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aio.c12-Mar-20159.7 KiB

Android.mk12-Mar-20156.5 KiB


bjm.c12-Mar-20155.7 KiB

block.c12-Mar-20156.7 KiB



ChangeLog-CVS12-Mar-2015199.3 KiB

configure.ac12-Mar-201517.5 KiB

COPYING12-Mar-20151.7 KiB

count.c12-Mar-20157.2 KiB

CREDITS.in12-Mar-20154.5 KiB

debian/12-Mar-20154 KiB

defs.h12-Mar-201526.5 KiB

desc.c12-Mar-201513.9 KiB

errnoent.sh12-Mar-20151.7 KiB


file.c12-Mar-201553 KiB



git-version-gen12-Mar-20157.8 KiB

gitlog-to-changelog12-Mar-20156.1 KiB

HACKING-scripts12-Mar-20152.8 KiB


INSTALL12-Mar-201512.7 KiB

io.c12-Mar-20159.2 KiB

ioctl.c12-Mar-20154.6 KiB

ioprio.c12-Mar-20151.4 KiB

ipc.c12-Mar-201510.2 KiB

kexec.c12-Mar-20152 KiB

keyctl.c12-Mar-20158 KiB

ldt.c12-Mar-20151.9 KiB

linux/12-Mar-20154 KiB

loop.c12-Mar-20154.9 KiB

m4/12-Mar-20154 KiB


Makefile.am12-Mar-20157.5 KiB

mem.c12-Mar-201513.3 KiB

mtd.c12-Mar-20159.6 KiB

net.c12-Mar-201526 KiB

NEWS12-Mar-201521 KiB

pathtrace.c12-Mar-20158.7 KiB

process.c12-Mar-201567.3 KiB

ptp.c12-Mar-20152.2 KiB

qemu_multiarch_testing/12-Mar-20154 KiB

quota.c12-Mar-201516.2 KiB


README-hacking12-Mar-20151.3 KiB

README-linux12-Mar-20151.3 KiB

README-linux-ptrace12-Mar-201523.8 KiB


resource.c12-Mar-20159.6 KiB

scsi.c12-Mar-20154.2 KiB

signal.c12-Mar-201533.4 KiB

signalent.sh12-Mar-20152 KiB

sock.c12-Mar-20156.9 KiB

strace-graph12-Mar-20158.1 KiB


strace.112-Mar-201521.3 KiB

strace.c12-Mar-201561.7 KiB

strace.spec12-Mar-201517.5 KiB

stream.c12-Mar-20159 KiB

syscall.c12-Mar-201567.6 KiB

syscallent.sh12-Mar-20152.6 KiB

system.c12-Mar-201514.9 KiB

term.c12-Mar-20156.8 KiB

test/12-Mar-20154 KiB

tests/12-Mar-20154 KiB

time.c12-Mar-201518.6 KiB

unwind.c12-Mar-201514.3 KiB

util.c12-Mar-201537.1 KiB

vsprintf.c12-Mar-201518 KiB

xlat/12-Mar-201516 KiB

xlate.el12-Mar-20153 KiB


1This is strace, a system call tracer for Linux.
3strace is released under a Berkeley-style license at the request
4of Paul Kranenburg; see the file COPYING for details.
6See the file CREDITS for a list of authors and other contributors.
8See the file INSTALL for compilation and installation instructions.
10See the file NEWS for information on what has changed in recent
13You can get the latest version of strace from its homepage at
14 .
16Please send bug reports and enhancements to the strace
17mailinglist at


3If you use the GIT version of strace there will be some files missing
4that you need to build strace.  These files are generated by tools from
5the GNU Autoconf and Automake packages.  You need recent versions, which
6provide the `autoreconf -i' command that will do everything you need.
9No more ChangeLog file
11Do not create or modify the ChangeLog files.  Starting at 2009-07-09, the
12policy changed.  Before, we would insert the exact same text (or worse,
13sometimes slightly differing) into both the ChangeLog file and the commit
14log.  Now we put that information only in the commit log, and generate
15the top-level ChangeLog file from logs at "make dist" time.  As such,
16there are strict requirements on the form of the commit log messages.
19Commit log requirements
21Each commit log should always start with a one-line summary, the second
22line should be blank, and the remaining lines are usually ChangeLog-style
23entries for all affected files, except the leading TABs which should
24be omitted.
29All noteworthy changes should be mentioned in NEWS.  All changes in strace
30command interface should be reflected in the man page.  New options should
31be documented both in -h output and in the man page.


