man-pages/man2/ptrace.2

.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" Copyright (c) 1993 Michael Haardt
.\" (michael@moria.de),
.\" Fri Apr  2 11:32:09 MET DST 1993
.\"
.\" changes Copyright 1999 Mike Coleman (mkc@acm.org)
.\" -- major revision to fully document ptrace semantics per recent Linux
.\"    kernel (2.2.10) and glibc (2.1.2)
.\" Sun Nov  7 03:18:35 CST 1999
.\"
.\" This is free documentation; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License as
.\" published by the Free Software Foundation; either version 2 of
.\" the License, or (at your option) any later version.
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.\" This manual is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" GNU General Public License for more details.
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.\" You should have received a copy of the GNU General Public
.\" License along with this manual; if not, write to the Free
.\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111,
.\" USA.
.\"
.\" Modified Fri Jul 23 23:47:18 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Fri Jan 31 16:46:30 1997 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Thu Oct  7 17:28:49 1999 by Andries Brouwer <aeb@cwi.nl>
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.\"
.\" 2006-03-24, Chuck Ebbert <76306.1226@compuserve.com>
.\"    Added    PTRACE_SETOPTIONS, PTRACE_GETEVENTMSG, PTRACE_GETSIGINFO,
.\"        PTRACE_SETSIGINFO, PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP
.\"    (Thanks to Blaisorblade, Daniel Jacobowitz and others who helped.)
.\"
.\" FIXME: Linux 3.1 adds PTRACE_SEIZE, PTRACE_INTERRUPT, and PTRACE_LISTEN.
.\"
.TH PTRACE 2 2009-03-30 "Linux" "Linux Programmer's Manual"
.SH NAME
ptrace \- process trace
.SH SYNOPSIS
.nf
.B #include <sys/ptrace.h>
.sp
.BI "long ptrace(enum __ptrace_request " request ", pid_t " pid ", "
.BI "            void *" addr ", void *" data );
.fi
.SH DESCRIPTION
The
.BR ptrace ()
system call provides a means by which a process (tracer) may observe
and control the execution of another processes (tracees),
and examine and change their core image and registers.
It is primarily used to implement breakpoint debugging and system
call tracing.
.LP
Tracees first need to be attached to the tracer.
Attachment and subsequent commands are per-thread: in
multi-threaded process, every thread can be individually attached to a
(potentially different) tracer, or left not attached and thus not
debugged.
Therefore, "tracee" always means "(one) thread", never "a
(possibly multi-threaded) process".
Ptrace commands are always sent to
a specific tracee using ptrace(PTRACE_foo, pid, ...), where pid is the
thread ID of the corresponding Linux thread.
.LP
The process can initiate a trace by calling
.BR fork (2)
and having the resulting child do a
.BR PTRACE_TRACEME ,
followed (typically) by an
.BR execve (2).
Alternatively, the process may commence trace of an existing process using
.BR PTRACE_ATTACH .
.LP
While being traced, the tracee will stop each time a signal is delivered,
even if the signal is being ignored.
(The exception is
.BR SIGKILL ,
which has its usual effect.)
The tracer will be notified at its next
.BR wait (2)
and may inspect and modify the tracee while it is stopped.
The tracer then causes the tracee to continue,
optionally ignoring the delivered signal
(or even delivering a different signal instead).
.LP
When the tracer is finished tracing, it can cause tracee to continue
executing in a normal, untraced mode via
.BR PTRACE_DETACH .
.LP
The value of \fIrequest\fP determines the action to be performed:
.TP
.B PTRACE_TRACEME
Indicates that this process is to be traced by its parent.
Any signal (except
.BR SIGKILL )
delivered to this process will cause it to stop and its
parent to be notified via
.BR wait (2).
Also, all subsequent calls to
.BR execve (2)
by this process will cause a
.B SIGTRAP
to be sent to it,
giving the parent a chance to gain control before the new program
begins execution.
A process probably shouldn't make this request if its parent
isn't expecting to trace it.
(\fIpid\fP, \fIaddr\fP, and \fIdata\fP are ignored.)
.LP
The above request is used only by the tracee;
the rest are used only by the tracer.
In the following requests, \fIpid\fP specifies the tracee
to be acted on.
For requests other than
.BR PTRACE_KILL ,
the tracee must be stopped.
.TP
.BR PTRACE_PEEKTEXT ", " PTRACE_PEEKDATA
Reads a word at the location
.I addr
in the tracee's memory, returning the word as the result of the
.BR ptrace ()
call.
Linux does not have separate text and data address spaces, so the two
requests are currently equivalent.
(The argument \fIdata\fP is ignored.)
.TP
.B PTRACE_PEEKUSER
.\" PTRACE_PEEKUSR in kernel source, but glibc uses PTRACE_PEEKUSER,
.\" and that is the name that seems common on other systems.
Reads a word at offset
.I addr
in the tracee's USER area,
which holds the registers and other information about the process
(see \fI<sys/user.h>\fP).
The word is returned as the result of the
.BR ptrace ()
call.
Typically the offset must be word-aligned, though this might vary by
architecture.
See NOTES.
(\fIdata\fP is ignored.)
.TP
.BR PTRACE_POKETEXT ", " PTRACE_POKEDATA
Copies the word
.I data
to location
.I addr
in the tracee's memory.
As above, the two requests are currently equivalent.
.TP
.B PTRACE_POKEUSER
.\" PTRACE_POKEUSR in kernel source, but glibc uses PTRACE_POKEUSER,
.\" and that is the name that seems common on other systems.
