. If the file is already open, the kernel module calls
<function>module_interruptible_sleep_on</function>.
<indexterm><primary>module_interruptible_sleep_on</primary></indexterm>
<indexterm><primary>interruptible_sleep_on</primary></indexterm>
<footnote>
<para>
The easiest way to keep a file open is to open it with <command>tail
-f</command>.
</para>
</footnote>
This function changes the status of the task (a task is the kernel data
structure which holds information about a process and the system call
it's in, if any) to <parameter>TASK_INTERRUPTIBLE</parameter>,
<indexterm><primary>TASK_INTERRUPTIBLE</primary></indexterm> which means
that the task will not run until it is woken up somehow, and adds it to
<structname>WaitQ</structname>, the queue of tasks waiting to access the
file. Then, the function calls the scheduler to context switch to a
different process, one which has some use for the <acronym>CPU</acronym>.
</para>
<indexterm><primary>putting processes to sleep</primary></indexterm>
<indexterm>
<primary>sleep</primary>
<secondary>putting processes to</secondary>
</indexterm>
<para>
When a process is done with the file, it closes it, and
<function>module_close</function> is called. That function wakes up all
the processes in the queue (there's no mechanism to only wake up one of
them). It then returns and the process which just closed the file can
continue to run. In time, the scheduler decides that that process has
had enough and gives control of the <acronym>CPU</acronym> to another
process. Eventually, one of the processes which was in the queue will be
given control of the <acronym>CPU</acronym> by the scheduler. It starts
at the point right after the call to
<function>module_interruptible_sleep_on</function>.
<footnote>
<para>
This means that the process is still in kernel mode -- as far as the
process is concerned, it issued the <function>open</function> system
call and the system call hasn't returned yet. The process doesn't
know somebody else used the <acronym>CPU</acronym> for most of the
time between the moment it issued the call and the moment it
returned.
</para>
</footnote>
It can then proceed to set a global variable to tell all the other
processes that the file is still open and go on with its life. When the
other processes get a piece of the <acronym>CPU</acronym>, they'll see
that global variable and go back to sleep.
</para>
<indexterm><primary>waking up processes</primary></indexterm>
<indexterm>
<primary>processes</primary>
<secondary>waking up</secondary>
</indexterm>
<indexterm><primary>multitasking</primary></indexterm>
<indexterm><primary>scheduler</primary></indexterm>
<para>
To make our life more interesting, <function>module_close</function>
doesn't have a monopoly on waking up the processes which wait to access
the file. A signal, such as <keycombo
action="simul"><keycap>Ctrl</keycap><keycap>c</keycap></keycombo>
<indexterm><primary>ctrl-c</primary></indexterm>
(<parameter>SIGINT</parameter>)
<indexterm><primary>signal</primary></indexterm>
<indexterm><primary>SIGINT</primary></indexterm>
can also wake up a process.
<indexterm><primary>module_wake_up</primary></indexterm>
<footnote>
<para>
This is because we used
<function>module_interruptible_sleep_on</function>. We could have
used <function>module_sleep_on</function>
<indexterm><primary>module_sleep_on</primary></indexterm>
<indexterm><primary>sleep_on</primary></indexterm>
instead, but that would have resulted is extremely angry users whose
<keycombo
action="simul"><keycap>Ctrl</keycap><keycap>c</keycap></keycombo>s
are ignored.
</para>
</footnote>
In that case, we want to return with <parameter>-EINTR</parameter>
<indexterm><primary>EINTR</primary></indexterm>
immediately. This is important so users can, for example, kill the
process before it receives the file.
</para>
<indexterm>
<primary>processes</primary>
<secondary>killing</secondary>
</indexterm>
<para>
There is one more point to remember. Some times processes don't want to
sleep, they want either to get what they want immediately, or to be told
it cannot be done. Such processes use the
<parameter>O_NONBLOCK</parameter>
<indexterm><primary>O_NONBLOCK</primary></indexterm>
<indexterm><primary>non-blocking</primary></indexterm> flag when opening
the file. The kernel is supposed to respond by returning with the error
code <parameter>-EAGAIN</parameter>
<indexterm><primary>EAGAIN</primary></indexterm> from operations which
would otherwise block, such as opening the file in this example. The
program <command>cat_noblock</command>, available in the source directory
for this chapter, can be used to open a file with
<parameter>O_NONBLOCK</parameter>.
</para>
<indexterm><primary>blocking, how to avoid</primary></indexterm>
<example>
<title>sleep.c</title>
<indexterm><primary>sleep.c</primary></indexterm>
<programlisting>
<![CDATA[
/* sleep.c - create a /proc file, and if several processes try to open it at
* the same time, put all but one to sleep
*
* Copyright (C) 2001 by Peter Jay Salzman
*/
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
/* Necessary because we use proc fs */
#include <linux/proc_fs.h>
/* For putting processes to sleep and waking them up */
#include <linux/sched.h>
#include <linux/wrapper.h>
/* In 2.2.3 /usr/include/linux/version.h includes a macro for this, but 2.0.35
* doesn't - so I add it here if necessary.
*/
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h> /* for get_user and put_user */
#endif
/* The module's file functions */
/* Here we keep the last message received, to prove that we can process our
* input
*/
#define MESSAGE_LENGTH 80
static char Message[MESSAGE_LENGTH];
/* Since we use the file operations struct, we can't use the special proc
* output provisions - we have to use a standard read function, which is this
* function
*/
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t module_output (
struct file *file, /* The file read */
char *buf, /* The buffer to put data to (in the user segment) */
size_t len, /* The length of the buffer */
loff_t *offset) /* Offset in the file - ignore */
#else
static int module_output (
struct inode *inode, /* The inode read */
struct file *file, /* The file read */
char *buf, /* The buffer to put data to (in the user segment) */
int len) /* The length of the buffer */
#endif
{
static int finished = 0;
int i;
char message[MESSAGE_LENGTH+30];
/* Return 0 to signify end of file - that we have nothing more to say at this
* point.
*/
if (finished) {
finished = 0;
return 0;
}
/* If you don't understand this by now, you're hopeless as a kernel
* programmer.