2Strace has been ported by Branko Lankester <>
3to run on Linux systems.  Since then it has been greatly modified
4by various other people.
6If you want to compile strace on a Linux system please make sure that
7you use recent kernel headers. Strace needs those to get the proper data
8structures and constatns used by the kernel, since these can be
9different from the structures that the C library uses. Currently you
10will need at least a 2.2.7 or newer kernel.
12To complicate things a bit further strace might not compile if you are
13using development kernels. These tend to have headers that conflict with
14the headers from libc which makes it impossible to use them.
16There are three ways to compile strace with other kernel headers:
17* Specify the location in CFLAGS when running configure
19     CFLAGS=-I/usr/src/linux/include ./configure
21* you can tell make where your kernel sources are. For example if you
22  have your kernelsource in /usr/src/linux, you can invoke make like
23  this:
25     make CFLAGS="\$CFLAGS -I/usr/src/linux/include"
27  (the extra \$CFLAGS is there to make sure we don't override any CFLAGS
28  settings that configure has found).
30* you can link /usr/include/linux and /usr/include/asm to the
31  corresponding directories in your kernel source-tree.


1This document describes Linux ptrace implementation in Linux kernels
2version 3.0.0. (Update this notice if you update the document
3to reflect newer kernels).
6		Ptrace userspace API.
8Ptrace API (ab)uses standard Unix parent/child signaling over waitpid.
9An unfortunate effect of it is that resulting API is complex and has
10subtle quirks. This document aims to describe these quirks.
12Debugged processes (tracees) first need to be attached to the debugging
13process (tracer). Attachment and subsequent commands are per-thread: in
14multi-threaded process, every thread can be individually attached to a
15(potentially different) tracer, or left not attached and thus not
16debugged. Therefore, "tracee" always means "(one) thread", never "a
17(possibly multi-threaded) process". Ptrace commands are always sent to
18a specific tracee using ptrace(PTRACE_foo, pid, ...), where pid is a
19TID of the corresponding Linux thread.
21After attachment, each tracee can be in two states: running or stopped.
23There are many kinds of states when tracee is stopped, and in ptrace
24discussions they are often conflated. Therefore, it is important to use
25precise terms.
27In this document, any stopped state in which tracee is ready to accept
28ptrace commands from the tracer is called ptrace-stop. Ptrace-stops can
29be further subdivided into signal-delivery-stop, group-stop,
30syscall-stop and so on. They are described in detail later.
33	1.x Death under ptrace.
35When a (possibly multi-threaded) process receives a killing signal (a
36signal set to SIG_DFL and whose default action is to kill the process),
37all threads exit. Tracees report their death to the tracer(s). This is
38not a ptrace-stop (because tracer can't query tracee status such as
39register contents, cannot restart tracee etc) but the notification
40about this event is delivered through waitpid API similarly to
43Note that killing signal will first cause signal-delivery-stop (on one
44tracee only), and only after it is injected by tracer (or after it was
45dispatched to a thread which isn't traced), death from signal will
46happen on ALL tracees within multi-threaded process.
48SIGKILL operates similarly, with exceptions. No signal-delivery-stop is
49generated for SIGKILL and therefore tracer can't suppress it. SIGKILL
50kills even within syscalls (syscall-exit-stop is not generated prior to
51death by SIGKILL). The net effect is that SIGKILL always kills the
52process (all its threads), even if some threads of the process are
55Tracer can kill a tracee with ptrace(PTRACE_KILL, pid, 0, 0). This
56operation is deprecated, use kill/tgkill(SIGKILL) instead.
58^^^ Oleg prefers to deprecate it instead of describing (and needing to
59support) PTRACE_KILL's quirks.
61When tracee executes exit syscall, it reports its death to its tracer.
62Other threads are not affected.
64When any thread executes exit_group syscall, every tracee in its thread
65group reports its death to its tracer.
67If PTRACE_O_TRACEEXIT option is on, PTRACE_EVENT_EXIT will happen
68before actual death. This applies to exits on exit syscall, group_exit