Copies the word
.I data
to offset
.I addr
in the tracee's USER area.
As above, the offset must typically be word-aligned.
In order to maintain the integrity of the kernel,
some modifications to the USER area are disallowed.
.TP
.BR PTRACE_GETREGS ", " PTRACE_GETFPREGS
Copies the tracee's general purpose or floating-point registers,
respectively, to location \fIdata\fP in the tracer.
See \fI<sys/user.h>\fP for information on
the format of this data.
(\fIaddr\fP is ignored.)
.TP
.BR PTRACE_GETSIGINFO " (since Linux 2.3.99-pre6)"
Retrieve information about the signal that caused the stop.
Copies a \fIsiginfo_t\fP structure (see
.BR sigaction (2))
from the tracee to location \fIdata\fP in the tracer.
(\fIaddr\fP is ignored.)
.TP
.BR PTRACE_SETREGS ", " PTRACE_SETFPREGS
Copies the tracee's general purpose or floating-point registers,
respectively, from location \fIdata\fP in the tracer.
As for
.BR PTRACE_POKEUSER ,
some general
purpose register modifications may be disallowed.
(\fIaddr\fP is ignored.)
.TP
.BR PTRACE_SETSIGINFO " (since Linux 2.3.99-pre6)"
Set signal information.
Copies a \fIsiginfo_t\fP structure from location \fIdata\fP in the
tracer to the tracee.
This will only affect signals that would normally be delivered to
the tracee and were caught by the tracer.
It may be difficult to tell
these normal signals from synthetic signals generated by
.BR ptrace ()
itself.
(\fIaddr\fP is ignored.)
.TP
.BR PTRACE_SETOPTIONS " (since Linux 2.4.6; see BUGS for caveats)"
Sets ptrace options from \fIdata\fP.
(\fIaddr\fP is ignored.)
\fIdata\fP is interpreted
as a bit mask of options, which are specified by the following flags:
.RS
.TP
.BR PTRACE_O_TRACESYSGOOD " (since Linux 2.4.6)"
When delivering syscall traps, set bit 7 in the signal number
(i.e., deliver \fISIGTRAP | 0x80\fP).
This makes it easy for the tracer to tell the difference
between normal traps and those caused by a syscall.
.RB ( PTRACE_O_TRACESYSGOOD
may not work on all architectures.)
.TP
.BR PTRACE_O_TRACEFORK " (since Linux 2.5.46)"
Stop the tracee at the next
.BR fork (2)
call with \fISIGTRAP | PTRACE_EVENT_FORK\ <<\ 8\fP and automatically
start tracing the newly forked process,
which will start with a
.BR SIGSTOP .
The PID for the new process can be retrieved with
.BR PTRACE_GETEVENTMSG .
.TP
.BR PTRACE_O_TRACEVFORK " (since Linux 2.5.46)"
Stop the tracee at the next
.BR vfork (2)
call with \fISIGTRAP | PTRACE_EVENT_VFORK\ <<\ 8\fP and automatically start
tracing the newly vforked process, which will start with a
.BR SIGSTOP .
The PID for the new process can be retrieved with
.BR PTRACE_GETEVENTMSG .
.TP
.BR PTRACE_O_TRACECLONE " (since Linux 2.5.46)"
Stop the tracee at the next
.BR clone (2)
call with \fISIGTRAP | PTRACE_EVENT_CLONE\ <<\ 8\fP and automatically start
tracing the newly cloned process, which will start with a
.BR SIGSTOP .
The PID for the new process can be retrieved with
.BR PTRACE_GETEVENTMSG .
This option may not catch
.BR clone (2)
calls in all cases.
If the tracee calls
.BR clone (2)
with the
.B CLONE_VFORK
flag,
.B PTRACE_EVENT_VFORK
will be delivered instead
if
.B PTRACE_O_TRACEVFORK
is set; otherwise if the tracee calls
.BR clone (2)
with the exit signal set to
.BR SIGCHLD ,
.B PTRACE_EVENT_FORK
will be delivered
if
.B PTRACE_O_TRACEFORK
is set.
.TP
.BR PTRACE_O_TRACEEXEC " (since Linux 2.5.46)"
Stop the tracee at the next
.BR execve (2)
call with \fISIGTRAP | PTRACE_EVENT_EXEC\ <<\ 8\fP.
.TP
.BR PTRACE_O_TRACEVFORKDONE " (since Linux 2.5.60)"
Stop the tracee at the completion of the next
.BR vfork (2)
call with \fISIGTRAP | PTRACE_EVENT_VFORK_DONE\ <<\ 8\fP.
.TP
.BR PTRACE_O_TRACEEXIT " (since Linux 2.5.60)"
Stop the tracee at exit with \fISIGTRAP | PTRACE_EVENT_EXIT\ <<\ 8\fP.
The tracee's exit status can be retrieved with
.BR PTRACE_GETEVENTMSG .
This stop will be done early during process exit when registers
are still available, allowing the tracer to see where the exit occurred,
whereas the normal exit notification is done after the process
is finished exiting.
Even though context is available, the tracer cannot prevent the exit from
happening at this point.
.RE
.TP
.BR PTRACE_GETEVENTMSG " (since Linux 2.5.46)"
Retrieve a message (as an
.IR "unsigned long" )
about the ptrace event
that just happened, placing it in the location \fIdata\fP in the tracer.
For
.B PTRACE_EVENT_EXIT
this is the tracee's exit status.