*/
sprintf(message, "Last input:%s\n", Message);
for (i = 0; i < len && message[i]; i++)
put_user(message[i], buf+i);
finished = 1;
return i; /* Return the number of bytes "read" */
}
/* This function receives input from the user when the user writes to the /proc
* file.
*/
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t module_input (
struct file *file, /* The file itself */
const char *buf, /* The buffer with input */
size_t length, /* The buffer's length */
loff_t *offset) /* offset to file - ignore */
#else
static int module_input (
struct inode *inode, /* The file's inode */
struct file *file, /* The file itself */
const char *buf, /* The buffer with the input */
int length) /* The buffer's length */
#endif
{
int i;
/* Put the input into Message, where module_output will later be able to use
* it
*/
for(i = 0; i < MESSAGE_LENGTH-1 && i < length; i++)
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
get_user(Message[i], buf+i);
#else
Message[i] = get_user(buf+i);
#endif
/* we want a standard, zero terminated string */
Message[i] = '\0';
/* We need to return the number of input characters used */
return i;
}
/* 1 if the file is currently open by somebody */
int Already_Open = 0;
/* Queue of processes who want our file */
static struct wait_queue *WaitQ = NULL;
/* Called when the /proc file is opened */
static int module_open(struct inode *inode, struct file *file)
{
/* If the file's flags include O_NONBLOCK, it means the process doesn't want
* to wait for the file. In this case, if the file is already open, we
* should fail with -EAGAIN, meaning "you'll have to try again", instead of
* blocking a process which would rather stay awake.
*/
if ((file->f_flags & O_NONBLOCK) && Already_Open)
return -EAGAIN;
/* This is the correct place for MOD_INC_USE_COUNT because if a process is
* in the loop, which is within the kernel module, the kernel module must
* not be removed.
*/
MOD_INC_USE_COUNT;
/* If the file is already open, wait until it isn't */
while (Already_Open)
{
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
int i, is_sig = 0;
#endif
/* This function puts the current process, including any system calls,
* such as us, to sleep. Execution will be resumed right after the
* function call, either because somebody called wake_up(&WaitQ) (only
* module_close does that, when the file is closed) or when a signal,
* such as Ctrl-C, is sent to the process
*/
module_interruptible_sleep_on(&WaitQ);
/* If we woke up because we got a signal we're not blocking, return
* -EINTR (fail the system call). This allows processes to be killed or
* stopped.
*/
/*
* Emmanuel Papirakis:
*
* This is a little update to work with 2.2.*. Signals now are contained in
* two words (64 bits) and are stored in a structure that contains an array of
* two unsigned longs. We now have to make 2 checks in our if.
*
* Ori Pomerantz:
*
* Nobody promised me they'll never use more than 64 bits, or that this book
* won't be used for a version of Linux with a word size of 16 bits. This code
* would work in any case.
*/
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
for (i = 0; i < _NSIG_WORDS && !is_sig; i++)
is_sig = current->signal.sig[i] & ~current->blocked.sig[i];
if (is_sig) {
#else
if (current->signal & ~current->blocked) {
#endif
/* It's important to put MOD_DEC_USE_COUNT here, because for processes
* where the open is interrupted there will never be a corresponding
* close. If we don't decrement the usage count here, we will be left
* with a positive usage count which we'll have no way to bring down
* to zero, giving us an immortal module, which can only be killed by
* rebooting the machine.
*/
MOD_DEC_USE_COUNT;
return -EINTR;
}
}
/* If we got here, Already_Open must be zero */
/* Open the file */
Already_Open = 1;
return 0; /* Allow the access */
}
/* Called when the /proc file is closed */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
int module_close(struct inode *inode, struct file *file)
#else
void module_close(struct inode *inode, struct file *file)
#endif
{
/* Set Already_Open to zero, so one of the processes in the WaitQ will be
* able to set Already_Open back to one and to open the file. All the other
* processes will be called when Already_Open is back to one, so they'll go
* back to sleep.
*/
Already_Open = 0;
/* Wake up all the processes in WaitQ, so if anybody is waiting for the
* file, they can have it.
*/
module_wake_up(&WaitQ);
MOD_DEC_USE_COUNT;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return 0; /* success */
#endif
}
/* This function decides whether to allow an operation (return zero) or not
* allow it (return a non-zero which indicates why it is not allowed).
*
* The operation can be one of the following values:
* 0 - Execute (run the "file" - meaningless in our case)
* 2 - Write (input to the kernel module)
* 4 - Read (output from the kernel module)
*
* This is the real function that checks file permissions. The permissions
* returned by ls -l are for referece only, and can be overridden here.
*/
static int module_permission(struct inode *inode, int op)
{
/* We allow everybody to read from our module, but only root (uid 0) may
* write to it
*/
if (op == 4 || (op == 2 && current->euid == 0))
return 0;
/* If it's anything else, access is denied */
return -EACCES;
}
/* Structures to register as the /proc file, with pointers to all the relevant
* functions.
*/
/* File operations for our proc file. This is where we place pointers to all
* the functions called when somebody tries to do something to our file. NULL
* means we don't want to deal with something.
*/
static struct file_operations File_Ops_4_Our_Proc_File = {
NULL, /* lseek */
module_output, /* "read" from the file */
module_input, /* "write" to the file */
NULL, /* readdir */
NULL, /* select */
NULL, /* ioctl */
NULL, /* mmap */
module_open, /* called when the /proc file is opened */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
NULL, /* flush */
#endif
module_close}; /* called when it's classed */
/* Inode operations for our proc file. We need it so we'll have somewhere to
* specify the file operations structure we want to use, and the function we
* use for permissions. It's also possible to specify functions to be called
* for anything else which could be done to an inode (although we don't bother,
* we just put NULL).
*/
static struct inode_operations Inode_Ops_4_Our_Proc_File = {
&File_Ops_4_Our_Proc_File,
NULL, /* create */
NULL, /* lookup */
NULL, /* link */
NULL, /* unlink */
NULL, /* symlink */
NULL, /* mkdir */
NULL, /* rmdir */
NULL, /* mknod */
NULL, /* rename */
NULL, /* readlink */
NULL, /* follow_link */
NULL, /* readpage */
NULL, /* writepage */
NULL, /* bmap */
NULL, /* truncate */
module_permission}; /* check for permissions */
/* Directory entry */
static struct proc_dir_entry Our_Proc_File = {
0, /* Inode number - ignore, it will be filled by
* proc_register[_dynamic]
*/
5, /* Length of the file name */
"sleep", /* The file name */
/* File mode - this is a regular file which can be read by its owner, its
* group, and everybody else. Also, its owner can write to it.