69syscall, signal deaths (except SIGKILL), and when threads are torn down
70on execve in multi-threaded process.
72Tracer cannot assume that ptrace-stopped tracee exists. There are many
73scenarios when tracee may die while stopped (such as SIGKILL).
74Therefore, tracer must always be prepared to handle ESRCH error on any
75ptrace operation. Unfortunately, the same error is returned if tracee
76exists but is not ptrace-stopped (for commands which require stopped
77tracee), or if it is not traced by process which issued ptrace call.
78Tracer needs to keep track of stopped/running state, and interpret
79ESRCH as "tracee died unexpectedly" only if it knows that tracee has
80been observed to enter ptrace-stop. Note that there is no guarantee
81that waitpid(WNOHANG) will reliably report tracee's death status if
82ptrace operation returned ESRCH. waitpid(WNOHANG) may return 0 instead.
83IOW: tracee may be "not yet fully dead" but already refusing ptrace ops.
85Tracer can not assume that tracee ALWAYS ends its life by reporting
86WIFEXITED(status) or WIFSIGNALED(status).
88??? or can it? Do we include such a promise into ptrace API?
91	1.x Stopped states.
93When running tracee enters ptrace-stop, it notifies its tracer using
94waitpid API. Tracer should use waitpid family of syscalls to wait for
95tracee to stop. Most of this document assumes that tracer waits with:
97	pid = waitpid(pid_or_minus_1, &status, __WALL);
99Ptrace-stopped tracees are reported as returns with pid > 0 and
100WIFSTOPPED(status) == true.
102??? Do we require __WALL usage, or will just using 0 be ok? Are the
103rules different if user wants to use waitid? Will waitid require
106__WALL value does not include WSTOPPED and WEXITED bits, but implies
107their functionality.
109Setting of WCONTINUED bit in waitpid flags is not recommended: the
110continued state is per-process and consuming it can confuse real parent
111of the tracee.
113Use of WNOHANG bit in waitpid flags may cause waitpid return 0 ("no
114wait results available yet") even if tracer knows there should be a
115notification. Example: kill(tracee, SIGKILL); waitpid(tracee, &status,
118??? waitid usage? WNOWAIT?
120??? describe how wait notifications queue (or not queue)
122The following kinds of ptrace-stops exist: signal-delivery-stops,
123group-stop, PTRACE_EVENT stops, syscall-stops [, SINGLESTEP, SYSEMU,
124SYSEMU_SINGLESTEP]. They all are reported as waitpid result with
125WIFSTOPPED(status) == true. They may be differentiated by checking
126(status >> 8) value, and if looking at (status >> 8) value doesn't
127resolve ambiguity, by querying PTRACE_GETSIGINFO. (Note:
128WSTOPSIG(status) macro returns ((status >> 8) & 0xff) value).
131	1.x.x Signal-delivery-stop
133When (possibly multi-threaded) process receives any signal except
134SIGKILL, kernel selects a thread which handles the signal (if signal is
135generated with t[g]kill, thread selection is done by user). If selected
136thread is traced, it enters signal-delivery-stop. By this point, signal
137is not yet delivered to the process, and can be suppressed by tracer.
138If tracer doesn't suppress the signal, it passes signal to tracee in
139the next ptrace request. This second step of signal delivery is called
140"signal injection" in this document. Note that if signal is blocked,
141signal-delivery-stop doesn't happen until signal is unblocked, with the
142usual exception that SIGSTOP can't be blocked.
144Signal-delivery-stop is observed by tracer as waitpid returning with
145WIFSTOPPED(status) == true, WSTOPSIG(status) == signal. If
146WSTOPSIG(status) == SIGTRAP, this may be a different kind of
147ptrace-stop - see "Syscall-stops" and "execve" sections below for
148details. If WSTOPSIG(status) == stopping signal, this may be a
149group-stop - see below.
152	1.x.x Signal injection and suppression.
154After signal-delivery-stop is observed by tracer, tracer should restart
155tracee with
157	ptrace(PTRACE_rest, pid, 0, sig)
159call, where PTRACE_rest is one of the restarting ptrace ops. If sig is
1600, then signal is not delivered. Otherwise, signal sig is delivered.
161This operation is called "signal injection" in this document, to
162distinguish it from signal-delivery-stop.
164Note that sig value may be different from WSTOPSIG(status) value -
165tracer can cause a different signal to be injected.