For
.BR PTRACE_EVENT_FORK ,
.B PTRACE_EVENT_VFORK
and
.B PTRACE_EVENT_CLONE
this
is the PID of the new process.
Since Linux 2.6.18, the PID of the new process is also available
for
.BR PTRACE_EVENT_VFORK_DONE .
(\fIaddr\fP is ignored.)
.TP
.B PTRACE_CONT
Restarts the stopped tracee process.
If \fIdata\fP is nonzero, it is interpreted as a signal to be delivered to the tracee;
otherwise, no signal is delivered.
Thus, for example, the tracer can control
whether a signal sent to the tracee is delivered or not.
(\fIaddr\fP is ignored.)
.TP
.BR PTRACE_SYSCALL ", " PTRACE_SINGLESTEP
Restarts the stopped tracee as for
.BR PTRACE_CONT ,
but arranges for
the tracee to be stopped at the next entry to or exit from a system call,
or after execution of a single instruction, respectively.
(The tracee will also, as usual, be stopped upon receipt of a signal.)
From the tracer's perspective, the tracee will appear to have been
stopped by receipt of a
.BR SIGTRAP .
So, for
.BR PTRACE_SYSCALL ,
for example, the idea is to inspect
the arguments to the system call at the first stop,
then do another
.B PTRACE_SYSCALL
and inspect the return value of
the system call at the second stop.
The
.I data
argument is treated as for
.BR PTRACE_CONT .
(\fIaddr\fP is ignored.)
.TP
.BR PTRACE_SYSEMU ", " PTRACE_SYSEMU_SINGLESTEP " (since Linux 2.6.14)"
For
.BR PTRACE_SYSEMU ,
continue and stop on entry to the next syscall,
which will not be executed.
For
.BR PTRACE_SYSEMU_SINGLESTEP ,
do the same
but also singlestep if not a syscall.
This call is used by programs like
User Mode Linux that want to emulate all the tracee's system calls.
The
.I data
argument is treated as for
.BR PTRACE_CONT .
(\fIaddr\fP is ignored;
not supported on all architectures.)
.TP
.B PTRACE_KILL
Sends the tracee a
.B SIGKILL
to terminate it.
(\fIaddr\fP and \fIdata\fP are ignored.)
This operation is deprecated, use kill(SIGKILL) or tgkill(SIGKILL) instead.
.TP
.B PTRACE_ATTACH
Attaches to the process specified in
.IR pid ,
making it a tracee of the calling process.
.\" Not true:
.\" ; the behavior of the tracee is as if it had done a
.\" .BR PTRACE_TRACEME .
.\" The calling process actually becomes the parent of the tracee
.\" process for most purposes (e.g., it will receive
.\" notification of tracee events and appears in
.\" .BR ps (1)
.\" output as the tracee's parent), but a
.\" .BR getppid (2)
.\" by the tracee will still return the PID of the original parent.
The tracee is sent a
.BR SIGSTOP ,
but will not necessarily have stopped
by the completion of this call; use
.BR wait (2)
to wait for the tracee to stop.
See "Attaching and detaching" subsection
for additional information.
(\fIaddr\fP and \fIdata\fP are ignored.)
.TP
.B PTRACE_DETACH
Restarts the stopped tracee as for
.BR PTRACE_CONT ,
but first detaches from it.
Under Linux a tracee can be
detached in this way regardless of which method was used to initiate
tracing.
(\fIaddr\fP is ignored.)
.\"
.\" In the text below, we decided to avoid prettifying the text with markup:
.\" it would make the source nearly impossible to edit, and we _do_ intend
.\" to edit it often, in order to keep it updated:
.\" ptrace API is full of quirks, no need to compound this situation by
.\" making it excruciatingly painful to document them!
.\"
.SS Death under ptrace
When a (possibly multi-threaded) process receives a killing signal (a
signal set to SIG_DFL and whose default action is to kill the process),
all threads exit.
Tracees report their death to their tracer(s).
The notification about this event is delivered through waitpid API.
.LP
Note that killing signal will first cause signal-delivery-stop (on one
tracee only), and only after it is injected by tracer (or after it was
dispatched to a thread which isn't traced), death from signal will
happen on ALL tracees within multi-threaded process.
.LP
SIGKILL operates similarly, with exceptions.
No signal-delivery-stop is
generated for SIGKILL and therefore tracer can't suppress it.
SIGKILL
kills even within syscalls (syscall-exit-stop is not generated prior to
death by SIGKILL).
The net effect is that SIGKILL always kills the
process (all its threads), even if some threads of the process are
ptraced.
.LP
Tracer can kill a tracee with ptrace(PTRACE_KILL, pid, 0, 0).
This
operation is deprecated, use kill(SIGKILL) or tgkill(SIGKILL) instead.
The problem with this operation is that it requires tracee to be in
signal-delivery-stop, otherwise it may not work (may complete
successfully but won't kill the tracee), whereas tgkill(SIGKILL)
has no such limitation.
.\" .LP
.\" [Note: deprecation suggested by Oleg Nesterov. He prefers to deprecate
.\" it instead of describing (and needing to support) PTRACE_KILL's quirks.]
.LP
When tracee executes exit syscall, it reports its death to its tracer.
Other threads are not affected.
.LP
When any thread executes exit_group syscall, every tracee in its thread
group reports its death to its tracer.
.LP
If PTRACE_O_TRACEEXIT option is on, PTRACE_EVENT_EXIT will happen
before actual death.
This applies to exits on exit syscall, group_exit
syscall, signal deaths (except SIGKILL), and when threads are torn down
on execve in multi-threaded process.