*
* Actually, this field is just for reference, it's module_permission that
* does the actual check. It could use this field, but in our
* implementation it doesn't, for simplicity.
*/
S_IFREG | S_IRUGO | S_IWUSR,
1, /* Number of links (directories where the file is referenced) */
0, 0, /* The uid and gid for the file - we give it to root */
80, /* The size of the file reported by ls. */
/* A pointer to the inode structure for the file, if we need it. In our
* case we do, because we need a write function.
*/
&Inode_Ops_4_Our_Proc_File,
/* The read function for the file. Irrelevant, because we put it in the
* inode structure above
*/
NULL};
/* Module initialization and cleanup */
/* Initialize the module - register the proc file */
int init_module()
{
/* Success if proc_register_dynamic is a success, failure otherwise */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return proc_register(&proc_root, &Our_Proc_File);
#else
return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
/* proc_root is the root directory for the proc fs (/proc). This is where
* we want our file to be located.
*/
}
/* Cleanup - unregister our file from /proc. This could get dangerous if
* there are still processes waiting in WaitQ, because they are inside our
* open function, which will get unloaded. I'll explain how to avoid removal
* of a kernel module in such a case in chapter 10.
*/
void cleanup_module()
{
proc_unregister(&proc_root, Our_Proc_File.low_ino);
}
]]>
</programlisting>
</example>
</sect2>
</sect1>

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<sect1><title>Replacing <function>printk</function></title>
<indexterm><primary>replacing printk</primary></indexterm>
<indexterm>
<primary>printk</primary>
<secondary>replacing</secondary>
</indexterm>
<para>Good writing style says we have a paragraph here.</para>
<sect2><title>Replacing <function>printk</function></title>
<para>
In the beginning (chapter \ref{hello-world}), I said that X and kernel
module programming don't mix. That's true while developing the kernel
module, but in actual use you want to be able to send messages to
whichever <acronym>tty</acronym>
<footnote>
<para>
<emphasis>T</emphasis>ele<emphasis>ty</emphasis>pe, originally a
combination keyboard-printer used to communicate with a Unix system,
and today an abstraction for the text stream used for a Unix program,
whether it's a physical terminal, an xterm on an X display, a network
connection used with telnet, etc.
</para>
</footnote>
the command to the module came from. This is important for identifying
errors after the kernel module is released, because it will be used
through all of them.
</para>
<para>
The way this is done is by using <parameter>current</parameter>,
<indexterm><primary>current task</primary></indexterm>
<indexterm>
<primary>task</primary>
<secondary>current></secondary>
</indexterm>
a pointer to the currently running task, to get the current task's
<structname>tty</structname> structure.
<indexterm><primary>tty_structure</primary></indexterm>
<indexterm>
<primary>struct</primary>
<secondary>tty</secondary>
</indexterm>
Then, we look inside that <structname>tty</structname> structure to find
a pointer to a string write function, which we use to write a string to
the <acronym>tty</acronym>.
</para>
<example><title>printk.c</title>
<indexterm><primary>printk.c</primary></indexterm>
<programlisting><![CDATA[
/* printk.c - send textual output to the tty you're running on, regardless of
* whether it's passed through X11, telnet, etc.
*
* Copyright (C) 2001 by Peter Jay Salzman
*/
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
/* Necessary here */
#include <linux/sched.h> /* For current */
#include <linux/tty.h> /* For the tty declarations */
/* Print the string to the appropriate tty, the one the current task uses */
void print_string(char *str)
{
struct tty_struct *my_tty;
/* The tty for the current task */
my_tty = current->tty;
/* If my_tty is NULL, it means that the current task has no tty you can print
* to (this is possible, for example, if it's a daemon). In this case,
* there's nothing we can do. */
if (my_tty != NULL) {
/* my_tty->driver is a struct which holds the tty's functions, one of
* which (write) is used to write strings to the tty. It can be used to
* take a string either from the user's memory segment or the kernel's
* memory segment.
*
* The function's first parameter is the tty to write to, because the
* same function would normally be used for all tty's of a certain type.
* The second parameter controls whether the function receives a string
* from kernel memory (false, 0) or from user memory (true, non zero).
* The third parameter is a pointer to a string, and the fourth
* parameter is the length of the string.
*/
(*(my_tty->driver).write)(
my_tty, /* The tty itself */
0, /* We don't take the string from user space */
str, /* String */
strlen(str)); /* Length */
/* ttys were originally hardware devices, which (usually) strictly
* followed the ASCII standard. In ASCII, to move to a new line you
* need two characters, a carriage return and a line feed. On Unix,
* the ASCII line feed is used for both purposes - so we can't just
* use \n, because it wouldn't have a carriage return and the next
* next line will start at the column right after the line feed.
*
* BTW, this is the reason why the text file is different between
* Unix and Windows. In CP/M and its derivatives, such as MS-DOS and
* Windows the ASCII standard was strictly adhered to, and therefore a
* newline requires both a line feed and a carriage return.
*/
(*(my_tty->driver).write)(my_tty, 0, "\015\012", 2);
}
}
/* Module initialization and cleanup */
/* Initialize the module - register the proc file */
int init_module()
{
print_string("Module Inserted");
return 0;
}
/* Cleanup - unregister our file from /proc */
void cleanup_module()
{
print_string("Module Removed");
}
]]></programlisting>
</example>
</sect2>
</sect1>

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<sect1><title>Scheduling Tasks</title>
<indexterm><primary>scheduling tasks</primary></indexterm>
<indexterm>
<primary>tasks</primary>
<secondary>scheduling</secondary>
</indexterm>
<para>Good writing style says we have a paragraph here.</para>
<sect2><title>Scheduling Tasks</title>
<para>
Very often, we have <quote>housekeeping</quote>
<indexterm><primary>housekeeping</primary></indexterm>
<indexterm><primary>crontab</primary></indexterm> tasks which have to be
done at a certain time, or every so often. If the task is to be done by a
process, we do it by putting it in the <filename>crontab</filename> file.
