ptrace

When a (possibly multithreaded) process receives a killing signal (one whose disposition is set to SIG_DFL and whose default action is to kill the process), all threads exit. Tracees report their death to their tracer(s). Notification of this event is delivered via waitpid(2).

Note that the killing signal will first cause signal-delivery-stop (on one tracee only), and only after it is injected by the tracer (or after it was dispatched to a thread which isn't traced), will death from the signal happen on all tracees within a multithreaded process. (The term "signal-delivery-stop" is explained below.)

SIGKILL does not generate signal-delivery-stop and therefore the tracer can't suppress it. SIGKILL kills even within system calls (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.

When the tracee calls _exit(2), it reports its death to its tracer. Other threads are not affected.

When any thread executes exit_group(2), every tracee in its thread group reports its death to its tracer.

If the PTRACE_O_TRACEEXIT option is on, PTRACE_EVENT_EXIT will happen before actual death. This applies to exits via exit(2), exit_group(2), and signal deaths (except SIGKILL, depending on the kernel version; see Bugs below), and when threads are torn down on execve(2) in a multithreaded process.

The tracer cannot assume that the ptrace-stopped tracee exists. There are many scenarios when the tracee may die while stopped (such as SIGKILL). Therefore, the tracer must be prepared to handle an ESRCH error on any ptrace operation. Unfortunately, the same error is returned if the tracee exists but is not ptrace-stopped (for commands which require a stopped tracee), or if it is not traced by the process which issued the ptrace call. The tracer needs to keep track of the stopped/running state of the tracee, and interpret ESRCH as "tracee died unexpectedly" only if it knows that the tracee has been observed to enter ptrace-stop. Note that there is no guarantee that waitpid(WNOHANG) will reliably report the tracee's death status if a ptrace operation returned ESRCH. waitpid(WNOHANG) may return 0 instead. In other words, the tracee may be "not yet fully dead", but already refusing ptrace requests.

The tracer can't assume that the tracee always ends its life by reporting WIFEXITED(status) or WIFSIGNALED(status); there are cases where this does not occur. For example, if a thread other than thread group leader does an execve(2), it disappears; its PID will never be seen again, and any subsequent ptrace stops will be reported under the thread group leader's PID.

A tracee can be in two states: running or stopped. For the purposes of ptrace, a tracee which is blocked in a system call (such as read(2), pause(2), etc.) is nevertheless considered to be running, even if the tracee is blocked for a long time. The state of the tracee after PTRACE_LISTEN is somewhat of a gray area: it is not in any ptrace-stop (ptrace commands won't work on it, and it will deliver waitpid(2) notifications), but it also may be considered "stopped" because it is not executing instructions (is not scheduled), and if it was in group-stop before PTRACE_LISTEN, it will not respond to signals until SIGCONT is received.

There are many kinds of states when the tracee is stopped, and in ptrace discussions they are often conflated. Therefore, it is important to use precise terms.

In this manual page, any stopped state in which the 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, PTRACE_EVENT stops, and so on. These stopped states are described in detail below.

When the running tracee enters ptrace-stop, it notifies its tracer using waitpid(2) (or one of the other "wait" system calls). Most of this manual page assumes that the tracer waits with:

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

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

The __WALL flag does not include the WSTOPPED and WEXITED flags, but implies their functionality.

Setting the WCONTINUED flag when calling waitpid(2) is not recommended: the "continued" state is per-process and consuming it can confuse the real parent of the tracee.

Use of the WNOHANG flag may cause waitpid(2) to return 0 ("no wait results available yet") even if the tracer knows there should be a notification. Example:

errno = 0;
ptrace(PTRACE_CONT, pid, 0L, 0L);
if (errno == ESRCH) {
    /* tracee is dead */
    r = waitpid(tracee, &status, __WALL | WNOHANG);
    /* r can still be 0 here! */
}

The following kinds of ptrace-stops exist: signal-delivery-stops, group-stops, PTRACE_EVENT stops, syscall-stops. They all are reported by waitpid(2) with WIFSTOPPED(status) true. They may be differentiated by examining the value status>>8, and if there is ambiguity in that value, by querying PTRACE_GETSIGINFO. (Note: the WSTOPSIG(status) macro can't be used to perform this examination, because it returns the value (status>>8) & 0xff.)