.LP
Tracer cannot assume that ptrace-stopped tracee exists.
There are many
scenarios when tracee may die while stopped (such as SIGKILL).
Therefore, tracer must always be prepared to handle ESRCH error on any
ptrace operation.
Unfortunately, the same error is returned if tracee
exists but is not ptrace-stopped (for commands which require stopped
tracee), or if it is not traced by process which issued ptrace call.
Tracer needs to keep track of stopped/running state, and interpret
ESRCH as "tracee died unexpectedly" only if it knows that tracee has
been observed to enter ptrace-stop.
Note that there is no guarantee
that waitpid(WNOHANG) will reliably report tracee's death status if
ptrace operation returned ESRCH.
waitpid(WNOHANG) may return 0 instead.
IOW: tracee may be "not yet fully dead" but already refusing ptrace ops.
.LP
Tracer can not assume that tracee ALWAYS ends its life by reporting
WIFEXITED(status) or WIFSIGNALED(status).
.LP
.\" or can it? Do we include such a promise into ptrace API?
.SS Stopped states
A tracee can be in two states: running or stopped.
.LP
There are many kinds of states when tracee is stopped, and in ptrace
discussions they are often conflated.
Therefore, it is important to use
precise terms.
.LP
In this document, any stopped state in which tracee is ready to accept
ptrace commands from the tracer is called ptrace-stop.
Ptrace-stops can
be further subdivided into signal-delivery-stop, group-stop,
syscall-stop and so on.
They are described in detail later.
.LP
When running tracee enters ptrace-stop, it notifies its tracer using
waitpid API.
Tracer should use waitpid family of syscalls to wait for
tracee to stop.
Most of this document assumes that tracer waits with:
.LP
	pid = waitpid(pid_or_minus_1, &status, __WALL);
.LP
Ptrace-stopped tracees are reported as returns with pid > 0 and
WIFSTOPPED(status) == true.
.LP
.\" Do we require __WALL usage, or will just using 0 be ok? Are the
.\" rules different if user wants to use waitid? Will waitid require
.\" WEXITED?
.LP
__WALL value does not include WSTOPPED and WEXITED bits, but implies
their functionality.
.LP
Setting of WCONTINUED bit in waitpid flags is not recommended: the
continued state is per-process and consuming it can confuse real parent
of the tracee.
.LP
Use of WNOHANG bit in waitpid flags may cause waitpid return 0 ("no
wait results available yet") even if tracer knows there should be a
notification.
Example: kill(tracee, SIGKILL); waitpid(tracee, &status,
__WALL | WNOHANG);
.\" waitid usage? WNOWAIT?
.\" describe how wait notifications queue (or not queue)
.LP
The following kinds of ptrace-stops exist: signal-delivery-stops,
group-stop, PTRACE_EVENT stops, syscall-stops [, SINGLESTEP, SYSEMU,
SYSEMU_SINGLESTEP].
They all are reported as waitpid result with
WIFSTOPPED(status) == true.
They may be differentiated by checking
(status >> 8) value, and if looking at (status >> 8) value doesn't
resolve ambiguity, by querying PTRACE_GETSIGINFO.
(Note:
WSTOPSIG(status) macro returns ((status >> 8) & 0xff) value).
.SS Signal-delivery-stop
When (possibly multi-threaded) process receives any signal except
SIGKILL, kernel selects a thread which handles the signal (if signal is
generated with t[g]kill, thread selection is done by user).
If selected
thread is traced, it enters signal-delivery-stop.
By this point, signal
is not yet delivered to the process, and can be suppressed by tracer.
If tracer doesn't suppress the signal, it passes signal to tracee in
the next ptrace request.
This second step of signal delivery is called
"signal injection" in this document.
Note that if signal is blocked,
signal-delivery-stop doesn't happen until signal is unblocked, with the
usual exception that SIGSTOP can't be blocked.
.LP
Signal-delivery-stop is observed by tracer as waitpid returning with
WIFSTOPPED(status) == true, WSTOPSIG(status) == signal.
If
WSTOPSIG(status) == SIGTRAP, this may be a different kind of
ptrace-stop - see "Syscall-stops" and "execve" sections below for
details.
If WSTOPSIG(status) == stopping signal, this may be a
group-stop - see below.
.SS Signal injection and suppression
After signal-delivery-stop is observed by tracer, tracer should restart
tracee with
.LP
	ptrace(PTRACE_rest, pid, 0, sig)
.LP
call, where PTRACE_rest is one of the restarting ptrace ops.
If sig is
0, then signal is not delivered.
Otherwise, signal sig is delivered.
This operation is called "signal injection" in this document, to
distinguish it from signal-delivery-stop.
.LP
Note that sig value may be different from WSTOPSIG(status) value -
tracer can cause a different signal to be injected.
.LP
Note that suppressed signal still causes syscalls to return
prematurely.
Restartable syscalls will be restarted (tracer will
observe tracee to execute restart_syscall(2) syscall if tracer uses
PTRACE_SYSCALL), non-restartable syscalls (for example, nanosleep) may
return with -EINTR even though no observable signal is injected to the
tracee.
.LP
Note that restarting ptrace commands issued in ptrace-stops other than
signal-delivery-stop are not guaranteed to inject a signal, even if sig
is nonzero.
No error is reported, nonzero sig may simply be ignored.
Ptrace users should not try to "create new signal" this way: use
tgkill(2) instead.
.LP
This is a cause of confusion among ptrace users.