If the task is to be done by a kernel module, we have two possibilities.
The first is to put a process in the <filename>crontab</filename> file
which will wake up the module by a system call when necessary, for
example by opening a file. This is terribly inefficient, however -- we
run a new process off of <filename>crontab</filename>, read a new
executable to memory, and all this just to wake up a kernel module which
is in memory anyway.
</para>
<para>
Instead of doing that, we can create a function that will be called once
for every timer interrupt. The way we do this is we create a task,
<indexterm><primary>task</primary></indexterm> held in a
<structname>tq_struct</structname>
<indexterm><primary>tq_struct</primary></indexterm> structure, which will
hold a pointer to the function. Then, we use
<function>queue_task</function>
<indexterm><primary>queue_task</primary></indexterm> to put that task on
a task list called <structname>tq_timer</structname>,
<indexterm><primary>tq_timer</primary></indexterm> which is the list of
tasks to be executed on the next timer interrupt. Because we want the
function to keep on being executed, we need to put it back on
<structname>tq_timer</structname> whenever it is called, for the next
timer interrupt.
</para>
<para>
There's one more point we need to remember here. When a module is
removed by <command>rmmod</command>,
<indexterm><primary>rmmod</primary></indexterm> first its reference count
<indexterm><primary>reference count</primary></indexterm> is checked. If
it is zero, <function>module_cleanup</function>
<indexterm><primary>module_cleanup</primary></indexterm> is called.
Then, the module is removed from memory with all its functions. Nobody
checks to see if the timer's task list happens to contain a pointer to
one of those functions, which will no longer be available. Ages later
(from the computer's perspective, from a human perspective it's nothing,
less than a hundredth of a second), the kernel has a timer interrupt and
tries to call the function on the task list. Unfortunately, the function
is no longer there. In most cases, the memory page where it sat is
unused, and you get an ugly error message. But if some other code is now
sitting at the same memory location, things could get
<emphasis>very</emphasis> ugly. Unfortunately, we don't have an easy way
to unregister a task from a task list.
</para>
<para>
Since <function>cleanup_module</function> can't return with an error code
(it's a void function), the solution is to not let it return at all.
Instead, it calls <function>sleep_on</function>
<indexterm><primary>sleep_on</primary></indexterm> or
<function>module_sleep_on</function>
<indexterm><primary>module_sleep_on</primary></indexterm>
<footnote><para>They're really the same.</para></footnote>
to put the <command>rmmod</command> process to sleep. Before that, it
informs the function called on the timer interrupt to stop attaching
itself by setting a global variable. Then, on the next timer interrupt,
the <command>rmmod</command> process will be woken up, when our function
is no longer in the queue and it's safe to remove the module.
</para>
<example><title>sched.c</title>
<indexterm><primary>sched.c</primary></indexterm>
<programlisting><![CDATA[
/* sched.c - scheduale a function to be called on every timer interrupt.
*
* Copyright (C) 2001 by Peter Jay Salzman
*/
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
/* Necessary because we use the proc fs */
#include <linux/proc_fs.h>
/* We scheduale tasks here */
#include <linux/tqueue.h>
/* We also need the ability to put ourselves to sleep and wake up later */
#include <linux/sched.h>
/* In 2.2.3 /usr/include/linux/version.h includes a macro for this, but
* 2.0.35 doesn't - so I add it here if necessary.
*/
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
/* The number of times the timer interrupt has been called so far */
static int TimerIntrpt = 0;
/* This is used by cleanup, to prevent the module from being unloaded while
* intrpt_routine is still in the task queue
*/
static struct wait_queue *WaitQ = NULL;
static void intrpt_routine(void *);
/* The task queue structure for this task, from tqueue.h */
static struct tq_struct Task = {
NULL, /* Next item in list - queue_task will do this for us */
0, /* A flag meaning we haven't been inserted into a task
* queue yet
*/
intrpt_routine, /* The function to run */
NULL /* The void* parameter for that function */
};
/* This function will be called on every timer interrupt. Notice the void*
* pointer - task functions can be used for more than one purpose, each time
* getting a different parameter.
*/
static void intrpt_routine(void *irrelevant)
{
/* Increment the counter */
TimerIntrpt++;
/* If cleanup wants us to die */
if (WaitQ != NULL)
wake_up(&WaitQ); /* Now cleanup_module can return */
else
/* Put ourselves back in the task queue */
queue_task(&Task, &tq_timer);
}
/* Put data into the proc fs file. */
int procfile_read(char *buffer,
char **buffer_location, off_t offset,
int buffer_length, int zero)
{
int len; /* The number of bytes actually used */
/* It's static so it will still be in memory when we leave this function
*/
static char my_buffer[80];
static int count = 1;
/* We give all of our information in one go, so if the anybody asks us
* if we have more information the answer should always be no.
*/
if (offset > 0)
return 0;
/* Fill the buffer and get its length */
len = sprintf(my_buffer, "Timer called %d times so far\n", TimerIntrpt);
count++;
/* Tell the function which called us where the buffer is */
*buffer_location = my_buffer;
/* Return the length */
return len;
}
struct proc_dir_entry Our_Proc_File = {
0, /* Inode number - ignore, it'll be filled by proc_register_dynamic */
5, /* Length of the file name */
"sched", /* The file name */
S_IFREG | S_IRUGO, /* File mode - this is a regular file which can be
* read by its owner, its group, and everybody else
*/
1, /* Number of links (directories where the file is referenced) */
0, 0, /* The uid and gid for the file - we give it to root */
80, /* The size of the file reported by ls. */
NULL, /* functions which can be done on the inode (linking, removing,
* etc). - we don't * support any.
*/
procfile_read, /* The read function for this file, the function called
* when somebody tries to read something from it.
*/
NULL /* We could have here a function to fill the file's inode, to
* enable us to play with permissions, ownership, etc.
*/
};
/* Initialize the module - register the proc file */
int init_module()
{
/* Put the task in the tq_timer task queue, so it will be executed at
* next timer interrupt
*/
queue_task(&Task, &tq_timer);
/* Success if proc_register_dynamic is a success, failure otherwise */
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,2,0)
return proc_register(&proc_root, &Our_Proc_File);
#else
return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
}
/* Cleanup */
void cleanup_module()
{
/* Unregister our /proc file */
proc_unregister(&proc_root, Our_Proc_File.low_ino);
/* Sleep until intrpt_routine is called one last time. This is necessary,
* because otherwise we'll deallocate the memory holding intrpt_routine
* and Task while tq_timer still references them. Notice that here we
* don't allow signals to interrupt us.