167Note that suppressed signal still causes syscalls to return
168prematurely. Kernel should always restart the syscall in this case:
169tracer would observe a new syscall-enter-stop for the same syscall,
170or, in case of syscalls returning ERESTART_RESTARTBLOCK,
171tracer would observe a syscall-enter-stop for restart_syscall(2)
172syscall. There may still be bugs in this area which cause some syscalls
173to instead return with -EINTR even though no observable signal
174was injected to the tracee.
176This is a cause of confusion among ptrace users. One typical scenario
177is that tracer observes group-stop, mistakes it for
178signal-delivery-stop, restarts tracee with ptrace(PTRACE_rest, pid, 0,
179stopsig) with the intention of injecting stopsig, but stopsig gets
180ignored and tracee continues to run.
182SIGCONT signal has a side effect of waking up (all threads of)
183group-stopped process. This side effect happens before
184signal-delivery-stop. Tracer can't suppress this side-effect (it can
185only suppress signal injection, which only causes SIGCONT handler to
186not be executed in the tracee, if such handler is installed). In fact,
187waking up from group-stop may be followed by signal-delivery-stop for
188signal(s) *other than* SIGCONT, if they were pending when SIGCONT was
189delivered. IOW: SIGCONT may be not the first signal observed by the
190tracee after it was sent.
192Stopping signals cause (all threads of) process to enter group-stop.
193This side effect happens after signal injection, and therefore can be
194suppressed by tracer.
196PTRACE_GETSIGINFO can be used to retrieve siginfo_t structure which
197corresponds to delivered signal. PTRACE_SETSIGINFO may be used to
198modify it. If PTRACE_SETSIGINFO has been used to alter siginfo_t,
199si_signo field and sig parameter in restarting command must match,
200otherwise the result is undefined.
203	1.x.x Group-stop
205When a (possibly multi-threaded) process receives a stopping signal,
206all threads stop. If some threads are traced, they enter a group-stop.
207Note that stopping signal will first cause signal-delivery-stop (on one
208tracee only), and only after it is injected by tracer (or after it was
209dispatched to a thread which isn't traced), group-stop will be
210initiated on ALL tracees within multi-threaded process. As usual, every
211tracee reports its group-stop separately to corresponding tracer.
213Group-stop is observed by tracer as waitpid returning with
214WIFSTOPPED(status) == true, WSTOPSIG(status) == signal. The same result
215is returned by some other classes of ptrace-stops, therefore the
216recommended practice is to perform
218	ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)
220call. The call can be avoided if signal number is not SIGSTOP, SIGTSTP,
221SIGTTIN or SIGTTOU - only these four signals are stopping signals. If
222tracer sees something else, it can't be group-stop. Otherwise, tracer
223needs to call PTRACE_GETSIGINFO. If PTRACE_GETSIGINFO fails with
224EINVAL, then it is definitely a group-stop. (Other failure codes are
225possible, such as ESRCH "no such process" if SIGKILL killed the tracee).
227As of kernel 2.6.38, after tracer sees tracee ptrace-stop and until it
228restarts or kills it, tracee will not run, and will not send
229notifications (except SIGKILL death) to tracer, even if tracer enters
230into another waitpid call.
232Currently, it causes a problem with transparent handling of stopping
233signals: if tracer restarts tracee after group-stop, SIGSTOP is
234effectively ignored: tracee doesn't remain stopped, it runs. If tracer
235doesn't restart tracee before entering into next waitpid, future
236SIGCONT will not be reported to the tracer. Which would make SIGCONT to
237have no effect.
240	1.x.x PTRACE_EVENT stops
242If tracer sets TRACE_O_TRACEfoo options, tracee will enter ptrace-stops
243called PTRACE_EVENT stops.
245PTRACE_EVENT stops are observed by tracer as waitpid returning with
246WIFSTOPPED(status) == true, WSTOPSIG(status) == SIGTRAP. Additional bit
247is set in a higher byte of status word: value ((status >> 8) & 0xffff)
248will be (SIGTRAP | PTRACE_EVENT_foo << 8). The following events exist:
250PTRACE_EVENT_VFORK - stop before return from vfork/clone+CLONE_VFORK.
251When tracee is continued after this, it will wait for child to
252exit/exec before continuing its execution (IOW: usual behavior on
