PTRACE(2) Linux Programmer's Manual PTRACE(2)
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 sub-
sequent 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 deliv-
ered, 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 pro-
gram begins execution.
When the tracer is finished tracing, it can cause the tracee to con-
tinue 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.)
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, return-
ing 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 modifica-
tions 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_GETF-
PREGS are not present on all architectures.
PTRACE_GETREGSET (since Linux 2.6.34)
Read the tracee's registers. addr specifies, in an architec-
ture-dependent way, the type of registers to be read. NT_PRSTA-
TUS (with numerical value 1) usually results in reading of gen-
eral-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 use-
ful 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 notifi-
cation is done after the process is finished exiting.
Even though context is available, the tracer cannot pre-
vent 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 sig-
nal 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 sta-
tus. 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, respec-
tively. (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_LIS-
TEN 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 inter-
rupted and syscall-exit-stop is reported. (The interrupted sys-
tem 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 WSTOP-
SIG(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 immedi-
ately.
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 wait-
pid(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 refus-
ing ptrace requests.
The tracer can't assume that the tracee always ends its life by report-
ing 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) noti-
fications), 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 impor-
tant 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 sig-
nal. (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(sta-
tus). 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 pre-
maturely. 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 sys-
tem 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-deliv-
ery-stop. The tracer can't suppress this side effect (it can only sup-
press signal injection, which only causes the SIGCONT handler to not be
executed in the tracee, if such a handler is installed). In fact, wak-
ing up from group-stop may be followed by signal-delivery-stop for sig-
nal(s) other than SIGCONT, if they were pending when SIGCONT was deliv-
ered. 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 WIF-
STOPPED(status) true, with the stopping signal available via WSTOP-
SIG(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 detec-
tion 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 prob-
lem with transparent handling of stopping signals. If the tracer
restarts the tracee after group-stop, the stopping signal is effec-
tively ignored--the tracee doesn't remain stopped, it runs. If the
tracer doesn't restart the tracee before entering into the next wait-
pid(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 execu-
tion (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 ini-
tial 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 inter-
rupted 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 SIG-
TRAP 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 suppres-
sion" 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 automati-
cally 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 wait-
pid(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 wrap-
per 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, dif-
ferent 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 pro-
cesses that they cannot send signals to or those running set-
user-ID/set-group-ID programs, for obvious reasons. Alterna-
tively, 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 proto-
type given, glibc currently declares ptrace() as a variadic function
with only the request argument fixed. It is recommended to always sup-
ply 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 oper-
ating-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 archi-
tecture.
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 com-
piled with 2.6 kernel headers failing when run on 2.4 kernels. This
can be worked around by redefining PTRACE_SETOPTIONS to PTRACE_OLDSE-
TOPTIONS, if that is defined.
Group-stop notifications are sent to the tracer, but not to real par-
ent. 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 sub-
sequent 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 oper-
ation: 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), sigac-
tion(2), tgkill(2), vfork(2), waitpid(2), exec(3), capabilities(7),
signal(7)
COLOPHON
This page is part of release 3.53 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/.
Linux 2013-07-11 PTRACE(2)
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