*
* Since WaitQ is now not NULL, this automatically tells the interrupt
* routine it's time to die.
*/
sleep_on(&WaitQ);
}
]]></programlisting>
</example>
</sect2>
</sect1>

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<sect1><title>Interrupt Handlers</title>
<indexterm><primary>interrupt handlers</primary></indexterm>
<indexterm>
<primary>handlers</primary>
<secondary>interrupt</secondary>
</indexterm>
<sect2><title>Interrupt Handlers</title>
<para>
Except for the last chapter, everything we did in the kernel so far we've
done as a response to a process asking for it, either by dealing with a
special file, sending an <function>ioctl</function>, or issuing a system
call. But the job of the kernel isn't just to respond to process
requests. Another job, which is every bit as important, is to speak to
the hardware connected to the machine.
</para>
<para>
There are two types of interaction between the <acronym>CPU</acronym> and
the rest of the computer's hardware. The first type is when the
<acronym>CPU</acronym> gives orders to the hardware, the other is when
the hardware needs to tell the <acronym>CPU</acronym> something. The
second, called interrupts, is much harder to implement because it has to
be dealt with when convenient for the hardware, not the
<acronym>CPU</acronym>. Hardware devices typically have a very small
amount of <acronym>RAM</acronym>, and if you don't read their information
when available, it is lost.
</para>
<para>
Under Linux, hardware interrupts are called <acronym>IRQ</acronym>s
(short for <emphasis>I</emphasis>nterrupt
<emphasis>R</emphasis>e<emphasis>q</emphasis>uests).
<footnote>
<para>
This is standard nomencalture on the Intel architecture where Linux
originated.
<para>
</footnote>
There are two types of <acronym>IRQ</acronym>s, short and long. A short
<acronym>IRQ</acronym> is one which is expected to take a
<emphasis>very</emphasis> short period of time, during which the rest of
the machine will be blocked and no other interrupts will be handled. A
long <acronym>IRQ</acronym> is one which can take longer, and during
which other interrupts may occur (but not interrupts from the same
device). If at all possible, it's better to declare an interrupt handler
to be long.
</para>
<para>
When the <acronym>CPU</acronym> receives an interrupt, it stops whatever
it's doing (unless it's processing a more important interrupt, in which
case it will deal with this one only when the more important one is
done), saves certain parameters on the stack and calls the interrupt
handler. This means that certain things are not allowed in the interrupt
handler itself, because the system is in an unknown state. The solution
to this problem is for the interrupt handler to do what needs to be done
immediately, usually read something from the hardware or send something
to the hardware, and then schedule the handling of the new information at
a later time (this is called the <quote>bottom half</quote>)
<indexterm><primary>bottom half</primary></indexterm> and return. The
kernel is then guaranteed to call the bottom half as soon as possible --
and when it does, everything allowed in kernel modules will be allowed.
</para>
<para>
The way to implement this is to call <function>request_irq</function>
<indexterm><primary>request_irq</primary></indexterm> to get your
interrupt handler called when the relevant <acronym>IRQ</acronym> is
received (there are 15 of them, plus 1 which is used to cascade the
interrupt controllers, on Intel platforms). This function receives the
<acronym>IRQ</acronym> number, the name of the function, flags, a name
for <filename>/proc/interrupts</filename>
<indexterm><primary>/proc/interrupts</primary></indexterm> and a
parameter to pass to the interrupt handler. The flags can include
<parameter>SA_SHIRQ</parameter>
<indexterm><primary>SA_SHIRQ</primary></indexterm> to indicate you're
willing to share the <acronym>IRQ</acronym> with other interrupt handlers
(usually because a number of hardware devices sit on the same
<acronym>IRQ</acronym>) and <parameter>SA_INTERRUPT</parameter>
<indexterm><primary>SA_INTERRUPT</primary></indexterm> to indicate this
is a fast interrupt. This function will only succeed if there isn't
already a handler on this <acronym>IRQ</acronym>, or if you're both
willing to share.
</para>
<para>
Then, from within the interrupt handler, we communicate with the hardware
and then use <function>queue_task_irq</function>
<indexterm><primary>queue_task_irq</primary></indexterm> with
<function>tq_immediate</function>
<indexterm><primary>tq_immediate</primary></indexterm> and
<function>mark_bh(BH_IMMEDIATE)</function>
<indexterm><primary>mark_bh</primary></indexterm>
<indexterm><primary>BH_IMMEDIATE</primary></indexterm> to schedule the
bottom half. The reason we can't use the standard
<function>queue_task</function>
<indexterm><primary>queue_task</primary></indexterm> in version 2.0 is
that the interrupt might happen right in the middle of somebody else's
<function>queue_task</function>.
<footnote>
<para>
<function>queue_task_irq</function> is protected from this by a
global lock -- in 2.2 there is no <function>queue_task_irq</function>
and <function>queue_task</function> is protected by a lock.
</para>
</footnote>
We need <function>mark_bh</function> because earlier versions of Linux
only had an array of 32 bottom halves, and now one of them
(<parameter>BH_IMMEDIATE</parameter>) is used for the linked list of
bottom halves for drivers which didn't get a bottom half entry assigned
to them.
</para>
</sect2>
<sect2 id="keyboard">
<title>Keyboards on the Intel Architecture</title>
<indexterm><primary>keyboard</primary></indexterm>
<indexterm>
<primary>Intel architecture</primary>
<secondary>keyboard</secondary>
</indexterm>
<warning>
<para>
The rest of this chapter is completely Intel specific. If you're not
running on an Intel platform, it will not work. Don't even try to
compile the code here.