When a (possibly multithreaded) process receives any signal except SIGKILL, the kernel selects an arbitrary thread which handles the signal. (If the signal is generated with tgkill(2), the target thread can be explicitly selected by the caller.) If the selected thread is traced, it enters signal-delivery-stop. At this point, the signal is not yet delivered to the process, and can be suppressed by the tracer. If the tracer doesn't suppress the signal, it passes the signal to the tracee in the next ptrace restart request. This second step of signal delivery is called signal injection in this manual page. Note that if the signal is blocked, signal-delivery-stop doesn't happen until the signal is unblocked, with the usual exception that SIGSTOP can't be blocked.

Signal-delivery-stop is observed by the tracer as waitpid(2) returning with WIFSTOPPED(status) true, with the signal returned by WSTOPSIG(status). If the signal is SIGTRAP, this may be a different kind of ptrace-stop; see the "Syscall-stops" and "execve" sections below for details. If WSTOPSIG(status) returns a stopping signal, this may be a group-stop; see below.

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

   ptrace(PTRACE_restart, pid, 0, sig)

where PTRACE_restart is one of the restarting ptrace requests. If sig is 0, then a signal is not delivered. Otherwise, the signal sig is delivered. This operation is called signal injection in this manual page, to distinguish it from signal-delivery-stop.

The sig value may be different from the WSTOPSIG(status) value: the tracer can cause a different signal to be injected.

Note that a suppressed signal still causes system calls to return prematurely. In this case, system calls will be restarted: the tracer will observe the tracee to reexecute the interrupted system call (or restart_syscall(2) system call for a few system calls which use a different mechanism for restarting) if the tracer uses PTRACE_SYSCALL. Even system calls (such as poll(2)) which are not restartable after signal are restarted after signal is suppressed; however, kernel bugs exist which cause some system calls to fail with EINTR even though no observable signal is injected to the tracee.

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; a nonzero sig may simply be ignored. Ptrace users should not try to "create a new signal" this way: use tgkill(2) instead.

The fact that signal injection requests may be ignored when restarting the tracee after ptrace stops that are not signal-delivery-stops is a cause of confusion among ptrace users. One typical scenario is that the tracer observes group-stop, mistakes it for signal-delivery-stop, restarts the tracee with

   ptrace(PTRACE_restart, pid, 0, stopsig)

with the intention of injecting stopsig, but stopsig gets ignored and the tracee continues to run.

The SIGCONT signal has a side effect of waking up (all threads of) a group-stopped process. This side effect happens before signal-delivery-stop. The tracer can't suppress this side effect (it can only suppress signal injection, which only causes the SIGCONT handler to not be executed in the tracee, if such a 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. In other words, SIGCONT may be not the first signal observed by the tracee after it was sent.

Stopping signals cause (all threads of) a process to enter group-stop. This side effect happens after signal injection, and therefore can be suppressed by the tracer.

In Linux 2.4 and earlier, the SIGSTOP signal can't be injected.

PTRACE_GETSIGINFO can be used to retrieve a siginfo_t structure which corresponds to the delivered signal. PTRACE_SETSIGINFO may be used to modify it. If PTRACE_SETSIGINFO has been used to alter siginfo_t, the si_signo field and the sig parameter in the restarting command must match, otherwise the result is undefined.

When a (possibly multithreaded) process receives a stopping signal, all threads stop. If some threads are traced, they enter a group-stop. Note that the stopping signal will first cause signal-delivery-stop (on one tracee only), and only after it is injected by the tracer (or after it was dispatched to a thread which isn't traced), will group-stop be initiated on all tracees within the multithreaded process. As usual, every tracee reports its group-stop separately to the corresponding tracer.

Group-stop is observed by the tracer as waitpid(2) returning with WIFSTOPPED(status) true, with the stopping signal available via WSTOPSIG(status). The same result is returned by some other classes of ptrace-stops, therefore the recommended practice is to perform the call

   ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

The call can be avoided if the signal is not SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU; only these four signals are stopping signals. If the tracer sees something else, it can't be a group-stop. Otherwise, the 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 a SIGKILL killed the tracee.)

