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NAME
ptrace - process trace
SYNOPSIS
#include <sys/ptrace.h>
long ptrace(enum __ptrace_request request, pid_t pid,
void *addr, void *data);
DESCRIPTION
The ptrace() system call provides a means by which one process (the "tracer") may observe
and control the execution of another process (the "tracee"), and examine and change the
tracee's memory and registers. It is primarily used to implement breakpoint debugging and
system call tracing.
A tracee first needs to be attached to the tracer. Attachment and subsequent commands are
per thread: in a multithreaded 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 multithreaded) process". Ptrace
commands are always sent to a specific tracee using a call of the form
ptrace(PTRACE_foo, pid, ...)
where pid is the thread ID of the corresponding Linux thread.
(Note that in this page, a "multithreaded process" means a thread group consisting of
threads created using the clone(2) CLONE_THREAD flag.)
A process can initiate a trace by calling fork(2) and having the resulting child do a
PTRACE_TRACEME, followed (typically) by an execve(2). Alternatively, one process may
commence tracing another process using PTRACE_ATTACH or PTRACE_SEIZE.
While being traced, the tracee will stop each time a signal is delivered, even if the
signal is being ignored. (An exception is SIGKILL, which has its usual effect.) The
tracer will be notified at its next call to waitpid(2) (or one of the related "wait"
system calls); that call will return a status value containing information that indicates
the cause of the stop in the tracee. While the tracee is stopped, the tracer can use
various ptrace requests to inspect and modify the tracee. The tracer then causes the
tracee to continue, optionally ignoring the delivered signal (or even delivering a
different signal instead).
If the PTRACE_O_TRACEEXEC option is not in effect, all successful calls to execve(2) by
the traced process will cause it to be sent a SIGTRAP signal, giving the parent a chance
to gain control before the new program begins execution.
When the tracer is finished tracing, it can cause the tracee to continue executing in a
normal, untraced mode via PTRACE_DETACH.
The value of request determines the action to be performed:
PTRACE_TRACEME
Indicate that this process is to be traced by its parent. A process probably
shouldn't make this request if its parent isn't expecting to trace it. (pid, addr,
and data are ignored.)
Since Ubuntu 10.10, PTRACE_ATTACH is not allowed against arbitrary matching-uid
processes. The traced "child" must be a descendant of the tracer or must have
called prctl(2) using PR_SET_PTRACER, with the pid of the tracer (or one of its
ancestors). For more details, see /etc/sysctl.d/10-ptrace.conf.
The PTRACE_TRACEME request is used only by the tracee; the remaining requests are used
only by the tracer. In the following requests, pid specifies the thread ID of the tracee
to be acted on. For requests other than PTRACE_ATTACH, PTRACE_SEIZE, PTRACE_INTERRUPT and
PTRACE_KILL, the tracee must be stopped.
PTRACE_PEEKTEXT, PTRACE_PEEKDATA
Read a word at the address addr in the tracee's memory, returning the word as the
result of the ptrace() call. Linux does not have separate text and data address
spaces, so these two requests are currently equivalent. (data is ignored.)
PTRACE_PEEKUSER
Read a word at offset addr in the tracee's USER area, which holds the registers and
other information about the process (see <sys/user.h>). The word is returned as
the result of the ptrace() call. Typically, the offset must be word-aligned,
though this might vary by architecture. See NOTES. (data is ignored.)
PTRACE_POKETEXT, PTRACE_POKEDATA
Copy the word data to the address addr in the tracee's memory. As for
PTRACE_PEEKTEXT and PTRACE_PEEKDATA, these two requests are currently equivalent.
PTRACE_POKEUSER
Copy the word data to offset addr in the tracee's USER area. As for
PTRACE_PEEKUSER, the offset must typically be word-aligned. In order to maintain
the integrity of the kernel, some modifications to the USER area are disallowed.