</para>
</warning>
<para>
I had a problem with writing the sample code for this chapter. On one
hand, for an example to be useful it has to run on everybody's computer
with meaningful results. On the other hand, the kernel already includes
device drivers for all of the common devices, and those device drivers
won't coexist with what I'm going to write. The solution I've found was
to write something for the keyboard interrupt, and disable the regular
keyboard interrupt handler first. Since it is defined as a static symbol
in the kernel source files (specifically,
<filename>drivers/char/keyboard.c</filename>), there is no way to restore
it. Before <userinput>insmod</userinput>'ing this code, do on another
terminal <userinput>sleep 120 ; reboot</userinput> if you value your
file system.
</para>
<para>
This code binds itself to <acronym>IRQ</acronym> 1, which is the
<acronym>IRQ</acronym> of the keyboard controlled under Intel
architectures. Then, when it receives a keyboard interrupt, it reads the
keyboard's status (that's the purpose of the
<userinput>inb(0x64)</userinput>)
<indexterm><primary>inb</primary></indexterm> and the scan code, which is
the value returned by the keyboard. Then, as soon as the kernel thinks
it's feasible, it runs <function>got_char</function> which gives the code
of the key used (the first seven bits of the scan code) and whether it
has been pressed (if the 8th bit is zero) or released (if it's one).
</para>
<example><title>intrpt.c</title>
<indexterm><primary>intrpt.c</primary></indexterm>
<programlisting><![CDATA[
/* intrpt.c - An interrupt handler.
*
* Copyright (C) 2001 by Peter Jay Salzman
*/
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
#include <linux/sched.h>
#include <linux/tqueue.h>
/* We want an interrupt */
#include <linux/interrupt.h>
#include <asm/io.h>
/* In 2.2.3 /usr/include/linux/version.h includes a macro for this, but
* 2.0.35 doesn't - so I add it here if necessary.
*/
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
/* Bottom Half - this will get called by the kernel as soon as it's safe
* to do everything normally allowed by kernel modules.
*/
static void got_char(void *scancode)
{
printk("Scan Code %x %s.\n",
(int) *((char *) scancode) & 0x7F,
*((char *) scancode) & 0x80 ? "Released" : "Pressed");
}
/* This function services keyboard interrupts. It reads the relevant
* information from the keyboard and then scheduales the bottom half
* to run when the kernel considers it safe.
*/
void irq_handler(int irq, void *dev_id, struct pt_regs *regs)
{
/* This variables are static because they need to be
* accessible (through pointers) to the bottom half routine.
*/
static unsigned char scancode;
static struct tq_struct task = {NULL, 0, got_char, &scancode};
unsigned char status;
/* Read keyboard status */
status = inb(0x64);
scancode = inb(0x60);
/* Scheduale bottom half to run */
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,2,0)
queue_task(&task, &tq_immediate);
#else
queue_task_irq(&task, &tq_immediate);
#endif
mark_bh(IMMEDIATE_BH);
}
/* Initialize the module - register the IRQ handler */
int init_module()
{
/* Since the keyboard handler won't co-exist with another handler,
* such as us, we have to disable it (free its IRQ) before we do
* anything. Since we don't know where it is, there's no way to
* reinstate it later - so the computer will have to be rebooted
* when we're done.
*/
free_irq(1, NULL);
/* Request IRQ 1, the keyboard IRQ, to go to our irq_handler.
* SA_SHIRQ means we're willing to have othe handlers on this IRQ.
* SA_INTERRUPT can be used to make the handler into a fast interrupt.
*/
return request_irq(1, /* The number of the keyboard IRQ on PCs */
irq_handler, /* our handler */
SA_SHIRQ,
"test_keyboard_irq_handler", NULL);
}
/* Cleanup */
void cleanup_module()
{
/* This is only here for completeness. It's totally irrelevant, since
* we don't have a way to restore the normal keyboard interrupt so the
* computer is completely useless and has to be rebooted.
*/
free_irq(1, NULL);
}
]]></programlisting>
</example>
</sect2>
</sect1>

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<sect1><title>Symmetrical Multi-Processing</title>
<indexterm><primary>SMP</primary></indexterm>
<indexterm><primary>multi-processing</primary></indexterm>
<indexterm><primary>symmetrical multi-processing</primary></indexterm>
<indexterm>
<primary>processing</primary>
<secondary>multi</secondary>
</indexterm>
<para>Good writing style says we have a paragraph here.</para>
<sect2><title>Symmetrical Multi-Processing</title>
<para>
One of the easiest (read, cheapest) ways to improve hardware performance
is to put more than one <acronym>CPU</acronym> on the board.
<indexterm>
<primary>CPU</primary>
<secondary>multiple</secondary>
</indexterm>
This can be done either making the different <acronym>CPU</acronym>s take
on different jobs (asymmetrical multi-processing) or by making them all
run in parallel, doing the same job (symmetrical multi-processing, a.k.a.
<acronym>SMP</acronym>). Doing asymmetrical multi-processing effectively
requires specialized knowledge about the tasks the computer should do,
which is unavailable in a general purpose operating system such as Linux.
On the other hand, symmetrical multi-processing is relatively easy to
implement.
</para>
<para>
By relatively easy, I mean exactly that -- not that it's
<emphasis>really</emphasis> easy. In a symmetrical multi-processing
environment, the <acronym>CPU</acronym>s share the same memory, and as a
result code running in one <acronym>CPU</acronym> can affect the memory
used by another. You can no longer be certain that a variable you've set
to a certain value in the previous line still has that value -- the other
<acronym>CPU</acronym> might have played with it while you weren't
looking. Obviously, it's impossible to program like this.
</para>
<para>
In the case of process programming this normally isn't an issue, because
a process will normally only run on one <acronym>CPU</acronym> at a time.
<footnote>
<para>
The exception is threaded processes, which can run on several
<acronym>CPU</acronym>s at once.
</para>
</footnote>
The kernel, on the other hand, could be called by different processes
running on different <acronym>CPU</acronym>s.
</para>
<para>
In version 2.0.x, this isn't a problem because the entire kernel is in
one big spinlock. This means that if one <acronym>CPU</acronym> is in
the kernel and another <acronym>CPU</acronym> wants to get in, for
example because of a system call, it has to wait until the first
<acronym>CPU</acronym> is done. This makes Linux <acronym>SMP</acronym>
safe,
<footnote>
<para>Meaning it is safe to use it with <acronym>SMP</acronym></para>
</footnote>
but terriably inefficient.