If tracee was attached using PTRACE_SEIZE, group-stop is indicated by PTRACE_EVENT_STOP: status>>16 == PTRACE_EVENT_STOP. This allows detection of group-stops without requiring an extra PTRACE_GETSIGINFO call.

As of Linux 2.6.38, after the tracer sees the tracee ptrace-stop and until it restarts or kills it, the tracee will not run, and will not send notifications (except SIGKILL death) to the tracer, even if the tracer enters into another waitpid(2) call.

The kernel behavior described in the previous paragraph causes a problem with transparent handling of stopping signals. If the tracer restarts the tracee after group-stop, the stopping signal is effectively ignored—the tracee doesn't remain stopped, it runs. If the tracer doesn't restart the tracee before entering into the next waitpid(2), future SIGCONT signals will not be reported to the tracer; this would cause the SIGCONT signals to have no effect on the tracee.

Since Linux 3.4, there is a method to overcome this problem: instead of PTRACE_CONT, a PTRACE_LISTEN command can be used to restart a tracee in a way where it does not execute, but waits for a new event which it can report via waitpid(2) (such as when it is restarted by a SIGCONT).

If the tracer sets PTRACE_O_TRACE_* options, the tracee will enter ptrace-stops called PTRACE_EVENT stops.

PTRACE_EVENT stops are observed by the tracer as waitpid(2) returning with WIFSTOPPED(status), and WSTOPSIG(status) returns SIGTRAP. An additional bit is set in the higher byte of the status word: the value status>>8 will be

   (SIGTRAP | PTRACE_EVENT_foo << 8).

The following events exist:

PTRACE_EVENT_VFORK

Stop before return from vfork(2) or clone(2) with the CLONE_VFORK flag. When the tracee is continued after this stop, it will wait for child to exit/exec before continuing its execution (in other words, the usual behavior on vfork(2)).

PTRACE_EVENT_FORK

Stop before return from fork(2) or clone(2) with the exit signal set to SIGCHLD.

PTRACE_EVENT_CLONE

Stop before return from clone(2).

PTRACE_EVENT_VFORK_DONE

Stop before return from vfork(2) or clone(2) with the CLONE_VFORK flag, but after the child unblocked this tracee by exiting or execing.

For all four stops described above, the stop occurs in the parent (i.e., the tracee), not in the newly created thread. PTRACE_GETEVENTMSG can be used to retrieve the new thread's ID.

PTRACE_EVENT_EXEC

Stop before return from execve(2). Since Linux 3.0, PTRACE_GETEVENTMSG returns the former thread ID.

PTRACE_EVENT_EXIT

Stop before exit (including death from exit_group(2)), signal death, or exit caused by execve(2) in a multithreaded process. PTRACE_GETEVENTMSG returns the 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 exiting.

PTRACE_EVENT_STOP

Stop induced by PTRACE_INTERRUPT command, or group-stop, or initial ptrace-stop when a new child is attached (only if attached using PTRACE_SEIZE).

PTRACE_EVENT_SECCOMP

Stop triggered by a seccomp(2) rule on tracee syscall entry when PTRACE_O_TRACESECCOMP has been set by the tracer. The seccomp event message data (from the SECCOMP_RET_DATA portion of the seccomp filter rule) can be retrieved with PTRACE_GETEVENTMSG. The semantics of this stop are described in detail in a separate section below.

PTRACE_GETSIGINFO on PTRACE_EVENT stops returns SIGTRAP in si_signo, with si_code set to (event<<8) | SIGTRAP.

If the tracee was restarted by PTRACE_SYSCALL or PTRACE_SYSEMU, the tracee enters syscall-enter-stop just prior to entering any system call (which will not be executed if the restart was using PTRACE_SYSEMU, regardless of any change made to registers at this point or how the tracee is restarted after this stop). No matter which method caused the syscall-entry-stop, if the tracer restarts the tracee with PTRACE_SYSCALL, the tracee enters syscall-exit-stop when the system call 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.). If the tracee is continued using any other method (including PTRACE_SYSEMU), no syscall-exit-stop occurs. Note that all mentions PTRACE_SYSEMU apply equally to PTRACE_SYSEMU_SINGLESTEP.