255PTRACE_EVENT_FORK - stop before return from fork/clone+SIGCHLD
257PTRACE_EVENT_CLONE - stop before return from clone
259PTRACE_EVENT_VFORK_DONE - stop before return from
260vfork/clone+CLONE_VFORK, but after vfork child unblocked this tracee by
261exiting or exec'ing.
263For all four stops described above: stop occurs in parent, not in newly
264created thread. PTRACE_GETEVENTMSG can be used to retrieve new thread's
267PTRACE_EVENT_EXEC - stop before return from exec.
269PTRACE_EVENT_EXIT - stop before exit (including death from exit_group),
270signal death, or exit caused by execve in multi-threaded process.
271PTRACE_GETEVENTMSG returns exit status. Registers can be examined
272(unlike when "real" exit happens). The tracee is still alive, it needs
273to be PTRACE_CONTed or PTRACE_DETACHed to finish exit.
275PTRACE_GETSIGINFO on PTRACE_EVENT stops returns si_signo = SIGTRAP,
276si_code = (event << 8) | SIGTRAP.
279	1.x.x Syscall-stops
281If tracee was restarted by PTRACE_SYSCALL, tracee enters
282syscall-enter-stop just prior to entering any syscall. If tracer
283restarts it with PTRACE_SYSCALL, tracee enters syscall-exit-stop when
284syscall is finished, or if it is interrupted by a signal. (That is,
285signal-delivery-stop never happens between syscall-enter-stop and
286syscall-exit-stop, it happens *after* syscall-exit-stop).
288Other possibilities are that tracee may stop in a PTRACE_EVENT stop,
289exit (if it entered exit or exit_group syscall), be killed by SIGKILL,
290or die silently (if execve syscall happened in another thread).
292Syscall-enter-stop and syscall-exit-stop are observed by tracer as
293waitpid returning with WIFSTOPPED(status) == true, WSTOPSIG(status) ==
294SIGTRAP. If PTRACE_O_TRACESYSGOOD option was set by tracer, then
295WSTOPSIG(status) == (SIGTRAP | 0x80).
297Syscall-stops can be distinguished from signal-delivery-stop with
298SIGTRAP by querying PTRACE_GETSIGINFO: si_code <= 0 if sent by usual
299suspects like [tg]kill/sigqueue/etc; or = SI_KERNEL (0x80) if sent by
300kernel, whereas syscall-stops have si_code = SIGTRAP or (SIGTRAP |
3010x80). However, syscall-stops happen very often (twice per syscall),
302and performing PTRACE_GETSIGINFO for every syscall-stop may be somewhat
305Some architectures allow to distinguish them by examining registers.
306For example, on x86 rax = -ENOSYS in syscall-enter-stop. Since SIGTRAP
307(like any other signal) always happens *after* syscall-exit-stop, and
308at this point rax almost never contains -ENOSYS, SIGTRAP looks like
309"syscall-stop which is not syscall-enter-stop", IOW: it looks like a
310"stray syscall-exit-stop" and can be detected this way. But such
311detection is fragile and is best avoided.
313Using PTRACE_O_TRACESYSGOOD option is a recommended method, since it is
314reliable and does not incur performance penalty.
316Syscall-enter-stop and syscall-exit-stop are indistinguishable from
317each other by tracer. Tracer needs to keep track of the sequence of
318ptrace-stops in order to not misinterpret syscall-enter-stop as
319syscall-exit-stop or vice versa. The rule is that syscall-enter-stop is
320always followed by syscall-exit-stop, PTRACE_EVENT stop or tracee's
321death - no other kinds of ptrace-stop can occur in between.
323If after syscall-enter-stop tracer uses restarting command other than
324PTRACE_SYSCALL, syscall-exit-stop is not generated.
326PTRACE_GETSIGINFO on syscall-stops returns si_signo = SIGTRAP, si_code
327= SIGTRAP or (SIGTRAP | 0x80).
335	1.x Informational and restarting ptrace commands.
337Most ptrace commands (all except ATTACH, TRACEME, KILL) require tracee
338to be in ptrace-stop, otherwise they fail with ESRCH.
340When tracee is in ptrace-stop, tracer can read and write data to tracee
341using informational commands. They leave tracee in ptrace-stopped state:
343longv = ptrace(PTRACE_PEEKTEXT/PEEKDATA/PEEKUSER, pid, addr, 0);
344	ptrace(PTRACE_POKETEXT/POKEDATA/POKEUSER, pid, addr, long_val);
345	ptrace(PTRACE_GETREGS/GETFPREGS, pid, 0, &struct);
346	ptrace(PTRACE_SETREGS/SETFPREGS, pid, 0, &struct);
347	ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo);
348	ptrace(PTRACE_SETSIGINFO, pid, 0, &siginfo);
349	ptrace(PTRACE_GETEVENTMSG, pid, 0, &long_var);
350	ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);
352Note that some errors are not reported. For example, setting siginfo