PTRACE_GETREGS, PTRACE_GETFPREGS
Copy the tracee's general-purpose or floating-point registers, respectively, to the
address data in the tracer. See <sys/user.h> for information on the format of this
data. (addr is ignored.) Note that SPARC systems have the meaning of data and
addr reversed; that is, data is ignored and the registers are copied to the address
addr. PTRACE_GETREGS and PTRACE_GETFPREGS are not present on all architectures.
PTRACE_GETREGSET (since Linux 2.6.34)
Read the tracee's registers. addr specifies, in an architecture-dependent way, the
type of registers to be read. NT_PRSTATUS (with numerical value 1) usually results
in reading of general-purpose registers. If the CPU has, for example, floating-
point and/or vector registers, they can be retrieved by setting addr to the
corresponding NT_foo constant. data points to a struct iovec, which describes the
destination buffer's location and length. On return, the kernel modifies iov.len
to indicate the actual number of bytes returned.
PTRACE_SETREGS, PTRACE_SETFPREGS
Modify the tracee's general-purpose or floating-point registers, respectively, from
the address data in the tracer. As for PTRACE_POKEUSER, some general-purpose
register modifications may be disallowed. (addr is ignored.) Note that SPARC
systems have the meaning of data and addr reversed; that is, data is ignored and
the registers are copied from the address addr. PTRACE_SETREGS and
PTRACE_SETFPREGS are not present on all architectures.
PTRACE_SETREGSET (since Linux 2.6.34)
Modify the tracee's registers. The meaning of addr and data is analogous to
PTRACE_GETREGSET.
PTRACE_GETSIGINFO (since Linux 2.3.99-pre6)
Retrieve information about the signal that caused the stop. Copy a siginfo_t
structure (see sigaction(2)) from the tracee to the address data in the tracer.
(addr is ignored.)
PTRACE_SETSIGINFO (since Linux 2.3.99-pre6)
Set signal information: copy a siginfo_t structure from the address data in the
tracer to the tracee. This will affect only signals that would normally be
delivered to the tracee and were caught by the tracer. It may be difficult to tell
these normal signals from synthetic signals generated by ptrace() itself. (addr is
ignored.)
PTRACE_SETOPTIONS (since Linux 2.4.6; see BUGS for caveats)
Set ptrace options from data. (addr is ignored.) data is interpreted as a bit
mask of options, which are specified by the following flags:
PTRACE_O_EXITKILL (since Linux 3.8)
If a tracer sets this flag, a SIGKILL signal will be sent to every tracee if
the tracer exits. This option is useful for ptrace jailers that want to
ensure that tracees can never escape the tracer's control.
PTRACE_O_TRACECLONE (since Linux 2.5.46)
Stop the tracee at the next clone(2) and automatically start tracing the
newly cloned process, which will start with a SIGSTOP, or PTRACE_EVENT_STOP
if PTRACE_SEIZE was used. A waitpid(2) by the tracer will return a status
value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_CLONE<<8))
The PID of the new process can be retrieved with PTRACE_GETEVENTMSG.
This option may not catch clone(2) calls in all cases. If the tracee calls
clone(2) with the CLONE_VFORK flag, PTRACE_EVENT_VFORK will be delivered
instead if PTRACE_O_TRACEVFORK is set; otherwise if the tracee calls
clone(2) with the exit signal set to SIGCHLD, PTRACE_EVENT_FORK will be
delivered if PTRACE_O_TRACEFORK is set.
PTRACE_O_TRACEEXEC (since Linux 2.5.46)
Stop the tracee at the next execve(2). A waitpid(2) by the tracer will
return a status value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_EXEC<<8))
If the execing thread is not a thread group leader, the thread ID is reset
to thread group leader's ID before this stop. Since Linux 3.0, the former
thread ID can be retrieved with PTRACE_GETEVENTMSG.
PTRACE_O_TRACEEXIT (since Linux 2.5.60)
Stop the tracee at exit. A waitpid(2) by the tracer will return a status
value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_EXIT<<8))
The tracee's exit status can be retrieved with PTRACE_GETEVENTMSG.
The tracee is stopped early during process exit, when registers are still
available, allowing the tracer to see where the exit occurred, whereas the
normal exit notification is done after the process is finished exiting.