However, even if the tracee was continued using PTRACE_SYSCALL, it is not guaranteed that the next stop will be a syscall-exit-stop. Other possibilities are that the tracee may stop in a PTRACE_EVENT stop (including seccomp stops), exit (if it entered _exit(2) or exit_group(2)), be killed by SIGKILL, or die silently (if it is a thread group leader, the execve(2) happened in another thread, and that thread is not traced by the same tracer; this situation is discussed later).

Syscall-enter-stop and syscall-exit-stop are observed by the tracer as waitpid(2) returning with WIFSTOPPED(status) true, and WSTOPSIG(status) giving SIGTRAP. If the PTRACE_O_TRACESYSGOOD option was set by the tracer, then WSTOPSIG(status) will give the value (SIGTRAP | 0x80).

Syscall-stops can be distinguished from signal-delivery-stop with SIGTRAP by querying PTRACE_GETSIGINFO for the following cases:

si_code <= 0

SIGTRAP was delivered as a result of a user-space action, for example, a system call (tgkill(2), kill(2), sigqueue(3), etc.), expiration of a POSIX timer, change of state on a POSIX message queue, or completion of an asynchronous I/O request.

si_code == SI_KERNEL (0x80)

SIGTRAP was sent by the kernel.

si_code == SIGTRAP or si_code == (SIGTRAP|0x80)

This is a syscall-stop.

However, syscall-stops happen very often (twice per system call), and performing PTRACE_GETSIGINFO for every syscall-stop may be somewhat expensive.

Some architectures allow the cases to be distinguished 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, the SIGTRAP looks like "syscall-stop which is not syscall-enter-stop"; in other words, it looks like a "stray syscall-exit-stop" and can be detected this way. But such detection is fragile and is best avoided.

Using the PTRACE_O_TRACESYSGOOD option is the recommended method to distinguish syscall-stops from other kinds of ptrace-stops, since it is reliable and does not incur a performance penalty.

Syscall-enter-stop and syscall-exit-stop are indistinguishable from each other by the tracer. The 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. In general, a syscall-enter-stop is always followed by syscall-exit-stop, PTRACE_EVENT stop, or the tracee's death; no other kinds of ptrace-stop can occur in between. However, note that seccomp stops (see below) can cause syscall-exit-stops, without preceding syscall-entry-stops. If seccomp is in use, care needs to be taken not to misinterpret such stops as syscall-entry-stops.

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

PTRACE_GETSIGINFO on syscall-stops returns SIGTRAP in si_signo, with si_code set to SIGTRAP or (SIGTRAP|0x80).

The behavior of PTRACE_EVENT_SECCOMP stops and their interaction with other kinds of ptrace stops has changed between kernel versions. This documents the behavior from their introduction until Linux 4.7 (inclusive). The behavior in later kernel versions is documented in the next section.

A PTRACE_EVENT_SECCOMP stop occurs whenever a SECCOMP_RET_TRACE rule is triggered. This is independent of which methods was used to restart the system call. Notably, seccomp still runs even if the tracee was restarted using PTRACE_SYSEMU and this system call is unconditionally skipped.

Restarts from this stop will behave as if the stop had occurred right before the system call in question. In particular, both PTRACE_SYSCALL and PTRACE_SYSEMU will normally cause a subsequent syscall-entry-stop. However, if after the PTRACE_EVENT_SECCOMP the system call number is negative, both the syscall-entry-stop and the system call itself will be skipped. This means that if the system call number is negative after a PTRACE_EVENT_SECCOMP and the tracee is restarted using PTRACE_SYSCALL, the next observed stop will be a syscall-exit-stop, rather than the syscall-entry-stop that might have been expected.

Starting with Linux 4.8, the PTRACE_EVENT_SECCOMP stop was reordered to occur between syscall-entry-stop and syscall-exit-stop. Note that seccomp no longer runs (and no PTRACE_EVENT_SECCOMP will be reported) if the system call is skipped due to PTRACE_SYSEMU.

