ptrace(2) — Linux manual page


ptrace(2)                  System Calls Manual                 ptrace(2)

NAME         top

       ptrace - process trace

LIBRARY         top

       Standard C library (libc, -lc)

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 flag.)

       A process can initiate a trace by calling fork(2) and having the
       resulting child do a PTRACE_TRACEME, followed (typically) by an
       execve(2).  Alternatively, one process may commence tracing
       another process using PTRACE_ATTACH or PTRACE_SEIZE.

       While being traced, the tracee will stop each time a signal is
       delivered, even if the signal is being ignored.  (An exception is
       SIGKILL, which has its usual effect.)  The tracer will be
       notified at its next call to waitpid(2) (or one of the related
       "wait" system calls); that call will return a status value
       containing information that indicates the cause of the stop in
       the tracee.  While the tracee is stopped, the tracer can use
       various ptrace requests to inspect and modify the tracee.  The
       tracer then causes the tracee to continue, optionally ignoring
       the delivered signal (or even delivering a different signal

       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 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_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

              Copy the tracee's general-purpose or floating-point
              registers, respectively, to the address data in the
              tracer.  See <sys/user.h> for information on the format of
              this data.  (addr is ignored.)  Note that SPARC systems
              have the meaning of data and addr reversed; that is, data
              is ignored and the registers are copied to the address
              addr.  PTRACE_GETREGS and PTRACE_GETFPREGS are not present
              on all architectures.

       PTRACE_GETREGSET (since Linux 2.6.34)
              Read the tracee's registers.  addr specifies, in an
              architecture-dependent way, the type of registers to be
              read.  NT_PRSTATUS (with numerical value 1) usually
              results in reading of general-purpose registers.  If the
              CPU has, for example, floating-point and/or vector
              registers, they can be retrieved by setting addr to the
              corresponding NT_foo constant.  data points to a struct
              iovec, which describes the destination buffer's location
              and length.  On return, the kernel modifies iov.len to
              indicate the actual number of bytes returned.

              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

       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

       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 space.

           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., sizeof(sigset_t)).

       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., sizeof(sigset_t)).

       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_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

                     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_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 capability, 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_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 triggered 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 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_SET_SYSCALL (since Linux 2.6.16)
              When in syscall-enter-stop, change the number of the
              system call that is about to be executed to the number
              specified in the data argument.  The addr argument is
              ignored.  This request is currently supported only on arm
              (and arm64, though only for backwards compatibility), but
              most other architectures have other means of accomplishing
              this (usually by changing the register that the userland
              code passed the system call number in).

              For PTRACE_SYSEMU, continue and stop on entry to the next
              system 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
              signal).  See the "group-stop" subsection for additional
              information.  PTRACE_LISTEN works only on tracees attached
              by PTRACE_SEIZE.

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

              This operation is deprecated; do not use it!  Instead,
              send a SIGKILL directly using kill(2) or tgkill(2).  The
              problem with PTRACE_KILL is that it requires the tracee to
              be in signal-delivery-stop, otherwise it may not work
              (i.e., may complete successfully but won't kill the
              tracee).  By contrast, sending a SIGKILL directly has no
              such limitation.

       PTRACE_INTERRUPT (since Linux 3.4)
              Stop a tracee.  If the tracee is running or sleeping in
              kernel space and PTRACE_SYSCALL is in effect, the system
              call is interrupted and syscall-exit-stop is reported.
              (The interrupted system call is restarted when the tracee
              is restarted.)  If the tracee was already stopped by a
              signal and PTRACE_LISTEN was sent to it, the tracee stops
              with PTRACE_EVENT_STOP and WSTOPSIG(status) returns the
              stop signal.  If any other ptrace-stop is generated at the
              same time (for example, if a signal is sent to the
              tracee), this ptrace-stop happens.  If none of the above
              applies (for example, if the tracee is running in 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 information.  (addr and data are ignored.)

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

       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 signal.  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

       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 EACCES 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_CHECKPOINT_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.)