One typical scenario
is that tracer observes group-stop, mistakes it for
signal-delivery-stop, restarts tracee with ptrace(PTRACE_rest, pid, 0,
stopsig) with the intention of injecting stopsig, but stopsig gets
ignored and tracee continues to run.
.LP
SIGCONT signal has a side effect of waking up (all threads of)
group-stopped process.
This side effect happens before
signal-delivery-stop.
Tracer can't suppress this side-effect (it can
only suppress signal injection, which only causes SIGCONT handler to
not be executed in the tracee, if such handler is installed).
In fact,
waking up from group-stop may be followed by signal-delivery-stop for
signal(s) *other than* SIGCONT, if they were pending when SIGCONT was
delivered.
IOW: SIGCONT may be not the first signal observed by the
tracee after it was sent.
.LP
Stopping signals cause (all threads of) process to enter group-stop.
This side effect happens after signal injection, and therefore can be
suppressed by tracer.
.LP
PTRACE_GETSIGINFO can be used to retrieve siginfo_t structure which
corresponds to delivered signal.
PTRACE_SETSIGINFO may be used to
modify it.
If PTRACE_SETSIGINFO has been used to alter siginfo_t,
si_signo field and sig parameter in restarting command must match,
otherwise the result is undefined.
.SS Group-stop
When a (possibly multi-threaded) process receives a stopping signal,
all threads stop.
If some threads are traced, they enter a group-stop.
Note that stopping signal will first cause signal-delivery-stop (on one
tracee only), and only after it is injected by tracer (or after it was
dispatched to a thread which isn't traced), group-stop will be
initiated on ALL tracees within multi-threaded process.
As usual, every
tracee reports its group-stop separately to corresponding tracer.
.LP
Group-stop is observed by tracer as waitpid returning with
WIFSTOPPED(status) == true, WSTOPSIG(status) == signal.
The same result
is returned by some other classes of ptrace-stops, therefore the
recommended practice is to perform
.LP
	ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)
.LP
call.
The call can be avoided if signal number is not SIGSTOP, SIGTSTP,
SIGTTIN or SIGTTOU - only these four signals are stopping signals.
If tracer sees something else, it can't be group-stop.
Otherwise, tracer
needs to call PTRACE_GETSIGINFO.
If PTRACE_GETSIGINFO fails with
EINVAL, then it is definitely a group-stop.
(Other failure codes are
possible, such as ESRCH "no such process" if SIGKILL killed the tracee).
.LP
As of kernel 2.6.38, after tracer sees tracee ptrace-stop and until it
restarts or kills it, tracee will not run, and will not send
notifications (except SIGKILL death) to tracer, even if tracer enters
into another waitpid call.
.LP
Currently, it causes a problem with transparent handling of stopping
signals: if tracer restarts tracee after group-stop, SIGSTOP is
effectively ignored: tracee doesn't remain stopped, it runs.
If tracer
doesn't restart tracee before entering into next waitpid, future
SIGCONT will not be reported to the tracer.
Which would make SIGCONT to
have no effect.
.SS PTRACE_EVENT stops
If tracer sets TRACE_O_TRACEfoo options, tracee will enter ptrace-stops
called PTRACE_EVENT stops.
.LP
PTRACE_EVENT stops are observed by tracer as waitpid returning with
WIFSTOPPED(status) == true, WSTOPSIG(status) == SIGTRAP.
Additional bit
is set in a higher byte of status word: value (status >> 8)
will be (SIGTRAP | PTRACE_EVENT_foo << 8).
The following events exist:
.LP
PTRACE_EVENT_VFORK - stop before return from vfork or clone+CLONE_VFORK.
When tracee is continued after this stop, it will wait for child to
exit/exec before continuing its execution (IOW: usual behavior on
vfork).
.LP
PTRACE_EVENT_FORK - stop before return from fork or clone+SIGCHLD
.LP
PTRACE_EVENT_CLONE - stop before return from clone
.LP
PTRACE_EVENT_VFORK_DONE - stop before return from
vfork or clone+CLONE_VFORK, but after vforked child unblocked this
tracee by exiting or exec'ing.
.LP
For all four stops described above: stop occurs in parent, not in newly
created thread.
PTRACE_GETEVENTMSG can be used to retrieve new thread's
tid.
.LP
PTRACE_EVENT_EXEC - stop before return from execve.
.LP
PTRACE_EVENT_EXIT - stop before exit (including death from exit_group),
signal death, or exit caused by execve in multi-threaded process.
PTRACE_GETEVENTMSG returns exit status.
Registers can be examined
(unlike when "real" exit happens).
The tracee is still alive, it needs
to be PTRACE_CONTed or PTRACE_DETACHed to finish exit.
.LP
PTRACE_GETSIGINFO on PTRACE_EVENT stops returns si_signo = SIGTRAP,
si_code = (event << 8) | SIGTRAP.
.SS Syscall-stops
If tracee was restarted by PTRACE_SYSCALL, tracee enters
syscall-enter-stop just prior to entering any syscall.
If tracer
restarts it with PTRACE_SYSCALL, tracee enters syscall-exit-stop when
syscall is finished, or if it is interrupted by a signal.
(That is,
signal-delivery-stop never happens between syscall-enter-stop and
syscall-exit-stop, it happens *after* syscall-exit-stop).
.LP
Other possibilities are that tracee may stop in a PTRACE_EVENT stop,
exit (if it entered exit or exit_group syscall), be killed by SIGKILL,
or die silently (if it is a thread group leader, execve syscall happened
in another thread, and that thread is not traced by the same tracer -
this sutuation is discussed later).