Even though context is available, the tracer cannot prevent the exit from
happening at this point.
PTRACE_O_TRACEFORK (since Linux 2.5.46)
Stop the tracee at the next fork(2) and automatically start tracing the
newly forked process, which will start with a SIGSTOP, or PTRACE_EVENT_STOP
if PTRACE_SEIZE was used. A waitpid(2) by the tracer will return a status
value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_FORK<<8))
The PID of the new process can be retrieved with PTRACE_GETEVENTMSG.
PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
When delivering system call traps, set bit 7 in the signal number (i.e.,
deliver SIGTRAP|0x80). This makes it easy for the tracer to distinguish
normal traps from those caused by a system call. (PTRACE_O_TRACESYSGOOD may
not work on all architectures.)
PTRACE_O_TRACEVFORK (since Linux 2.5.46)
Stop the tracee at the next vfork(2) and automatically start tracing the
newly vforked process, which will start with a SIGSTOP, or PTRACE_EVENT_STOP
if PTRACE_SEIZE was used. A waitpid(2) by the tracer will return a status
value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_VFORK<<8))
The PID of the new process can be retrieved with PTRACE_GETEVENTMSG.
PTRACE_O_TRACEVFORKDONE (since Linux 2.5.60)
Stop the tracee at the completion of the next vfork(2). A waitpid(2) by the
tracer will return a status value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_VFORK_DONE<<8))
The PID of the new process can (since Linux 2.6.18) be retrieved with
PTRACE_GETEVENTMSG.
PTRACE_GETEVENTMSG (since Linux 2.5.46)
Retrieve a message (as an unsigned long) about the ptrace event that just happened,
placing it at the address data in the tracer. For PTRACE_EVENT_EXIT, this is the
tracee's exit status. For PTRACE_EVENT_FORK, PTRACE_EVENT_VFORK,
PTRACE_EVENT_VFORK_DONE, and PTRACE_EVENT_CLONE, this is the PID of the new
process. (addr is ignored.)
PTRACE_CONT
Restart the stopped tracee process. If data is nonzero, it is interpreted as the
number of a signal to be delivered to the tracee; otherwise, no signal is
delivered. Thus, for example, the tracer can control whether a signal sent to the
tracee is delivered or not. (addr is ignored.)
PTRACE_SYSCALL, PTRACE_SINGLESTEP
Restart the stopped tracee as for PTRACE_CONT, but arrange for the tracee to be
stopped at the next entry to or exit from a system call, or after execution of a
single instruction, respectively. (The tracee will also, as usual, be stopped upon
receipt of a signal.) From the tracer's perspective, the tracee will appear to
have been stopped by receipt of a SIGTRAP. So, for PTRACE_SYSCALL, for example,
the idea is to inspect the arguments to the system call at the first stop, then do
another PTRACE_SYSCALL and inspect the return value of the system call at the
second stop. The data argument is treated as for PTRACE_CONT. (addr is ignored.)
PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP (since Linux 2.6.14)
For PTRACE_SYSEMU, continue and stop on entry to the next system call, which will
not be executed. For PTRACE_SYSEMU_SINGLESTEP, do the same but also singlestep if
not a system call. This call is used by programs like User Mode Linux that want to
emulate all the tracee's system calls. The data argument is treated as for
PTRACE_CONT. The addr argument is ignored. These requests are currently supported
only on x86.
PTRACE_LISTEN (since Linux 3.4)
Restart the stopped tracee, but prevent it from executing. The resulting state of
the tracee is similar to a process which has been stopped by a SIGSTOP (or other
stopping signal). See the "group-stop" subsection for additional information.
PTRACE_LISTEN works only on tracees attached by PTRACE_SEIZE.
PTRACE_KILL
Send the tracee a SIGKILL to terminate it. (addr and data are ignored.)
This operation is deprecated; do not use it! Instead, send a SIGKILL directly
using kill(2) or tgkill(2). The problem with PTRACE_KILL is that it requires the
tracee to be in signal-delivery-stop, otherwise it may not work (i.e., may complete
successfully but won't kill the tracee). By contrast, sending a SIGKILL directly
has no such limitation.
