PTRACE(2)                 Linux Programmer's Manual                PTRACE(2)

NAME         top

       ptrace - process trace

SYNOPSIS         top

       #include <sys/ptrace.h>

       long ptrace(enum __ptrace_request request, pid_t pid,
                   void *addr, void *data);

DESCRIPTION         top

       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

       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:

              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

              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.

              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; but
              see NOTES.)

              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; but see NOTES.)

              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.

              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.

              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

              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_PEEKSIGINFO (since Linux 3.10)
              Retrieve siginfo_t structures without removing signals from a
              queue.  addr points to a ptrace_peeksiginfo_args structure
              that specifies the ordinal position from which copying of
              signals should start, and the number of signals to copy.
              siginfo_t structures are copied into the buffer pointed to by
              data.  The return value contains the number of copied signals
              (zero indicates that there is no signal corresponding to the
              specified ordinal position).  Within the returned siginfo
              structures, the si_code field includes information (__SI_CHLD,
              __SI_FAULT, etc.) that are not otherwise exposed to user

           struct ptrace_peeksiginfo_args {
               u64 off;    /* Ordinal position in queue at which
                              to start copying signals */
               u32 flags;  /* PTRACE_PEEKSIGINFO_SHARED or 0 */
               s32 nr;     /* Number of signals to copy */

              Currently, there is only one flag, PTRACE_PEEKSIGINFO_SHARED,
              for dumping signals from the process-wide signal queue.  If
              this flag is not set, signals are read from the per-thread
              queue of the specified thread.

       PTRACE_GETSIGMASK (since Linux 3.11)
              Place a copy of the mask of blocked signals (see
              sigprocmask(2)) in the buffer pointed to by data, which should
              be a pointer to a buffer of type sigset_t.  The addr argument
              contains the size of the buffer pointed to by data (i.e.,

       PTRACE_SETSIGMASK (since Linux 3.11)
              Change the mask of blocked signals (see sigprocmask(2)) to the
              value specified in the buffer pointed to by data, which should
              be a pointer to a buffer of type sigset_t.  The addr argument
              contains the size of the buffer pointed to by data (i.e.,

       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)
                     Send a SIGKILL signal to the 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

                     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_TRACE‐
                     FORK 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

                     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_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_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_O_TRACESECCOMP (since Linux 3.5)
                     Stop the tracee when a seccomp(2) SECCOMP_RET_TRACE
                     rule is triggered.  A waitpid(2) by the tracer will
                     return a status value such that

                       status>>8 == (SIGTRAP | (PTRACE_EVENT_SECCOMP<<8))

                     While this triggers a PTRACE_EVENT stop, it is similar
                     to a syscall-enter-stop.  For details, see the note on
                     PTRACE_EVENT_SECCOMP below.  The seccomp event message
                     data (from the SECCOMP_RET_DATA portion of the seccomp
                     filter rule) can be retrieved with PTRACE_GETEVENTMSG.

              PTRACE_O_SUSPEND_SECCOMP (since Linux 4.3)
                     Suspend the tracee's seccomp protections.  This applies
                     regardless of mode, and can be used when the tracee has
                     not yet installed seccomp filters.  That is, a valid
                     use case is to suspend a tracee's seccomp protections
                     before they are installed by the tracee, let the tracee
                     install the filters, and then clear this flag when the
                     filters should be resumed.  Setting this option
                     requires that the tracer have the CAP_SYS_ADMIN capa‐
                     bility, not have any seccomp protections installed, and
                     not have PTRACE_O_SUSPEND_SECCOMP set on itself.

       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.  For PTRACE_EVENT_SECCOMP, this is the
              seccomp(2) filter's SECCOMP_RET_DATA associated with the trig‐
              gered rule.  (addr is ignored.)

              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.)

              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 SIG‐
              TRAP.  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.)

              For PTRACE_SYSEMU, continue and stop on entry to the next sys‐
              tem call, which will not be executed.  See the documentation
              on syscall-stops below.  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 sig‐
              nal).  See the "group-stop" subsection for additional informa‐
              tion.  PTRACE_LISTEN works only on tracees attached by

              Send the tracee a SIGKILL to terminate it.  (addr and data are

              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 inter‐
              rupted system call is restarted when the tracee is restarted.)
              If the tracee was already stopped by a signal and PTRACE_LIS‐
              TEN 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 run‐
              ning in user space), it stops with PTRACE_EVENT_STOP with
              WSTOPSIG(status) == SIGTRAP.  PTRACE_INTERRUPT only works on
              tracees attached by PTRACE_SEIZE.