</para>
<para>
In version 2.2.x, several <acronym>CPU</acronym>s can be in the kernel at
the same time. This is something module writers need to be aware of. I
got somebody to give me access to an <acronym>SMP</acronym> box, so
hopefully the next version of this book will include more information.
</para>
<!-- Unfortunately, I don't have access to an SMP box to test things, so I
can't write a chapter about how to do it right. It anybody out there has
access to one and is willing to help me with this, I'll be grateful. If a
company will provide me with this access, I'll give them a free one
paragraph ad at the top of this chapter.
-->
</sect2>
</sect1>

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<sect1><title>Common Pitfalls</title>
<sect2><title>Common Pitfalls</title>
<para>Before I send you on your way to go out into the world and write
kernel modules, there are a few things I need to warn you about. If I
fail to warn you and something bad happens, please report the problem to
me for a full refund of the amount I was paid for your copy of the book.
</para>
<indexterm><primary>refund policy</primary></indexterm>
<variablelist>
<varlistentry>
<term>Using standard libraries</term>
<indexterm><primary>standard libraries</primary></indexterm>
<indexterm>
<primary>libraries</primary>
<secondary>standard</secondary>
</indexterm>
<listitem>
<para>
You can't do that. In a kernel module you can only use kernel
functions, which are the functions you can see in
<filename>/proc/ksyms</filename>.
<indexterm><primary>/proc/ksyms</primary></indexterm>
<indexterm>
<primary>proc file</primary>
<secondary>ksyms</secondary>
</indexterm>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Disabling interrupts</term>
<indexterm>
<primary>interrupts</primary>
<secondary>disabling</secondary>
</indexterm>
<listitem>
<para>
You might need to do this for a short time and that is OK, but if
you don't enable them afterwards, your system will be stuck and
you'll have to power it off.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Sticking your head inside a large carnivore</term>
<listitem>
<para>
I probably don't have to warn you about this, but I figured I will
anyway, just in case.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>

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<sect1>
<title>Changes between 2.0 and 2.2</title>
<indexterm><primary>2.2 changes</primary></indexterm>
<indexterm>
<primary>kernel</primary>
<secondary>versions</secondary>
</indexterm>
<para>Good writing style says we have a paragraph here.</para>
<sect2>
<title>Changes between 2.0 and 2.2</title>
<para>
I don't know the entire kernel well enough do document all of the
changes. In the course of converting the examples (or actually, adapting
Emmanuel Papirakis's changes) I came across the following differences. I
listed all of them here together to help module programmers, especially
those who learned from previous versions of this book and are most
familiar with the techniques I use, convert to the new version.
</para>
<para>
An additional resource for people who wish to convert to 2.2 is located
on
<ulink
url="http://www.atnf.csiro.au/~rgooch/linux/docs/porting-to-2.2.html">
Richard Gooch's site
</ulink>.
</para>
<variablelist>
<varlistentry>
<term><filename class="headerfile">asm/uaccess.h</filename></term>
<indexterm><primary>asm/uaccess.h</primary></indexterm>
<indexterm>
<primary>asm</primary>
<secondary>uaccess.h</secondary>
</indexterm>
<listitem>
<para>
If you need <function>put_user</function>
<indexterm><primary>put_user</primary></indexterm> or
<function>get_user</function>
<indexterm><primary>get_user</primary></indexterm> you have to
<userinput>#include</userinput> it.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>get_user</function></term>
<listitem>
<para>
In version 2.2, <function>get_user</function> receives both the
pointer into user memory and the variable in kernel memory to fill
with the information. The reason for this is that
<function>get_user</function> can now read two or four bytes at a
time if the variable we read is two or four bytes long.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><structname>file_operations</structname></term>
<indexterm>
<primary>structure</primary>
<secondary>file_operations</secondary>
</indexterm>
<listitem>
<para>
This structure now has a flush
<indexterm><primary>flush</primary></indexterm> function between
the <function>open</function> and <function>close</function>
functions.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>close</function> in
<structname>file_operations</structname>
</term>
<indexterm><primary>close</primary></indexterm>
<listitem>
<para>
In version 2.2, the <function>close</function> function returns an
integer, so it's allowed to fail.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>read</function> and <function>write</function> in
<structname>file_operations</structname>
</term>
<indexterm><primary>read</primary></indexterm>
<indexterm><primary>write</primary></indexterm>
<indexterm><primary>ssize_t</primary></indexterm>
<listitem>
<para>
The headers for these functions changed. They now return
<userinput>ssize_t</userinput> instead of an integer, and their
parameter list is different. The inode is no longer a parameter,
and on the other hand the offset into the file is.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>proc_register_dynamic</function></term>
<indexterm><primary>proc_register_dynamic</primary></indexterm>
<listitem>
<para>
This function no longer exists. Instead, you call the regular
<function>proc_register</function>
<indexterm><primary>proc_register</primary></indexterm> and put
zero in the inode field of the structure.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Signals</term>
<indexterm><primary>signals</primary></indexterm>
<listitem>
<para>
The signals in the task structure are no longer a 32 bit integer,
but an array of <parameter>_NSIG_WORDS</parameter>
<indexterm><primary>_NSIG_WORDS</primary></indexterm> integers.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>queue_task_irq</function></term>
<indexterm><primary>queue_task_irq</primary></indexterm>
<listitem>
<para>
Even if you want to scheduale a task to happen from inside an
interrupt handler, you use <function>queue_task</function>,
<indexterm><primary>queue_task</primary></indexterm> not
<function>queue_task_irq</function>.
</para>
</listitem>
</varlistentry>
<indexterm><primary>interrupts</primary></indexterm>
<indexterm><primary>irqs</primary></indexterm>
<varlistentry>
<term>Module Parameters</term>
<indexterm>
<primary>module</primary>
<secondary>parameters</secondary>
</indexterm>
<indexterm><primary>module parameters</primary></indexterm>
<listitem>
<para>
You no longer just declare module parameters as global variables.