PTRACE_INTERRUPT (since Linux 3.4)
Stop a tracee. If the tracee is running or sleeping in kernel space and
PTRACE_SYSCALL is in effect, the system call is interrupted and syscall-exit-stop
is reported. (The interrupted system call is restarted when the tracee is
restarted.) If the tracee was already stopped by a signal and PTRACE_LISTEN was
sent to it, the tracee stops with PTRACE_EVENT_STOP and WSTOPSIG(status) returns
the stop signal. If any other ptrace-stop is generated at the same time (for
example, if a signal is sent to the tracee), this ptrace-stop happens. If none of
the above applies (for example, if the tracee is running in userspace), it stops
with PTRACE_EVENT_STOP with WSTOPSIG(status) == SIGTRAP. PTRACE_INTERRUPT only
works on tracees attached by PTRACE_SEIZE.
PTRACE_ATTACH
Attach to the process specified in pid, making it a tracee of the calling process.
The tracee is sent a SIGSTOP, but will not necessarily have stopped by the
completion of this call; use waitpid(2) to wait for the tracee to stop. See the
"Attaching and detaching" subsection for additional information. (addr and data
are ignored.)
PTRACE_SEIZE (since Linux 3.4)
Attach to the process specified in pid, making it a tracee of the calling process.
Unlike PTRACE_ATTACH, PTRACE_SEIZE does not stop the process. Only a PTRACE_SEIZEd
process can accept PTRACE_INTERRUPT and PTRACE_LISTEN commands. addr must be zero.
data contains a bit mask of ptrace options to activate immediately.
PTRACE_DETACH
Restart the stopped tracee as for PTRACE_CONT, but first detach from it. Under
Linux, a tracee can be detached in this way regardless of which method was used to
initiate tracing. (addr is ignored.)
Death under 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), 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.
Stopped states
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, 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.)
Signal-delivery-stop
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.
Signal injection and suppression
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 syscalls 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 syscalls 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.
Group-stop
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).
PTRACE_EVENT stops
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), or
PTRACE_EVENT_STOP if PTRACE_SEIZE was used.
PTRACE_GETSIGINFO on PTRACE_EVENT stops returns SIGTRAP in si_signo, with si_code set to
(event<<8) | SIGTRAP.
Syscall-stops
If the tracee was restarted by PTRACE_SYSCALL, the tracee enters syscall-enter-stop just
prior to entering any system call. 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.)
Other possibilities are that the tracee may stop in a PTRACE_EVENT stop, 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. The rule is that
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.
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).
PTRACE_SINGLESTEP, PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP stops
[Details of these kinds of stops are yet to be documented.]
Informational and restarting ptrace commands
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.)
Attaching and detaching
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.
execve(2) under ptrace
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, 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 and thus suppressing this extra
SIGTRAP is the recommended approach.
Real parent
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.
RETURN VALUE
On success, PTRACE_PEEK* requests return the requested data, while other requests return
zero. (On Linux, this is done in the libc wrapper around ptrace system call. On the
system call level, PTRACE_PEEK* requests have a different API: they store the result at
the address specified by data parameter, and return value is the error flag.)
On error, all requests return -1, and errno is set appropriately. Since the value
returned by a successful PTRACE_PEEK* request may be -1, the caller must clear errno
before the call, and then check it afterward to determine whether or not an error
occurred.
ERRORS
EBUSY (i386 only) There was an error with allocating or freeing a debug register.
EFAULT There was an attempt to read from or write to an invalid area in the tracer's or
the tracee's memory, probably because the area wasn't mapped or accessible.
Unfortunately, under Linux, different variations of this fault will return EIO or
EFAULT more or less arbitrarily.
EINVAL An attempt was made to set an invalid option.
EIO request is invalid, or an attempt was made to read from or write to an invalid area
in the tracer's or the tracee's memory, or there was a word-alignment violation, or
an invalid signal was specified during a restart request.