.LP
Syscall-enter-stop and syscall-exit-stop are observed by tracer as
waitpid returning with WIFSTOPPED(status) == true, WSTOPSIG(status) ==
SIGTRAP.
If PTRACE_O_TRACESYSGOOD option was set by tracer, then
WSTOPSIG(status) == (SIGTRAP | 0x80).
.LP
Syscall-stops can be distinguished from signal-delivery-stop with
SIGTRAP by querying PTRACE_GETSIGINFO: si_code <= 0 if SIGTRAP was sent by usual
suspects like [tg]kill/sigqueue/etc; or = SI_KERNEL (0x80) if sent by
kernel, whereas syscall-stops have si_code = SIGTRAP or (SIGTRAP |
0x80).
However, syscall-stops happen very often (twice per syscall),
and performing PTRACE_GETSIGINFO for every syscall-stop may be somewhat
expensive.
.LP
Some architectures allow to distinguish them by examining registers.
For example, on x86 rax = -ENOSYS in syscall-enter-stop.
Since SIGTRAP
(like any other signal) always happens *after* syscall-exit-stop, and
at this point rax almost never contains -ENOSYS, SIGTRAP looks like
"syscall-stop which is not syscall-enter-stop", IOW: it looks like a
"stray syscall-exit-stop" and can be detected this way.
But such
detection is fragile and is best avoided.
.LP
Using PTRACE_O_TRACESYSGOOD option is a recommended method, since it is
reliable and does not incur performance penalty.
.LP
Syscall-enter-stop and syscall-exit-stop are indistinguishable from
each other by tracer.
Tracer needs to keep track of the sequence of
ptrace-stops in order to not misinterpret syscall-enter-stop as
syscall-exit-stop or vice versa.
The rule is that syscall-enter-stop is
always followed by syscall-exit-stop, PTRACE_EVENT stop or tracee's
death - no other kinds of ptrace-stop can occur in between.
.LP
If after syscall-enter-stop tracer uses restarting command other than
PTRACE_SYSCALL, syscall-exit-stop is not generated.
.LP
PTRACE_GETSIGINFO on syscall-stops returns si_signo = SIGTRAP, si_code
= SIGTRAP or (SIGTRAP | 0x80).
.SS SINGLESTEP, SYSEMU, SYSEMU_SINGLESTEP stops
(TODO: document stops occurring with PTRACE_SINGLESTEP, PTRACE_SYSEMU,
PTRACE_SYSEMU_SINGLESTEP)
.SS Informational and restarting ptrace commands
Most ptrace commands (all except ATTACH, TRACEME, KILL) require tracee
to be in a ptrace-stop, otherwise they fail with ESRCH.
.LP
When tracee is in ptrace-stop, tracer can read and write data to tracee
using informational commands.
They leave tracee in ptrace-stopped state:
.LP
.nf
longv = ptrace(PTRACE_PEEKTEXT/PEEKDATA/PEEKUSER, pid, addr, 0);
	ptrace(PTRACE_POKETEXT/POKEDATA/POKEUSER, pid, addr, long_val);
	ptrace(PTRACE_GETREGS/GETFPREGS, pid, 0, &struct);
	ptrace(PTRACE_SETREGS/SETFPREGS, pid, 0, &struct);
	ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo);
	ptrace(PTRACE_SETSIGINFO, pid, 0, &siginfo);
	ptrace(PTRACE_GETEVENTMSG, pid, 0, &long_var);
	ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);
.fi
.LP
Note that some errors are not reported.
For example, setting siginfo
may have no effect in some ptrace-stops, yet the call may succeed
(return 0 and don't set errno); querying GETEVENTMSG may succeed
and return some random value if current ptrace-stop is not documented
as returning meaningful event message.
.LP
ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags) affects one tracee.
Current flags are replaced.
Flags are inherited by new tracees created
and "auto-attached" via active PTRACE_O_TRACE[V]FORK or
PTRACE_O_TRACECLONE options.
.LP
Another group of commands makes ptrace-stopped tracee run.
They have
the form:
.LP
	ptrace(PTRACE_cmd, pid, 0, sig);
.LP
where cmd is CONT, DETACH, SYSCALL, SINGLESTEP, SYSEMU, or
SYSEMU_SINGLESTEP.
If tracee is in signal-delivery-stop, sig is the
signal to be injected.
Otherwise, sig may be ignored (recommended
practice is to always pass 0 in these cases).
.SS Attaching and detaching
A thread can be attached to tracer using ptrace(PTRACE_ATTACH, pid, 0,
0) call.
This also sends SIGSTOP to this thread.
If tracer wants this
SIGSTOP to have no effect, it needs to suppress it.
Note that if other
signals are concurrently sent to this thread during attach, tracer may
see tracee enter signal-delivery-stop with other signal(s) first! The
usual practice is to reinject these signals until SIGSTOP is seen, then
suppress SIGSTOP injection.
The design bug here is that attach and
concurrent SIGSTOP are racing and concurrent SIGSTOP may be lost.
.\" Describe how to attach to a thread which is already group-stopped.
.LP
Since attaching sends SIGSTOP and tracer usually suppresses it, this
may cause stray EINTR return from the currently executing syscall in
the tracee, as described in "signal injection and suppression" section.
.LP
ptrace(PTRACE_TRACEME, 0, 0, 0) request turns current thread into a
tracee.
It continues to run (doesn't enter ptrace-stop).