Functionally, a PTRACE_EVENT_SECCOMP stop functions comparably to a syscall-entry-stop (i.e., continuations using PTRACE_SYSCALL will cause syscall-exit-stops, the system call number may be changed and any other modified registers are visible to the to-be-executed system call as well). Note that there may be, but need not have been a preceding syscall-entry-stop.

After a PTRACE_EVENT_SECCOMP stop, seccomp will be rerun, with a SECCOMP_RET_TRACE rule now functioning the same as a SECCOMP_RET_ALLOW. Specifically, this means that if registers are not modified during the PTRACE_EVENT_SECCOMP stop, the system call will then be allowed.

[Details of these kinds of stops are yet to be documented.]

Most ptrace commands (all except PTRACE_ATTACH, PTRACE_SEIZE, PTRACE_TRACEME, PTRACE_INTERRUPT, and PTRACE_KILL) require the tracee to be in a ptrace-stop, otherwise they fail with ESRCH.

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

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_GETREGSET, pid, NT_foo, &iov);
ptrace(PTRACE_SETREGSET, pid, NT_foo, &iov);
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 signal information (siginfo) may have no effect in some ptrace-stops, yet the call may succeed (return 0 and not set errno); querying PTRACE_GETEVENTMSG may succeed and return some random value if current ptrace-stop is not documented as returning a meaningful event message.

The call

   ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

affects one tracee. The tracee's current flags are replaced. Flags are inherited by new tracees created and "auto-attached" via active PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, or PTRACE_O_TRACECLONE options.

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

   ptrace(cmd, pid, 0, sig);

where cmd is PTRACE_CONT, PTRACE_LISTEN, PTRACE_DETACH, PTRACE_SYSCALL, PTRACE_SINGLESTEP, PTRACE_SYSEMU, or PTRACE_SYSEMU_SINGLESTEP. If the tracee is in signal-delivery-stop, sig is the signal to be injected (if it is nonzero). Otherwise, sig may be ignored. (When restarting a tracee from a ptrace-stop other than signal-delivery-stop, recommended practice is to always pass 0 in sig.)

A thread can be attached to the tracer using the call

   ptrace(PTRACE_ATTACH, pid, 0, 0);

or

   ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);

PTRACE_ATTACH sends SIGSTOP to this thread. If the 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, the tracer may see the 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 a ptrace attach and a concurrently delivered SIGSTOP may race and the concurrent SIGSTOP may be lost.

Since attaching sends SIGSTOP and the tracer usually suppresses it, this may cause a stray EINTR return from the currently executing system call in the tracee, as described in the "Signal injection and suppression" section.

Since Linux 3.4, PTRACE_SEIZE can be used instead of PTRACE_ATTACH. PTRACE_SEIZE does not stop the attached process. If you need to stop it after attach (or at any other time) without sending it any signals, use PTRACE_INTERRUPT command.

The request

   ptrace(PTRACE_TRACEME, 0, 0, 0);

turns the calling thread into a tracee. The thread continues to run (doesn't enter ptrace-stop). A common practice is to follow the PTRACE_TRACEME with

   raise(SIGSTOP);

and allow the parent (which is our tracer now) to observe our signal-delivery-stop.

If the PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, or PTRACE_O_TRACECLONE options are in effect, then children created by, respectively, vfork(2) or clone(2) with the CLONE_VFORK flag, fork(2) or clone(2) with the exit signal set to SIGCHLD, and other kinds of clone(2), are automatically attached to the same tracer which traced their parent. SIGSTOP is delivered to the children, causing them to enter signal-delivery-stop after they exit the system call which created them.

Detaching of the tracee is performed by:

   ptrace(PTRACE_DETACH, pid, 0, sig);

PTRACE_DETACH is a restarting operation; therefore it requires the tracee to be in ptrace-stop. If the tracee is in signal-delivery-stop, a signal can be injected. Otherwise, the sig parameter may be silently ignored.

If the tracee is running when the tracer wants to detach it, the usual solution is to send SIGSTOP (using tgkill(2), to make sure it goes to the correct thread), wait for the tracee to stop in signal-delivery-stop for SIGSTOP and then detach it (suppressing SIGSTOP injection). A design bug is that this can race with concurrent SIGSTOPs. Another complication is that the 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 the tracee is not already ptrace-stopped, because no signal delivery happens while it is—not even SIGSTOP.

