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Ptrace documentation, draft #6

From:  Denys Vlasenko <>
To:  Michael Kerrisk <>, Tejun Heo <>,,
Subject:  Ptrace documentation, draft #6
Date:  Wed, 8 Jun 2011 14:45:04 +0200
Message-ID:  <>
Archive-link:  Article

Hello Michael,

Oleg suggested that instead of dropping ptrace API doc
into Documentation/* (where it will successfully bit-rot),
it may be a better idea to put it into the ptrace manpage.
What do you think about it?

I resent the latest draft below, slightly edited.

Feel free to incorporate it into the manpage already.
Even better if after reading it you have questions :)
and we can improve this doc further.


Ptrace discussions repeatedly display a higher than average amount
of misunderstanding and confusion. New ptrace users and even people
who already worked with it are repeatedly confused by details
which are not documented anywhere and knowledge about which exists
mostly in the brains of strace/gdb/other_such_tools developers.

This document is meant as a brain dump of this knowledge.
It assumes that the reader has basic understanding what ptrace is.


		Ptrace userspace API.

Ptrace API (ab)uses standard Unix parent/child signaling over waitpid.
An unfortunate effect of it is that resulting API is complex and has
subtle quirks. This document aims to describe these quirks.

Debugged processes (tracees) first need to be attached to the debugging
process (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 a
TID of the corresponding Linux thread.

After attachment, each tracee can be in two states: running or stopped.

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.

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.

	1.x 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 the tracer(s). This is
not a ptrace-stop (because tracer can't query tracee status such as
register contents, cannot restart tracee etc) but the notification
about this event is delivered through waitpid API similarly to

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.

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

Tracer can kill a tracee with ptrace(PTRACE_KILL, pid, 0, 0). This
operation is deprecated, use kill/tgkill(SIGKILL) instead.

^^^ Oleg prefers to deprecate it instead of describing (and needing to
support) PTRACE_KILL's quirks.

When tracee executes exit syscall, it reports its death to its tracer.
Other threads are not affected.

When any thread executes exit_group syscall, every tracee in its thread
group reports its death to its tracer.

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.

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.

Tracer can not assume that tracee ALWAYS ends its life by reporting
WIFEXITED(status) or WIFSIGNALED(status).

??? or can it? Do we include such a promise into ptrace API?

	1.x Stopped states.

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:

	pid = waitpid(pid_or_minus_1, &status, __WALL);

Ptrace-stopped tracees are reported as returns with pid > 0 and
WIFSTOPPED(status) == true.

??? 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

__WALL value does not include WSTOPPED and WEXITED bits, but implies
their functionality.

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.

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,

??? waitid usage? WNOWAIT?

??? describe how wait notifications queue (or not queue)

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).

	1.x.x 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.

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.

	1.x.x Signal injection and suppression.

After signal-delivery-stop is observed by tracer, tracer should restart
tracee with

	ptrace(PTRACE_rest, pid, 0, sig)

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.

Note that sig value may be different from WSTOPSIG(status) value -
tracer can cause a different signal to be injected.

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

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.

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.

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.

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.

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.

	1.x.x 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.

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

	ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

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
EINVAL, then it is definitely a group-stop. (Other failure codes are
possible, such as ESRCH "no such process" if SIGKILL killed the tracee).

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.

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.

	1.x.x PTRACE_EVENT stops

If tracer sets TRACE_O_TRACEfoo options, tracee will enter ptrace-stops
called PTRACE_EVENT stops.

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:

PTRACE_EVENT_VFORK - stop before return from vfork/clone+CLONE_VFORK.
When tracee is continued after this, it will wait for child to
exit/exec before continuing its execution (IOW: usual behavior on

PTRACE_EVENT_FORK - stop before return from fork/clone+SIGCHLD

PTRACE_EVENT_CLONE - stop before return from clone

PTRACE_EVENT_VFORK_DONE - stop before return from
vfork/clone+CLONE_VFORK, but after vfork child unblocked this tracee by
exiting or exec'ing.

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

PTRACE_EVENT_EXEC - stop before return from exec.

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.

si_code = (event << 8) | SIGTRAP.

	1.x.x 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).

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).

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).

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

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.

Using PTRACE_O_TRACESYSGOOD option is a recommended method, since it is
reliable and does not incur performance penalty.

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.

If after syscall-enter-stop tracer uses restarting command other than
PTRACE_SYSCALL, syscall-exit-stop is not generated.

PTRACE_GETSIGINFO on syscall-stops returns si_signo = SIGTRAP, si_code
= SIGTRAP or (SIGTRAP | 0x80).



	1.x Informational and restarting ptrace commands.

Most ptrace commands (all except ATTACH, TRACEME, KILL) require tracee
to be in ptrace-stop, otherwise they fail with ESRCH.

When tracee is in ptrace-stop, tracer can read and write data to tracee
using informational commands. They leave tracee in ptrace-stopped state:

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);

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.

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

Another group of commands makes ptrace-stopped tracee run. They have
the form:

	ptrace(PTRACE_cmd, pid, 0, sig);

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).

	1.x 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.

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.

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

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.

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.

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.

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.

	1.x 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:

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?).

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.

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.

PTRACE_O_TRACEEXEC option is the recommended tool for dealing with this
case. It enables PTRACE_EVENT_EXEC stop which occurs before execve
syscall return.

Pid change happens before PTRACE_EVENT_EXEC stop, not after.

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.

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.

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.

Example: two threads execve at the same time:

  ** 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

In this situation there is no way to know which execve succeeded.

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.

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.

	1.x 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.

Many of these bugs have been fixed, but as of 2.6.38 several still

As of 2.6.38, the following is believed to work correctly:

- 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.

	1.x Known bugs

Following bugs still exist:

Group-stop notifications are sent to tracer, but not to real parent.
Last confirmed on

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

??? need to test this scenario

One possible workaround is to detach thread group leader instead of
restarting it in this case. Last confirmed on

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
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