              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 informa‐
              tion.  (addr and data are ignored.)

              Permission to perform a PTRACE_ATTACH is governed by a ptrace
              access mode PTRACE_MODE_ATTACH_REALCREDS check; see below.

       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.  Group-stops are reported as
              PTRACE_EVENT_STOP and WSTOPSIG(status) returns the stop sig‐
              nal.  Automatically attached children stop with
              PTRACE_EVENT_STOP and WSTOPSIG(status) returns SIGTRAP instead
              of having SIGSTOP signal delivered to them.  execve(2) does
              not deliver an extra SIGTRAP.  Only a PTRACE_SEIZEd process
              can accept PTRACE_INTERRUPT and PTRACE_LISTEN commands.  The
              "seized" behavior just described is inherited by children that
              are automatically attached using PTRACE_O_TRACEFORK,
              PTRACE_O_TRACEVFORK, and PTRACE_O_TRACECLONE.  addr must be
              zero.  data contains a bit mask of ptrace options to activate

              Permission to perform a PTRACE_SEIZE is governed by a ptrace
              access mode PTRACE_MODE_ATTACH_REALCREDS check; see below.

       PTRACE_SECCOMP_GET_FILTER (since Linux 4.4)
              This operation allows the tracer to dump the tracee's classic
              BPF filters.

              addr is an integer specifying the index of the filter to be
              dumped.  The most recently installed filter has the index 0.
              If addr is greater than the number of installed filters, the
              operation fails with the error ENOENT.

              data is either a pointer to a struct sock_filter array that is
              large enough to store the BPF program, or NULL if the program
              is not to be stored.

              Upon success, the return value is the number of instructions
              in the BPF program.  If data was NULL, then this return value
              can be used to correctly size the struct sock_filter array
              passed in a subsequent call.

              This operation fails with the error EACCESS if the caller does
              not have the CAP_SYS_ADMIN capability or if the caller is in
              strict or filter seccomp mode.  If the filter referred to by
              addr is not a classic BPF filter, the operation fails with the
              error EMEDIUMTYPE.

              This operation is available if the kernel was configured with
              both the CONFIG_SECCOMP_FILTER and the CONFIG_CHECK‐
              POINT_RESTORE options.

              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.)

       PTRACE_GET_THREAD_AREA (since Linux 2.6.0)
              This operation performs a similar task to get_thread_area(2).
              It reads the TLS entry in the GDT whose index is given in
              addr, placing a copy of the entry into the struct user_desc
              pointed to by data.  (By contrast with get_thread_area(2), the
              entry_number of the struct user_desc is ignored.)

       PTRACE_SET_THREAD_AREA (since Linux 2.6.0)
              This operation performs a similar task to set_thread_area(2).
              It sets the TLS entry in the GDT whose index is given in addr,
              assigning it the data supplied in the struct user_desc pointed
              to by data.  (By contrast with set_thread_area(2), the
              entry_number of the struct user_desc is ignored; in other
              words, this ptrace operation can't be used to allocate a free
              TLS entry.)

   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

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

       SIGKILL does not generate signal-delivery-stop and therefore the
       tracer can't suppress it.  SIGKILL kills even within system calls
       (syscall-exit-stop is not generated prior to death by SIGKILL).  The
       net effect is that SIGKILL always kills the process (all its
       threads), even if some threads of the process are ptraced.

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

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

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

       The tracer cannot assume that the ptrace-stopped tracee exists.
       There are many scenarios when the tracee may die while stopped (such
       as SIGKILL).  Therefore, the tracer must be prepared to handle an
       ESRCH error on any ptrace operation.  Unfortunately, the same error
       is returned if the tracee exists but is not ptrace-stopped (for com‐
       mands 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 sched‐
       uled), 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, PTRACE_EVENTstops, 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 recom‐
       mended: the "continued" state is per-process and consuming it can
       confuse the real parent of the tracee.

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

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

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

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

       Signal-delivery-stop is observed by the tracer as waitpid(2) return‐
       ing with WIFSTOPPED(status) true, with the signal returned by WSTOP‐
       SIG(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 man‐
       ual page, to distinguish it from signal-delivery-stop.