A common
practice is to follow ptrace(PTRACE_TRACEME) with raise(SIGSTOP) and
allow parent (which is our tracer now) to observe our
signal-delivery-stop.
.LP
If PTRACE_O_TRACE[V]FORK or PTRACE_O_TRACECLONE options are in effect,
then children created by (vfork or clone(CLONE_VFORK)), (fork or
clone(SIGCHLD)) and (other kinds of clone) respectively are
automatically attached to the same tracer which traced their parent.
SIGSTOP is delivered to them, causing them to enter
signal-delivery-stop after they exit syscall which created them.
.LP
Detaching of tracee is performed by ptrace(PTRACE_DETACH, pid, 0, sig).
PTRACE_DETACH is a restarting operation, therefore it requires tracee
to be in ptrace-stop.
If tracee is in signal-delivery-stop, signal can
be injected.
Otherwise, sig parameter may be silently ignored.
.LP
If tracee is running when tracer wants to detach it, the usual solution
is to send SIGSTOP (using tgkill, to make sure it goes to the correct
thread), wait for tracee to stop in signal-delivery-stop for SIGSTOP
and then detach it (suppressing SIGSTOP injection).
Design bug is that
this can race with concurrent SIGSTOPs.
Another complication is that
tracee may enter other ptrace-stops and needs to be restarted and
waited for again, until SIGSTOP is seen.
Yet another complication is to
be sure that tracee is not already ptrace-stopped, because no signal
delivery happens while it is - not even SIGSTOP.
.\" Describe how to detach from a group-stopped tracee so that it
.\" doesn't run, but continues to wait for SIGCONT.
.LP
If tracer dies, all tracees are automatically detached and restarted,
unless they were in group-stop.
Handling of restart from group-stop is
currently buggy, but "as planned" behavior is to leave tracee stopped
and waiting for SIGCONT.
If tracee is restarted from
signal-delivery-stop, pending signal is injected.
.SS execve under ptrace
During execve, kernel destroys all other threads in the process, and
resets execve'ing thread tid to tgid (process id).
This looks very
confusing to tracers:
.LP
All other threads stop in PTRACE_EXIT stop, if requested by active
ptrace option.
Then all other threads except thread group leader report
death as if they exited via exit syscall with exit code 0.
Then
PTRACE_EVENT_EXEC stop happens, if requested by active ptrace option.
.\" (on which tracee - leader? execve-ing one?)
.LP
The execve-ing tracee changes its pid while it is in execve syscall.
(Remember, under ptrace 'pid' returned from waitpid, or fed into ptrace
calls, is tracee's tid).
That is, pid is reset to process id, which
coincides with thread group leader tid.
.LP
If thread group leader has reported its death by this time, for tracer
this looks like dead thread leader "reappears from nowhere".
If thread
group leader was still alive, for tracer this may look as if thread
group leader returns from a different syscall than it entered, or even
"returned from syscall even though it was not in any syscall".
If
thread group leader was not traced (or was traced by a different
tracer), during execve it will appear as if it has become a tracee of
the tracer of execve-ing tracee.
All these effects are the artifacts of
pid change.
.LP
PTRACE_O_TRACEEXEC option is the recommended tool for dealing with this
case.
It enables PTRACE_EVENT_EXEC stop which occurs before execve
syscall returns.
.LP
Pid change happens before PTRACE_EVENT_EXEC stop, not after.
.LP
When tracer receives PTRACE_EVENT_EXEC stop notification, it is
guaranteed that except this tracee and thread group leader, no other
threads from the process are alive.
.LP
On receiving this notification, tracer should clean up all its internal
data structures about all threads of this process, and retain only one
data structure, one which describes single still running tracee, with
pid = tgid = process id.
.LP
Currently, there is no way to retrieve former pid of execve-ing tracee.
If tracer doesn't keep track of its tracees' thread group relations, it
may be unable to know which tracee execve-ed and therefore no longer
exists under old pid due to pid change.
.LP
Example: two threads execve at the same time:
.LP
.nf
*** we get syscall-entry-stop in thread 1: **
PID1 execve("/bin/foo", "foo" <unfinished ...>
*** we issue PTRACE_SYSCALL for thread 1 **
*** we get syscall-entry-stop in thread 2: **
PID2 execve("/bin/bar", "bar" <unfinished ...>
*** we issue PTRACE_SYSCALL for thread 2 **
*** we get PTRACE_EVENT_EXEC for PID0, we issue PTRACE_SYSCALL **
*** we get syscall-exit-stop for PID0: **
PID0 <... execve resumed> )             = 0
.fi
.LP
In this situation there is no way to know which execve succeeded.
.LP
If PTRACE_O_TRACEEXEC option is NOT in effect for the execve-ing
tracee, kernel delivers an extra SIGTRAP to tracee after execve syscall
returns.
This is an ordinary signal (similar to one which can be
generated by "kill -TRAP"), not a special kind of ptrace-stop.
GETSIGINFO on it has si_code = 0 (SI_USER).
It can be blocked by signal
mask, and thus can happen (much) later.
.LP
Usually, tracer (for example, strace) would not want to show this extra
post-execve SIGTRAP signal to the user, and would suppress its delivery
to the tracee (if SIGTRAP is set to SIG_DFL, it is a killing signal).
However, determining *which* SIGTRAP to suppress is not easy.
Setting
PTRACE_O_TRACEEXEC option and thus suppressing this extra SIGTRAP is
the recommended approach.
.SS Real parent
Ptrace API (ab)uses standard Unix parent/child signaling over waitpid.
