man-pages/man2/mprotect.2

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.\" Copyright (C) 2007 Michael Kerrisk <mtk-manpages@gmx.net>
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.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1997-05-31 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 2003-08-24 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 2004-08-16 by Andi Kleen <ak@muc.de>
.\" 2007-06-02, mtk: Fairly substantial rewrites and additions, and
.\" a much improved example program.
.\"
.TH MPROTECT 2 2007-06-02 "Linux" "Linux Programmer's Manual"
.SH NAME
mprotect \- set protection on a region of memory
.SH SYNOPSIS
.nf
.B #include <sys/mman.h>
.sp
\fBint mprotect(const void *\fIaddr\fB, size_t \fIlen\fB, int \fIprot\fB);
.fi
.SH DESCRIPTION
.BR mprotect ()
changes protection for the calling process's memory page(s)
containing any part of the address range in the 
interval [\fIaddr\fP,\fIaddr\fP+\fIlen\fP\-1].
.I addr
must be aligned to a page boundary.

If the calling process tries to access memory in a manner
that violates the protection, then the kernel generates a 
.B SIGSEGV
signal for the process.
.PP
.I prot
is either
.B PROT_NONE
or a bitwise-or of the other values in the following list:
.TP 1.1i
.B PROT_NONE
The memory cannot be accessed at all.
.TP
.B PROT_READ
The memory can be read.
.TP
.B PROT_WRITE
The memory can be modified.
.TP
.B PROT_EXEC
The memory can contain be executed.
.\" FIXME
.\" Document MAP_GROWSUP and MAP_GROWSDOWN
.SH "RETURN VALUE"
On success,
.BR mprotect ()
returns zero.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EACCES
The memory cannot be given the specified access.
This can happen, for example, if you
.BR mmap (2)
a file to which you have read-only access, then ask
.BR mprotect ()
to mark it
.BR PROT_WRITE .
.TP
.B EFAULT
The memory cannot be accessed.
.TP
.B EINVAL
\fIaddr\fR is not a valid pointer,
or not a multiple of the system page size.
.TP
.B ENOMEM
Internal kernel structures could not be allocated.
Or: addresses  in the range
.RI [ addr ,
.IR addr + len ]
are invalid for the address space of the process,
or specify one or more pages that are not mapped.
.SH "CONFORMING TO"
SVr4, POSIX.1-2001.
.\" SVr4 defines an additional error
.\" code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's.
POSIX says that the behaviour of
.BR mprotect ()
is unspecified if it is applied to a region of memory that
was not obtained via
.BR mmap (2).
.SH NOTES
On Linux it is always legal to call
.BR mprotect ()
on any address in a process' address space (except for the
kernel vsyscall area).
In particular it can be used
to change existing code mappings to be writable.

Whether
.B PROT_EXEC
has any effect different from
.B PROT_READ
is architecture and kernel version dependent.

POSIX.1-2001 says that an implementation may permit access
other than that specified in
.IR prot ,
but at a minimum can only allow write access if
.B PROT_WRITE
has been set, and must not allow any access if
.B PROT_NONE
has been set.
.SH EXAMPLE
.PP
The program below allocates four pages of memory, makes the third
of these pages read-only, and then executes a loop that walks upwards
through the allocated region modifying bytes.

An example of what we might see when running the program is the
following:

.in +0.5i
.nf
$ ./a.out
Start of region:        0x804c000
Got SIGSEGV at address: 0x804e000
.fi
.in
.nf

#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>

char *buffer;

static void
handler(int sig, siginfo_t *si, void *unused)
{
    printf("Got SIGSEGV at address: 0x%lx\\n", 
            (long) si->si_addr);
    exit(EXIT_FAILURE);
}

int
main(int argc, char *argv[])
{
    char *p;
    int pagesize;
    struct sigaction sa;

    sa.sa_flags = SA_SIGINFO;
    sigemptyset(&sa.sa_mask);
    sa.sa_sigaction = handler;
    if (sigaction(SIGSEGV, &sa, NULL) == -1) {
        perror("sigaction");
        exit(EXIT_FAILURE);
    }

    pagesize = sysconf(_SC_PAGE_SIZE);
    if (pagesize == -1) {
        perror("sysconf");
        exit(EXIT_FAILURE);
    }

    /* Allocate a buffer aligned on a page boundary; 
       initial protection is PROT_READ | PROT_WRITE */

    buffer = memalign(pagesize, 4 * pagesize);
    if (buffer == NULL) {
        perror("memalign");
        exit(EXIT_FAILURE);
    }

    printf("Start of region:        0x%lx\\n", (long) buffer);

    if (mprotect(buffer + pagesize * 2, pagesize, 
                PROT_NONE) == -1) {
        perror("mprotect");
        exit(EXIT_FAILURE);
    }

    for (p = buffer ; ; )
        *(p++) = 'a';

    printf("Loop completed\\n");     /* Should never happen */
    exit(EXIT_SUCCESS);
}
.fi
.SH "SEE ALSO"
.BR mmap (2),
.BR sysconf (3)