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

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

       Restarting ptrace commands issued in ptrace-stops other than signal-
       delivery-stop are not guaranteed to inject a signal, even if sig is
       nonzero.  No error is reported; a nonzero sig may simply be ignored.
       Ptrace users should not try to "create a new signal" this way: use
       tgkill(2) instead.

       The fact that signal injection requests may be ignored when restart‐
       ing 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
       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 SIG‐
       CONT was delivered.  In other words, SIGCONT may be not the first
       signal observed by the tracee after it was sent.

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

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

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

       When a (possibly multithreaded) process receives a stopping signal,
       all threads stop.  If some threads are traced, they enter a group-
       stop.  Note that the stopping signal will first cause signal-deliv‐
       ery-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

           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

       The call can be avoided if the signal is not SIGSTOP, SIGTSTP, SIGT‐
       TIN, 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 fail‐
       ure 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

       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 effec‐
       tively 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

       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

       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:

              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

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

              Stop before return from clone(2).

              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.

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

              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.

              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).

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

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

       If the tracee was restarted by PTRACE_SYSCALL or PTRACE_SYSEMU, the
       tracee enters syscall-enter-stop just prior to entering any system
       call (which will not be executed if the restart was using
       PTRACE_SYSEMU, regardless of any change made to registers at this
       point or how the tracee is restarted after this stop).  No matter
       which method caused the syscall-entry-stop, if the tracer restarts
       the tracee with PTRACE_SYSCALL, the tracee enters syscall-exit-stop
       when the system call is finished, or if it is interrupted by a sig‐
       nal.  (That is, signal-delivery-stop never happens between syscall-
       enter-stop and syscall-exit-stop; it happens after syscall-exit-
       stop.).  If the tracee is continued using any other method (including
       PTRACE_SYSEMU), no syscall-exit-stop occurs.  Note that all mentions

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

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

       Some architectures allow the cases to be distinguished by examining
       registers.  For example, on x86, rax == -ENOSYS in syscall-enter-
       stop.  Since SIGTRAP (like any other signal) always happens after
       syscall-exit-stop, and at this point rax almost never contains
       -ENOSYS, the SIGTRAP looks like "syscall-stop which is not syscall-
       enter-stop"; in other words, it looks like a "stray syscall-exit-
       stop" and can be detected this way.  But such detection is fragile
       and is best avoided.

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

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

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

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

   PTRACE_EVENT_SECCOMP stops (Linux 3.5 to 4.7)
       The behavior of PTRACE_EVENT_SECCOMP stops and their interaction with
       other kinds of ptrace stops has changed between kernel versions.
       This documents the behavior from their introduction until Linux 4.7
       (inclusive).  The behavior in later kernel versions is documented in
       the next section.

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

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

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

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

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

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

   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 cur‐
       rent ptrace-stop is not documented as returning a meaningful event

       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

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

           ptrace(cmd, pid, 0, sig);

       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);


           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 sig‐
       nal(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 sys‐
       tem 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 sig‐
       nals, 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


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

       CLONE 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 hap‐
       pens 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 ker‐
       nel 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

       *  Then a PTRACE_EVENT_EXEC stop happens, if the PTRACE_O_TRACEEXEC
          option was turned on.

       *  If the thread group leader has reported its PTRACE_EVENT_EXIT stop
          by this time, it appears to the tracer that the dead thread leader
          "reappears from nowhere".  (Note: the thread group leader does not
          report death via WIFEXITED(status) until there is at least one
          other live thread.  This eliminates the possibility that the
          tracer will see it dying and then reappearing.)  If the thread
          group leader was still alive, for the tracer this may look as if
          thread group leader returns from a different system call than it
          entered, or even "returned from a system call even though it was
          not in any system call".  If the thread group leader was not
          traced (or was traced by a different tracer), then during
          execve(2) it will appear as if it has become a tracee of the
          tracer of the execing tracee.