If the 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 the "as planned" behavior is to leave tracee stopped and waiting for SIGCONT. If the tracee is restarted from signal-delivery-stop, the pending signal is injected.

When one thread in a multithreaded process calls execve(2), the kernel destroys all other threads in the process, and resets the thread ID of the execing thread to the thread group ID (process ID). (Or, to put things another way, when a multithreaded process does an execve(2), at completion of the call, it appears as though the execve(2) occurred in the thread group leader, regardless of which thread did the execve(2).) This resetting of the thread ID looks very confusing to tracers:

• All other threads stop in PTRACE_EVENT_EXIT stop, if the PTRACE_O_TRACEEXIT option was turned on. Then all other threads except the thread group leader report death as if they exited via _exit(2) with exit code 0.
• The execing tracee changes its thread ID while it is in the execve(2). (Remember, under ptrace, the "pid" returned from waitpid(2), or fed into ptrace calls, is the tracee's thread ID.) That is, the tracee's thread ID is reset to be the same as its process ID, which is the same as the thread group leader's thread ID.
• Then a PTRACE_EVENT_EXEC stop happens, if the PTRACE_O_TRACEEXEC option was turned on.
• If the thread group leader has reported its PTRACE_EVENT_EXIT stop by this time, it appears to the tracer that the dead thread leader "reappears from nowhere". (Note: the thread group leader does not report death via WIFEXITED(status) until there is at least one other live thread. This eliminates the possibility that the tracer will see it dying and then reappearing.) If the thread group leader was still alive, for the tracer this may look as if thread group leader returns from a different system call than it entered, or even "returned from a system call even though it was not in any system call". If the thread group leader was not traced (or was traced by a different tracer), then during execve(2) it will appear as if it has become a tracee of the tracer of the execing tracee.

All of the above effects are the artifacts of the thread ID change in the tracee.

The PTRACE_O_TRACEEXEC option is the recommended tool for dealing with this situation. First, it enables PTRACE_EVENT_EXEC stop, which occurs before execve(2) returns. In this stop, the tracer can use PTRACE_GETEVENTMSG to retrieve the tracee's former thread ID. (This feature was introduced in Linux 3.0.) Second, the PTRACE_O_TRACEEXEC option disables legacy SIGTRAP generation on execve(2).

When the tracer receives PTRACE_EVENT_EXEC stop notification, it is guaranteed that except this tracee and the thread group leader, no other threads from the process are alive.

On receiving the PTRACE_EVENT_EXEC stop notification, the tracer should clean up all its internal data structures describing the threads of this process, and retain only one data structure—one which describes the single still running tracee, with

   thread ID == thread group ID == process ID.

Example: two threads call execve(2) at the same time:

*** we get syscall-enter-stop in thread 1: **
PID1 execve("/bin/foo", "foo" <unfinished ...>
*** we issue PTRACE_SYSCALL for thread 1 **
*** we get syscall-enter-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

If the PTRACE_O_TRACEEXEC option is not in effect for the execing tracee, and if the tracee was PTRACE_ATTACHed rather that PTRACE_SEIZEd, the kernel delivers an extra SIGTRAP to the tracee after execve(2) returns. This is an ordinary signal (similar to one which can be generated by kill -TRAP), not a special kind of ptrace-stop. Employing PTRACE_GETSIGINFO for this signal returns si_code set to 0 (SI_USER). This signal may be blocked by signal mask, and thus may be delivered (much) later.

Usually, the tracer (for example, strace(1)) 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 the PTRACE_O_TRACEEXEC option or using PTRACE_SEIZE and thus suppressing this extra SIGTRAP is the recommended approach.

The ptrace API (ab)uses the standard UNIX parent/child signaling over waitpid(2). This used to cause the real parent of the process to stop receiving several kinds of waitpid(2) notifications when the child process is traced by some other process.

Many of these bugs have been fixed, but as of Linux 2.6.38 several still exist; see Bugs below.