353may have no effect in some ptrace-stops, yet the call may succeed
354(return 0 and don't set errno).
356ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags) affects one tracee.
357Current flags are replaced. Flags are inherited by new tracees created
358and "auto-attached" via active PTRACE_O_TRACE[V]FORK or
361Another group of commands makes ptrace-stopped tracee run. They have
362the form:
364	ptrace(PTRACE_cmd, pid, 0, sig);
367SYSEMU_SINGLESTEP. If tracee is in signal-delivery-stop, sig is the
368signal to be injected. Otherwise, sig may be ignored.
371	1.x Attaching and detaching
373A thread can be attached to tracer using ptrace(PTRACE_ATTACH, pid, 0,
3740) call. This also sends SIGSTOP to this thread. If tracer wants this
375SIGSTOP to have no effect, it needs to suppress it. Note that if other
376signals are concurrently sent to this thread during attach, tracer may
377see tracee enter signal-delivery-stop with other signal(s) first! The
378usual practice is to reinject these signals until SIGSTOP is seen, then
379suppress SIGSTOP injection. The design bug here is that attach and
380concurrent SIGSTOP are racing and SIGSTOP may be lost.
382??? Describe how to attach to a thread which is already group-stopped.
384Since attaching sends SIGSTOP and tracer usually suppresses it, this
385may cause stray EINTR return from the currently executing syscall in
386the tracee, as described in "signal injection and suppression" section.
388ptrace(PTRACE_TRACEME, 0, 0, 0) request turns current thread into a
389tracee. It continues to run (doesn't enter ptrace-stop). A common
390practice is to follow ptrace(PTRACE_TRACEME) with raise(SIGSTOP) and
391allow parent (which is our tracer now) to observe our
394If PTRACE_O_TRACE[V]FORK or PTRACE_O_TRACECLONE options are in effect,
395then children created by (vfork or clone(CLONE_VFORK)), (fork or
396clone(SIGCHLD)) and (other kinds of clone) respectively are
397automatically attached to the same tracer which traced their parent.
398SIGSTOP is delivered to them, causing them to enter
399signal-delivery-stop after they exit syscall which created them.
401Detaching of tracee is performed by ptrace(PTRACE_DETACH, pid, 0, sig).
402PTRACE_DETACH is a restarting operation, therefore it requires tracee
403to be in ptrace-stop. If tracee is in signal-delivery-stop, signal can
404be injected. Othervice, sig parameter may be silently ignored.
406If tracee is running when tracer wants to detach it, the usual solution
407is to send SIGSTOP (using tgkill, to make sure it goes to the correct
408thread), wait for tracee to stop in signal-delivery-stop for SIGSTOP
409and then detach it (suppressing SIGSTOP injection). Design bug is that
410this can race with concurrent SIGSTOPs. Another complication is that
411tracee may enter other ptrace-stops and needs to be restarted and
412waited for again, until SIGSTOP is seen. Yet another complication is to
413be sure that tracee is not already ptrace-stopped, because no signal
414delivery happens while it is - not even SIGSTOP.
416??? Describe how to detach from a group-stopped tracee so that it
417    doesn't run, but continues to wait for SIGCONT.
419If tracer dies, all tracees are automatically detached and restarted,
420unless they were in group-stop. Handling of restart from group-stop is
421currently buggy, but "as planned" behavior is to leave tracee stopped
422and waiting for SIGCONT. If tracee is restarted from
423signal-delivery-stop, pending signal is injected.
426	1.x execve under ptrace.
428During execve, kernel destroys all other threads in the process, and
429resets execve'ing thread tid to tgid (process id). This looks very
430confusing to tracers:
432All other threads stop in PTRACE_EXIT stop, if requested by active
433ptrace option. Then all other threads except thread group leader report
434death as if they exited via exit syscall with exit code 0. Then
435PTRACE_EVENT_EXEC stop happens, if requested by active ptrace option
436(on which tracee - leader? execve-ing one?).
438The execve-ing tracee changes its pid while it is in execve syscall.
439(Remember, under ptrace 'pid' returned from waitpid, or fed into ptrace
440calls, is tracee's tid). That is, pid is reset to process id, which
441coincides with thread group leader tid.
443If thread group leader has reported its death by this time, for tracer