       PTRACE_GET_SYSCALL_INFO (since Linux 5.3)
              Retrieve information about the system call that caused the
              stop.  The information is placed into the buffer pointed
              by the data argument, which should be a pointer to a
              buffer of type struct ptrace_syscall_info.  The addr
              argument contains the size of the buffer pointed to by the
              data argument (i.e., sizeof(struct ptrace_syscall_info)).
              The return value contains the number of bytes available to
              be written by the kernel.  If the size of the data to be
              written by the kernel exceeds the size specified by the
              addr argument, the output data is truncated.

              The ptrace_syscall_info structure contains the following

                  struct ptrace_syscall_info {
                      __u8 op;        /* Type of system call stop */
                      __u32 arch;     /* AUDIT_ARCH_* value; see seccomp(2) */
                      __u64 instruction_pointer; /* CPU instruction pointer */
                      __u64 stack_pointer;    /* CPU stack pointer */
                      union {
                          struct {    /* op == PTRACE_SYSCALL_INFO_ENTRY */
                              __u64 nr;       /* System call number */
                              __u64 args[6];  /* System call arguments */
                          } entry;
                          struct {    /* op == PTRACE_SYSCALL_INFO_EXIT */
                              __s64 rval;     /* System call return value */
                              __u8 is_error;  /* System call error flag;
                                                 Boolean: does rval contain
                                                 an error value (-ERRCODE) or
                                                 a nonerror return value? */
                          } exit;
                          struct {    /* op == PTRACE_SYSCALL_INFO_SECCOMP */
                              __u64 nr;       /* System call number */
                              __u64 args[6];  /* System call arguments */
                              __u32 ret_data; /* SECCOMP_RET_DATA portion
                                                 of SECCOMP_RET_TRACE
                                                 return value */
                          } seccomp;

              The op, arch, instruction_pointer, and stack_pointer
              fields are defined for all kinds of ptrace system call
              stops.  The rest of the structure is a union; one should
              read only those fields that are meaningful for the kind of
              system call stop specified by the op field.

              The op field has one of the following values (defined in
              <linux/ptrace.h>) indicating what type of stop occurred
              and which part of the union is filled:

                     The entry component of the union contains
                     information relating to a system call entry stop.

                     The exit component of the union contains
                     information relating to a system call exit stop.

                     The seccomp component of the union contains
                     information relating to a PTRACE_EVENT_SECCOMP

                     No component of the union contains relevant

              In case of system call entry or exit stops, the data
              returned by PTRACE_GET_SYSCALL_INFO is limited to type
              option is set before the corresponding system call stop
              has occurred.

   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, depending on
       the kernel version; see BUGS below), and when threads are torn
       down on execve(2) in a multithreaded process.

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

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

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

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

       In this manual page, any stopped state in which the tracee is
       ready to accept ptrace commands from the tracer is called ptrace-
       stop.  Ptrace-stops can be further subdivided into signal-
       delivery-stop, group-stop, syscall-stop, PTRACE_EVENT stops, and
       so on.  These stopped states are described in detail below.

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

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

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

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

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

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

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

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

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

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

   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-

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

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

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

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

           ptrace(PTRACE_restart, pid, 0, stopsig)

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

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

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

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

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

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

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

           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

       The call can be avoided if the signal is not SIGSTOP, SIGTSTP,
       SIGTTIN, or SIGTTOU; only these four signals are stopping
       signals.  If the tracer sees something else, it can't be a group-
       stop.  Otherwise, the tracer needs to call PTRACE_GETSIGINFO.  If
       PTRACE_GETSIGINFO fails with EINVAL, then it is definitely a
       group-stop.  (Other failure codes are possible, such as ESRCH
       ("no such process") if a SIGKILL killed the tracee.)

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

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

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

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

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

       PTRACE_EVENT stops are observed by the tracer as waitpid(2)
       returning with WIFSTOPPED(status), and WSTOPSIG(status) returns
       SIGTRAP (or for PTRACE_EVENT_STOP, returns the stopping signal if
       tracee is in a group-stop).  An additional bit is set in the
       higher byte of the status word: the value status>>8 will be

           ((PTRACE_EVENT_foo<<8) | SIGTRAP).

       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 vfork(2)).