In 2.2 you have to also use <parameter>MODULE_PARM</parameter>
<indexterm><primary>MODULE_PARM</primary></indexterm> to declare
their type. This is a big improvement, because it allows the module
to receive string parameters which start with a digits, for
example, without getting confused.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Symmetrical Multi-Processing</term>
<indexterm><primary>Symmetrical Multi-Processing</primary></indexterm>
<indexterm><primary>SMP</primary></indexterm>
<listitem>
<para>
The kernel is no longer inside one huge spinlock, which means that
kernel modules have to be aware of <acronym>SMP</acronym>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>

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<sect1>
<title>Where From Here?</title>
<para>Good writing style says we have a paragraph here.</para>
<sect2>
<title>Where From Here?</title>
<para>
I could easily have squeezed a few more chapters into this book. I could
have added a chapter about creating new file systems, or about adding new
protocol stacks (as if there's a need for that -- you'd have to dig
underground to find a protocol stack not supported by Linux). I could
have added explanations of the kernel mechanisms we haven't touched upon,
such as bootstrapping or the disk interface.
</para>
<para>
However, I chose not to. My purpose in writing this book was to provide
initiation into the mysteries of kernel module programming and to teach
the common techniques for that purpose. For people seriously interested
in kernel programming, I recommend Juan-Mariano de Goyeneche's
<ulink
url="http://jungla.dit.upm.es/~jmseyas/linux/kernel/hackers-docs.html">
list of kernel resources
</ulink>. Also, as Linus said, the best way to learn the kernel is to
read the source code yourself.
</para>
<para>
If you're interested in more examples of short kernel modules, I
recommend Phrack magazine. Even if you're not interested in security,
and as a programmer you should be, the kernel modules there are good
examples of what you can do inside the kernel, and they're short enough
not to require too much effort to understand.
</para>
<para>
I hope I have helped you in your quest to become a better programmer, or
at least to have fun through technology. And, if you do write useful
kernel modules, I hope you publish them under the GPL, so I can use them
too.
</para>
</sect2>
</sect1>

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<!ENTITY Forward SYSTEM "00-Forward.sgml">
<!ENTITY Introduction SYSTEM "01-Introduction.sgml">
<!ENTITY HelloWorld SYSTEM "02-HelloWorld.sgml">
<!ENTITY Preliminaries SYSTEM "03-Preliminaries.sgml">
<!ENTITY CharDevFiles SYSTEM "04-CharacterDeviceFiles.sgml">
<!ENTITY TheProcFileSystem SYSTEM "05-TheProcFileSystem.sgml">
<!ENTITY UsingProcForInput SYSTEM "06-UsingProcForInput.sgml">
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<!ENTITY SystemCalls SYSTEM "08-SystemCalls.sgml">
<!ENTITY BlockingProcesses SYSTEM "09-BlockingProcesses.sgml">
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<!ENTITY InterruptHandlers SYSTEM "12-InterruptHandlers.sgml">
<!ENTITY SymmetricMultiProc SYSTEM "13-SymmetricMultiProcessing.sgml">
<!ENTITY CommonPitfalls SYSTEM "14-CommonPitfalls.sgml">
<!ENTITY Changes20-22 SYSTEM "A1-ChangesBet20And22.sgml">
<!ENTITY WhereFromHere SYSTEM "A2-WhereToGoFromHere.sgml">
]>
<book>
<bookinfo>
<title>The Linux Kernel Module Programming Guide</title>
<titleabbrev>MPG</titleabbrev>
<authorgroup>
<collab>
<collabname>Peter Jay Salzman</collabname>
</collab>
<collab><collabname>Ori Pomerantz</collabname></collab>
</authorgroup>
<copyright>
<year>2001</year>
<holder>Peter Jay Salzman</holder>
</copyright>
<legalnotice>
<para>The Linux Kernel Module Programming Guide is a free book; you may
reproduce and/or modify it under the terms of the Open Software
License, version 1.1. You can obtain a copy of this license at <ulink
url="http://opensource.org/licenses/osl.php"
>http://opensource.org/licenses/osl.php</ulink>.</para>
<para>This book is distributed in the hope it will be useful, but
without any warranty, without even the implied warranty of
merchantability or fitness for a particular purpose.</para>
<para>The author encourages wide distribution of this book for personal
or commercial use, provided the above copyright notice remains intact
and the method adheres to the provisions of the Open Software License.
In summary, you may copy and distribute this book free of charge or for
a profit. No explicit permission is required from the author for
reproduction of this book in any medium, physical or electronic.</para>
<para>Derivative works and translations of this document must be placed
under the Open Software License, and the original copyright notice
must remain intact. If you have contributed new material to this book,
you must make the material and source code available for your
revisions. Please make revisions and updates available directly to the
document maintainer, Peter Jay Salzman <email>p@dirac.org</email>.
This will allow for the merging of updates and provide consistent
revisions to the Linux community.</para>
<para>If you publish or distribute this book commercially, donations,
royalties, and/or printed copies are greatly appreciated by the author
and the <ulink url="http://www.tldp.org">Linux Documentation
Project</ulink> (LDP). Contributing in this way shows your support for
free software and the LDP. If you have questions or comments, please
contact the address above.</para>
</legalnotice>
</bookinfo>
<preface><title>Foreword</title> &Forward;</preface>
<chapter><title>Introduction</title> &Introduction;</chapter>
<chapter><title>Hello World</title> &HelloWorld;</chapter>
<chapter><title>Preliminaries</title> &Preliminaries;</chapter>
<chapter><title>Character Device Files</title> &CharDevFiles;</chapter>
<chapter><title>The /proc File System</title> &TheProcFileSystem;</chapter>
<chapter><title>Using /proc For Input</title> &UsingProcForInput;</chapter>
<chapter><title>Talking To Device Files</title> &TalkingToDevFiles;</chapter>
<chapter><title>System Calls</title> &SystemCalls;</chapter>
<chapter><title>Blocking Processes</title> &BlockingProcesses;</chapter>
<chapter><title>Replacing Printks</title> &ReplacingPrintks;</chapter>
<chapter><title>Scheduling Tasks</title> &SchedulingTasks;</chapter>
<chapter><title>Interrupt Handlers</title> &InterruptHandlers;</chapter>
<chapter><title>Symmetric Multi Processing</title>&SymmetricMultiProc;</chapter>
<chapter><title>Common Pitfalls</title> &CommonPitfalls;</chapter>
<appendix><title>Changes: 2.0 To 2.2</title> &Changes20-22;</appendix>
<appendix><title>Where To Go From Here</title> &WhereFromHere;</appendix>
</book>