EPERM The specified process cannot be traced. This could be because the tracer has
insufficient privileges (the required capability is CAP_SYS_PTRACE); unprivileged
processes cannot trace processes that they cannot send signals to or those running
set-user-ID/set-group-ID programs, for obvious reasons. Alternatively, the process
may already be being traced, or (on kernels before 2.6.26) be init(8) (PID 1).
ESRCH The specified process does not exist, or is not currently being traced by the
caller, or is not stopped (for requests that require a stopped tracee).
CONFORMING TO
SVr4, 4.3BSD.
NOTES
Although arguments to ptrace() are interpreted according to the prototype given, glibc
currently declares ptrace() as a variadic function with only the request argument fixed.
It is recommended to always supply four arguments, even if the requested operation does
not use them, setting unused/ignored arguments to 0L or (void *) 0.
In Linux kernels before 2.6.26, init(8), the process with PID 1, may not be traced.
The layout of the contents of memory and the USER area are quite operating-system- and
architecture-specific. The offset supplied, and the data returned, might not entirely
match with the definition of struct user.
The size of a "word" is determined by the operating-system variant (e.g., for 32-bit Linux
it is 32 bits).
This page documents the way the ptrace() call works currently in Linux. Its behavior
differs noticeably on other flavors of UNIX. In any case, use of ptrace() is highly
specific to the operating system and architecture.
BUGS
On hosts with 2.6 kernel headers, PTRACE_SETOPTIONS is declared with a different value
than the one for 2.4. This leads to applications compiled with 2.6 kernel headers failing
when run on 2.4 kernels. This can be worked around by redefining PTRACE_SETOPTIONS to
PTRACE_OLDSETOPTIONS, if that is defined.
Group-stop notifications are sent to the tracer, but not to real parent. Last confirmed
on 2.6.38.6.
If a thread group leader is traced and exits by calling _exit(2), a PTRACE_EVENT_EXIT stop
will happen for it (if requested), but the subsequent WIFEXITED notification will not be
delivered until all other threads exit. As explained above, if one of other threads calls
execve(2), the death of the thread group leader will never be reported. If the execed
thread is not traced by this tracer, the tracer will never know that execve(2) happened.
One possible workaround is to PTRACE_DETACH the thread group leader instead of restarting
it in this case. Last confirmed on 2.6.38.6.
A SIGKILL signal may still cause a PTRACE_EVENT_EXIT stop before actual signal death.
This may be changed in the future; SIGKILL is meant to always immediately kill tasks even
under ptrace. Last confirmed on 2.6.38.6.
Some system calls return with EINTR if a signal was sent to a tracee, but delivery was
suppressed by the tracer. (This is very typical operation: it is usually done by
debuggers on every attach, in order to not introduce a bogus SIGSTOP). As of Linux 3.2.9,
the following system calls are affected (this list is likely incomplete): epoll_wait(2),
and read(2) from an inotify(7) file descriptor. The usual symptom of this bug is that
when you attach to a quiescent process with the command
strace -p <process-ID>
then, instead of the usual and expected one-line output such as
restart_syscall(<... resuming interrupted call ...>_
or
select(6, [5], NULL, [5], NULL_
('_' denotes the cursor position), you observe more than one line. For example:
clock_gettime(CLOCK_MONOTONIC, {15370, 690928118}) = 0
epoll_wait(4,_
What is not visible here is that the process was blocked in epoll_wait(2) before strace(1)
has attached to it. Attaching caused epoll_wait(2) to return to user space with the error
EINTR. In this particular case, the program reacted to EINTR by checking the current
time, and then executing epoll_wait(2) again. (Programs which do not expect such "stray"
EINTR errors may behave in an unintended way upon an strace(1) attach.)
SEE ALSO
gdb(1), strace(1), clone(2), execve(2), fork(2), gettid(2), sigaction(2), tgkill(2),
vfork(2), waitpid(2), exec(3), capabilities(7), signal(7)
COLOPHON
This page is part of release 3.54 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.