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

• exit/death by signal is reported first to the tracer, then, when the tracer consumes the waitpid(2) result, to the real parent (to the real parent only when the whole multithreaded process exits). If the tracer and the real parent are the same process, the report is sent only once.

Various parts of the kernel-user-space API (not just ptrace() operations), require so-called "ptrace access mode" checks, whose outcome determines whether an operation is permitted (or, in a few cases, causes a "read" operation to return sanitized data). These checks are performed in cases where one process can inspect sensitive information about, or in some cases modify the state of, another process. The checks are based on factors such as the credentials and capabilities of the two processes, whether or not the "target" process is dumpable, and the results of checks performed by any enabled Linux Security Module (LSM)—for example, SELinux, Yama, or Smack—and by the commoncap LSM (which is always invoked).

Prior to Linux 2.6.27, all access checks were of a single type. Since Linux 2.6.27, two access mode levels are distinguished:

PTRACE_MODE_READ

For "read" operations or other operations that are less dangerous, such as: get_robust_list(2); kcmp(2); reading /proc/[pid]/auxv, /proc/[pid]/environ, or /proc/[pid]/stat; or readlink(2) of a /proc/[pid]/ns/* file.

PTRACE_MODE_ATTACH

For "write" operations, or other operations that are more dangerous, such as: ptrace attaching (PTRACE_ATTACH) to another process or calling process_vm_writev(2). (PTRACE_MODE_ATTACH was effectively the default before Linux 2.6.27.)

Since Linux 4.5, the above access mode checks are combined (ORed) with one of the following modifiers:

PTRACE_MODE_FSCREDS

Use the caller's filesystem UID and GID (see credentials(7)) or effective capabilities for LSM checks.

PTRACE_MODE_REALCREDS

Use the caller's real UID and GID or permitted capabilities for LSM checks. This was effectively the default before Linux 4.5.

Because combining one of the credential modifiers with one of the aforementioned access modes is typical, some macros are defined in the kernel sources for the combinations:

PTRACE_MODE_READ_FSCREDS

Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.

PTRACE_MODE_READ_REALCREDS

Defined as PTRACE_MODE_READ | PTRACE_MODE_REALCREDS.

PTRACE_MODE_ATTACH_FSCREDS

Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS.

PTRACE_MODE_ATTACH_REALCREDS

Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS.

One further modifier can be ORed with the access mode:

PTRACE_MODE_NOAUDIT (since Linux 3.3)

Don't audit this access mode check. This modifier is employed for ptrace access mode checks (such as checks when reading /proc/[pid]/stat) that merely cause the output to be filtered or sanitized, rather than causing an error to be returned to the caller. In these cases, accessing the file is not a security violation and there is no reason to generate a security audit record. This modifier suppresses the generation of such an audit record for the particular access check.

Note that all of the PTRACE_MODE_* constants described in this subsection are kernel-internal, and not visible to user space. The constant names are mentioned here in order to label the various kinds of ptrace access mode checks that are performed for various system calls and accesses to various pseudofiles (e.g., under /proc). These names are used in other manual pages to provide a simple shorthand for labeling the different kernel checks.

The algorithm employed for ptrace access mode checking determines whether the calling process is allowed to perform the corresponding action on the target process. (In the case of opening /proc/[pid] files, the "calling process" is the one opening the file, and the process with the corresponding PID is the "target process".) The algorithm is as follows:

1. If the calling thread and the target thread are in the same thread group, access is always allowed.

2. If the access mode specifies PTRACE_MODE_FSCREDS, then, for the check in the next step, employ the caller's filesystem UID and GID. (As noted in credentials(7), the filesystem UID and GID almost always have the same values as the corresponding effective IDs.)

   Otherwise, the access mode specifies PTRACE_MODE_REALCREDS, so use the caller's real UID and GID for the checks in the next step. (Most APIs that check the caller's UID and GID use the effective IDs. For historical reasons, the PTRACE_MODE_REALCREDS check uses the real IDs instead.)