              Stop before return from fork(2) or clone(2) with the exit
              signal 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

       si_signo, with si_code set to (event<<8) | SIGTRAP.

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

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

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

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

       si_code <= 0
              SIGTRAP was delivered as a result of a user-space action,
              for example, a system call (tgkill(2), kill(2),
              sigqueue(3), etc.), expiration of a POSIX timer, change of
              state on a POSIX message queue, or completion of an
              asynchronous I/O request.

       si_code == SI_KERNEL (0x80)
              SIGTRAP was sent by the kernel.

       si_code == SIGTRAP or si_code == (SIGTRAP|0x80)
              This is a syscall-stop.

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

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

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

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

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

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

   PTRACE_EVENT_SECCOMP stops (Linux 3.5 to Linux 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 preceding syscall-entry-stop.

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

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

   Informational and restarting ptrace commands
       Most ptrace commands (all except PTRACE_ATTACH, PTRACE_SEIZE,
       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"
       PTRACE_O_TRACECLONE options.

       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 signal(s) first!  The usual practice is to reinject
       these signals until SIGSTOP is seen, then suppress SIGSTOP
       injection.  The design bug here is that a ptrace attach and a
       concurrently delivered SIGSTOP may race and the concurrent
       SIGSTOP may be lost.

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

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

       The request

           ptrace(PTRACE_TRACEME, 0, 0, 0);

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


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

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

       Detaching of the tracee is performed by:

           ptrace(PTRACE_DETACH, pid, 0, sig);

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

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

       If the tracer dies, all tracees are automatically detached and
       restarted, unless they were in group-stop.  Handling of restart
       from group-stop is currently buggy, but the "as planned" behavior
       is to leave tracee stopped and waiting for SIGCONT.  If the
       tracee is restarted from signal-delivery-stop, the pending signal
       is injected.

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

       •  All other threads stop in PTRACE_EVENT_EXIT stop, if the
          PTRACE_O_TRACEEXIT option was turned on.  Then all other
          threads except the thread group leader report death as if they
          exited via _exit(2) with exit code 0.

       •  The execing tracee changes its thread ID while it is in the
          execve(2).  (Remember, under ptrace, the "pid" returned from
          waitpid(2), or fed into ptrace calls, is the tracee's thread
          ID.)  That is, the tracee's thread ID is reset to be the same
          as its process ID, which is the same as the thread group
          leader's thread ID.

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

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

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

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

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

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

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

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

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

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

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

   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

       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), the PTRACE_SECCOMP_GET_FILTER request returns
       the number of instructions in the BPF program, the
       PTRACE_GET_SYSCALL_INFO request returns the number of bytes
       available to be written by the kernel, and other requests return

       On error, all requests return -1, and errno is set to indicate
       the error.  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

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 (before Linux 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).

STANDARDS         top


HISTORY         top

       SVr4, 4.3BSD.

       Before Linux 2.6.26, init(1), the process with PID 1, may not be

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.

       A tracees parent continues to be the tracer even if that tracer
       calls execve(2).

       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

       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 2.6.27.)

       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 4.5.

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

              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 check.

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

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

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

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

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

       (3)  Deny access if neither of the following is true:

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

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

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

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

            (5.1)  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.

            (5.2)  Deny access if neither of the following is true:

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

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

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

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

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

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

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

       •  ptrace() PTRACE_TRACEME.

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

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

              The use of PTRACE_TRACEME is unchanged.

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

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

              The use of PTRACE_TRACEME is unchanged.

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

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

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

       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 Linux 2.6 kernel headers, PTRACE_SETOPTIONS is
       declared with a different value than the one for Linux 2.4.  This
       leads to applications compiled with Linux 2.6 kernel headers
       failing when run on Linux 2.4.  This can be worked around by
       redefining PTRACE_SETOPTIONS to PTRACE_OLDSETOPTIONS, if that is

       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.)

       Contrary to the normal rules, the glibc wrapper for ptrace() can
       set errno to zero.

SEE ALSO         top

       gdb(1), ltrace(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)

Linux man-pages (unreleased)     (date)                        ptrace(2)

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