This used to cause real parent of the process to stop receiving several
kinds of waitpid notifications when child process is traced by some
other process.
.LP
Many of these bugs have been fixed, but as of 2.6.38 several still
exist - see BUGS section below.
.LP
As of 2.6.38, the following is believed to work correctly:
.LP
* exit/death by signal is reported first to tracer, then, when tracer
consumes waitpid result, to real parent (to real parent only when the
whole multi-threaded process exits).
If they are the same process, the
report is sent only once.
.SH "RETURN VALUE"
On success,
.B PTRACE_PEEK*
requests return the requested data,
while other requests return zero.
On error, all requests return \-1, and
.I errno
is set appropriately.
Since the value returned by a successful
.B PTRACE_PEEK*
request may be \-1, the caller must check
.I errno
after such requests to determine whether or not an error occurred.
.SH ERRORS
.TP
.B EBUSY
(i386 only) There was an error with allocating or freeing a debug
register.
.TP
.B EFAULT
There was an attempt to read from or write to an invalid area in
the tracer's or tracee's memory,
probably because the area wasn't mapped or accessible.
Unfortunately, under Linux, different variations of this fault
will return
.B EIO
or
.B EFAULT
more or less arbitrarily.
.TP
.B EINVAL
An attempt was made to set an invalid option.
.TP
.B EIO
\fIrequest\fP is invalid, or an attempt was made to read from or
write to an invalid area in the tracer's or tracee's memory,
or there was a word-alignment violation,
or an invalid signal was specified during a restart request.
.TP
.B EPERM
The specified process cannot be traced.
This could be because the
tracer has insufficient privileges (the required capability is
.BR CAP_SYS_PTRACE );
unprivileged processes cannot trace processes that they
cannot send signals to or those running
set-user-ID/set-group-ID programs, for obvious reasons.
Alternatively, the process may already be being traced, or be
.BR init (8)
(PID 1).
.TP
.B ESRCH
The specified process does not exist, or is not currently being traced
by the caller, or is not stopped (for requests that require that).
.SH "CONFORMING TO"
SVr4, 4.3BSD.
.SH NOTES
Although arguments to
.BR ptrace ()
are interpreted according to the prototype given,
glibc currently declares
.BR ptrace ()
as a variadic function with only the \fIrequest\fP argument fixed.
This means that unneeded trailing arguments may be omitted,
though doing so makes use of undocumented
.BR gcc (1)
behavior.
.\" Not true anymore:
.\" .LP
.\" .BR init (8),
.\" the process with PID 1, may not be traced.
.\" .LP
The layout of the contents of memory and the USER area are quite OS- and
architecture-specific.
The offset supplied, and the data returned,
might not entirely match with the definition of
.IR "struct user" .
.\" See http://lkml.org/lkml/2008/5/8/375
.LP
The size of a "word" is determined by the OS variant
(e.g., for 32-bit Linux it is 32 bits, etc.).
.\" Covered in more details above:
.\" .LP
.\" Tracing causes a few subtle differences in the semantics of
.\" traced processes.
.\" For example, if a process is attached to with
.\" .BR PTRACE_ATTACH ,
.\" its original parent can no longer receive notification via
.\" .BR wait (2)
.\" when it stops, and there is no way for the new parent to
.\" effectively simulate this notification.
.\" .LP
.\" When the parent receives an event with
.\" .B PTRACE_EVENT_*
.\" set,
.\" the tracee is not in the normal signal delivery path.
.\" This means the parent cannot do
.\" .BR ptrace (PTRACE_CONT)
.\" with a signal or
.\" .BR ptrace (PTRACE_KILL).
.\" .BR kill (2)
.\" with a
.\" .B SIGKILL
.\" signal can be used instead to kill the tracee
.\" after receiving one of these messages.
.\" .LP
This page documents the way the
.BR ptrace ()
call works currently in Linux.
Its behavior differs noticeably on other flavors of UNIX.
In any case, use of
.BR ptrace ()
is highly OS- and architecture-specific.
.SH BUGS
On hosts with 2.6 kernel headers,
.B PTRACE_SETOPTIONS
is declared
with a different value than the one for 2.4.
This leads to applications compiled with such
headers failing when run on 2.4 kernels.
This can be worked around by redefining
.B PTRACE_SETOPTIONS
to
.BR PTRACE_OLDSETOPTIONS ,
if that is defined.
.LP
Group-stop notifications are sent to tracer, but not to real parent.
Last confirmed on 2.6.38.6.
.LP
If thread group leader is traced and exits by calling exit syscall,
PTRACE_EVENT_EXIT stop will happen for it (if requested), but
subsequent WIFEXITED notification will not be delivered until all other
threads exit.
As explained above, if one of other threads execve's,
thread group leader death will *never* be reported.
If execve-ed thread
is not traced by this tracer, tracer will never know that execve
happened.
One possible workaround is to detach thread group leader instead of
restarting it in this case.
Last confirmed on 2.6.38.6.
.\" ^^^ need to test/verify this scenario
.LP
SIGKILL signal may still cause PTRACE_EVENT_EXIT stop before actual
signal death.
This may be changed in the future - SIGKILL is meant to
always immediately kill tasks even under ptrace.
Last confirmed on
2.6.38.6.
.SH "SEE ALSO"
.BR gdb (1),
.BR strace (1),
.BR execve (2),
.BR fork (2),
.BR signal (2),
.BR wait (2),
.BR exec (3),
.BR capabilities (7)