3. Deny access if neither of the following is true:

   • The real, effective, and saved-set user IDs of the target match the caller's user ID, and the real, effective, and saved-set group IDs of the target match the caller's group ID.
   • The caller has the CAP_SYS_PTRACE capability in the user namespace of the target.
4. Deny access if the target process "dumpable" attribute has a value other than 1 (SUID_DUMP_USER; see the discussion of PR_SET_DUMPABLE in prctl(2)), and the caller does not have the CAP_SYS_PTRACE capability in the user namespace of the target process.

5. The kernel LSM security_ptrace_access_check() interface is invoked to see if ptrace access is permitted. The results depend on the LSM(s). The implementation of this interface in the commoncap LSM performs the following steps:

   a)

   If the access mode includes PTRACE_MODE_FSCREDS, then use the caller's effective capability set in the following check; otherwise (the access mode specifies PTRACE_MODE_REALCREDS, so) use the caller's permitted capability set.

   b)

   Deny access if neither of the following is true:

   • The caller and the target process are in the same user namespace, and the caller's capabilities are a superset of the target process's permitted capabilities.
   • The caller has the CAP_SYS_PTRACE capability in the target process's user namespace.

   Note that the commoncap LSM does not distinguish between PTRACE_MODE_READ and PTRACE_MODE_ATTACH.

6. If access has not been denied by any of the preceding steps, then access is allowed.

On systems with the Yama Linux Security Module (LSM) installed (i.e., the kernel was configured with CONFIG_SECURITY_YAMA), the /proc/sys/kernel/yama/ptrace_scope file (available since Linux 3.4) can be used to restrict the ability to trace a process with ptrace() (and thus also the ability to use tools such as strace(1) and gdb(1)). The goal of such restrictions is to prevent attack escalation whereby a compromised process can ptrace-attach to other sensitive processes (e.g., a GPG agent or an SSH session) owned by the user in order to gain additional credentials that may exist in memory and thus expand the scope of the attack.

More precisely, the Yama LSM limits two types of operations:

• Any operation that performs a ptrace access mode PTRACE_MODE_ATTACH check—for example, ptrace() PTRACE_ATTACH. (See the "Ptrace access mode checking" discussion above.)
• ptrace() PTRACE_TRACEME.

A process that has the CAP_SYS_PTRACE capability can update the /proc/sys/kernel/yama/ptrace_scope file with one of the following values:

0 ("classic ptrace permissions")

No additional restrictions on operations that perform PTRACE_MODE_ATTACH checks (beyond those imposed by the commoncap and other LSMs).

The use of PTRACE_TRACEME is unchanged.

1 ("restricted ptrace") [default value]

When performing an operation that requires a PTRACE_MODE_ATTACH check, the calling process must either have the CAP_SYS_PTRACE capability in the user namespace of the target process or it must have a predefined relationship with the target process. By default, the predefined relationship is that the target process must be a descendant of the caller.

A target process can employ the prctl(2) PR_SET_PTRACER operation to declare an additional PID that is allowed to perform PTRACE_MODE_ATTACH operations on the target. See the kernel source file Documentation/admin-guide/LSM/Yama.rst (or Documentation/security/Yama.txt before Linux 4.13) for further details.

The use of PTRACE_TRACEME is unchanged.

2 ("admin-only attach")

Only processes with the CAP_SYS_PTRACE capability in the user namespace of the target process may perform PTRACE_MODE_ATTACH operations or trace children that employ PTRACE_TRACEME.

3 ("no attach")

No process may perform PTRACE_MODE_ATTACH operations or trace children that employ PTRACE_TRACEME.

Once this value has been written to the file, it cannot be changed.

With respect to values 1 and 2, note that creating a new user namespace effectively removes the protection offered by Yama. This is because a process in the parent user namespace whose effective UID matches the UID of the creator of a child namespace has all capabilities (including CAP_SYS_PTRACE) when performing operations within the child user namespace (and further-removed descendants of that namespace). Consequently, when a process tries to use user namespaces to sandbox itself, it inadvertently weakens the protections offered by the Yama LSM.

At the system call level, the PTRACE_PEEKTEXT, PTRACE_PEEKDATA, and PTRACE_PEEKUSER requests have a different API: they store the result at the address specified by the data parameter, and the return value is the error flag. The glibc wrapper function provides the API given in Description above, with the result being returned via the function return value.