444this looks like dead thread leader "reappears from nowhere". If thread
445group leader was still alive, for tracer this may look as if thread
446group leader returns from a different syscall than it entered, or even
447"returned from syscall even though it was not in any syscall". If
448thread group leader was not traced (or was traced by a different
449tracer), during execve it will appear as if it has become a tracee of
450the tracer of execve'ing tracee. All these effects are the artifacts of
451pid change.
453PTRACE_O_TRACEEXEC option is the recommended tool for dealing with this
454case. It enables PTRACE_EVENT_EXEC stop which occurs before execve
455syscall return.
457Pid change happens before PTRACE_EVENT_EXEC stop, not after.
459When tracer receives PTRACE_EVENT_EXEC stop notification, it is
460guaranteed that except this tracee and thread group leader, no other
461threads from the process are alive.
463On receiving this notification, tracer should clean up all its internal
464data structures about all threads of this process, and retain only one
465data structure, one which describes single still running tracee, with
466pid = tgid = process id.
468Currently, there is no way to retrieve former pid of execve-ing tracee.
469If tracer doesn't keep track of its tracees' thread group relations, it
470may be unable to know which tracee  execve-ed and therefore no longer
471exists under old pid due to pid change.
473Example: two threads execve at the same time:
475  ** we get syscall-entry-stop in thread 1: **
476 PID1 execve("/bin/foo", "foo" <unfinished ...>
477  ** we issue PTRACE_SYSCALL for thread 1 **
478  ** we get syscall-entry-stop in thread 2: **
479 PID2 execve("/bin/bar", "bar" <unfinished ...>
480  ** we issue PTRACE_SYSCALL for thread 2 **
481  ** we get PTRACE_EVENT_EXEC for PID0, we issue PTRACE_SYSCALL **
482  ** we get syscall-exit-stop for PID0: **
483 PID0 <... execve resumed> )             = 0
485In this situation there is no way to know which execve succeeded.
487If PTRACE_O_TRACEEXEC option is NOT in effect for the execve'ing
488tracee, kernel delivers an extra SIGTRAP to tracee after execve syscall
489returns. This is an ordinary signal (similar to one which can be
490generated by "kill -TRAP"), not a special kind of ptrace-stop.
491GETSIGINFO on it has si_code = 0 (SI_USER). It can be blocked by signal
492mask, and thus can happen (much) later.
494Usually, tracer (for example, strace) would not want to show this extra
495post-execve SIGTRAP signal to the user, and would suppress its delivery
496to the tracee (if SIGTRAP is set to SIG_DFL, it is a killing signal).
497However, determining *which* SIGTRAP to suppress is not easy. Setting
498PTRACE_O_TRACEEXEC option and thus suppressing this extra SIGTRAP is
499the recommended approach.
502	1.x Real parent
504Ptrace API (ab)uses standard Unix parent/child signaling over waitpid.
505This used to cause real parent of the process to stop receiving several
506kinds of waitpid notifications when child process is traced by some
507other process.
509Many of these bugs have been fixed, but as of 2.6.38 several still
512As of 2.6.38, the following is believed to work correctly:
514- exit/death by signal is reported first to tracer, then, when tracer
515consumes waitpid result, to real parent (to real parent only when the
516whole multi-threaded process exits). If they are the same process, the
517report is sent only once.
520	1.x Known bugs
522Following bugs still exist:
524Group-stop notifications are sent to tracer, but not to real parent.
525Last confirmed on
527If thread group leader is traced and exits by calling exit syscall,
528PTRACE_EVENT_EXIT stop will happen for it (if requested), but subsequent
529WIFEXITED notification will not be delivered until all other threads
530exit. As explained above, if one of other threads execve's, thread
531group leader death will *never* be reported. If execve-ed thread is not
532traced by this tracer, tracer will never know that execve happened.
534??? need to test this scenario
536One possible workaround is to detach thread group leader instead of
537restarting it in this case. Last confirmed on
539SIGKILL signal may still cause PTRACE_EVENT_EXIT stop before actual
540signal death. This may be changed in the future - SIGKILL is meant to
541always immediately kill tasks even under ptrace. Last confirmed on