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

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

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

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

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

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

       *** we get syscall-enter-stop in thread 1: **
       PID1 execve("/bin/foo", "foo" <unfinished ...>
       *** we issue PTRACE_SYSCALL for thread 1 **
       *** we get syscall-enter-stop in thread 2: **
       PID2 execve("/bin/bar", "bar" <unfinished ...>
       *** we issue PTRACE_SYSCALL for thread 2 **
       *** we get PTRACE_EVENT_EXEC for PID0, we issue PTRACE_SYSCALL **
       *** we get syscall-exit-stop for PID0: **
       PID0 <... execve resumed> )             = 0

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

       Usually, the tracer (for example, strace(1)) would not want to show
       this extra post-execve SIGTRAP signal to the user, and would suppress
       its delivery to the tracee (if SIGTRAP is set to SIG_DFL, it is a
       killing signal).  However, determining which SIGTRAP to suppress is
       not easy.  Setting the PTRACE_O_TRACEEXEC option or using
       PTRACE_SEIZE and thus suppressing this extra SIGTRAP is the recom‐
       mended 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         top

       On success, the PTRACE_PEEK* requests return the requested data (but
       see NOTES), while other requests return zero.

       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         top

       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(1) (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         top

       SVr4, 4.3BSD.

NOTES         top

       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(1), the process with PID 1, may
       not be traced.

       A tracees parent continues to be the tracer even if that tracer calls

       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 significantly on other flavors of UNIX.
       In any case, use of ptrace() is highly specific to the operating
       system and architecture.

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

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

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

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

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

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

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

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

              Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.




       One further modifier can be ORed with the access mode:

       PTRACE_MODE_NOAUDIT (since Linux 3.3)
              Don't audit this access mode check.  This modifier is employed
              for ptrace access mode checks (such as checks when reading
              /proc/[pid]/stat) that merely cause the output to be filtered
              or sanitized, rather than causing an error to be returned to
              the caller.  In these cases, accessing the file is not a
              security violation and there is no reason to generate a
              security audit record.  This modifier suppresses the
              generation of such an audit record for the particular access

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

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

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

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

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

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

           · The real, effective, and saved-set user IDs of the target match
             the caller's user ID, and the real, effective, and saved-set
             group IDs of the target match the caller's group ID.

           · The caller has the CAP_SYS_PTRACE capability in the user
             namespace of the target.

       4.  Deny access if the target process "dumpable" attribute has a
           value other than 1 (SUID_DUMP_USER; see the discussion of
           PR_SET_DUMPABLE in prctl(2)), and the caller does not have the
           CAP_SYS_PTRACE capability in the user namespace of the target

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

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

           b) Deny access if neither of the following is true:

              · The caller and the target process are in the same user
                namespace, and the caller's capabilities are a proper
                superset of the target process's permitted capabilities.

              · The caller has the CAP_SYS_PTRACE capability in the target
                process's user namespace.

              Note that the commoncap LSM does not distinguish between

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

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

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

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

       *  ptrace() PTRACE_TRACEME.

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

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

              The use of PTRACE_TRACEME is unchanged.

       1 ("restricted ptrace") [default value]
              When performing an operation that requires a
              PTRACE_MODE_ATTACH check, the calling process must either have
              the CAP_SYS_PTRACE capability in the user namespace of the
              target process or it must have a predefined relationship with
              the target process.  By default, the predefined relationship
              is that the target process must be a descendant of the caller.

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

              The use of PTRACE_TRACEME is unchanged.

       2 ("admin-only attach")
              Only processes with the CAP_SYS_PTRACE capability in the user
              namespace of the target process may perform PTRACE_MODE_ATTACH
              operations or trace children that employ PTRACE_TRACEME.

       3 ("no attach")
              No process may perform PTRACE_MODE_ATTACH operations or trace
              children that employ PTRACE_TRACEME.

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

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

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

BUGS         top

       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

       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

       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 Linux 3.13.

       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 ...>_


           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

       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         top

       gdb(1), strace(1), clone(2), execve(2), fork(2), gettid(2), prctl(2),
       seccomp(2), sigaction(2), tgkill(2), vfork(2), waitpid(2), exec(3),
       capabilities(7), signal(7)

COLOPHON         top

       This page is part of release 4.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2017-09-15                        PTRACE(2)

Pages that refer to this page: ltrace(1)strace(1)clone(2)execve(2)get_robust_list(2)kcmp(2)move_pages(2)perf_event_open(2)prctl(2)process_vm_readv(2)seccomp(2)sigaction(2)syscalls(2)wait(2)exec(3)seccomp_init(3)seccomp_rule_add(3)proc(5)systemd.exec(5)capabilities(7)credentials(7)namespaces(7)user_namespaces(7)stapdyn(8)