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

       ptrace - process trace

       #include <sys/ptrace.h>

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

       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

       A tracee first needs to be attached to the tracer.  Attachment and
       subsequent commands are per thread: in a multithreaded process, every
       thread can be individually attached to a (potentially different) tracer,
       or left not attached and thus not debugged.  Therefore, "tracee" always
       means "(one) thread", never "a (possibly multithreaded) process".  Ptrace
       commands are always sent to a specific tracee using a call of the form

           ptrace(PTRACE_foo, pid, ...)

       where pid is the thread ID of the corresponding Linux thread.

       (Note that in this page, a "multithreaded process" means a thread group
       consisting of threads created using the clone(2) CLONE_THREAD flag.)

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

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

       If the PTRACE_O_TRACEEXEC option is not in effect, all successful calls
       to execve(2) by the traced process will cause it to be sent a SIGTRAP
       signal, giving the parent a chance to gain control before the new program
       begins execution.

       When the tracer is finished tracing, it can cause the tracee to continue
       executing in a normal, untraced mode via PTRACE_DETACH.

       The value of request determines the action to be performed:

              Indicate that this process is to be traced by its parent.  A
              process probably shouldn't make this request if its parent isn't
              expecting to trace it.  (pid, addr, and data are ignored.)

              The PTRACE_TRACEME request is used only by the tracee; the
              remaining requests are used only by the tracer.  In the following
              requests, pid specifies the thread ID of the tracee to be acted
              on.  For requests other than PTRACE_ATTACH, PTRACE_SEIZE,
              PTRACE_INTERRUPT, and PTRACE_KILL, the tracee must be stopped.

              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

              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 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 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 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.,

       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

                       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

                       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
              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 triggered rule.  (addr is

              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

              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

              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 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 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 below.

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

              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

              This operation is available if the kernel was configured with both

              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

       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 fields:

                  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 stop.

                     No component of the union contains relevant information.

   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

   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

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

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

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

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

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

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

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

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

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

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

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

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

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

           ptrace(PTRACE_restart, pid, 0, sig)

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

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

       Note that a suppressed signal still causes system calls to return
       prematurely.  In this case, system calls will be restarted: the tracer
       will observe the tracee to reexecute the interrupted system call (or
       restart_syscall(2) system call for a few 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

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

           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

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

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

       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

       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

              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 signal.  (That is, signal-
       delivery-stop never happens between syscall-enter-stop and syscall-exit-
       stop; it happens after syscall-exit-stop.).  If the tracee is continued
       using any other method (including PTRACE_SYSEMU), no syscall-exit-stop
       occurs.  Note that all mentions PTRACE_SYSEMU apply equally to

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

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

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

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

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

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

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

       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 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

       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

       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,
       PTRACE_TRACEME, PTRACE_INTERRUPT, and PTRACE_KILL) require the tracee to
       be in a ptrace-stop, otherwise they fail with ESRCH.

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

           ptrace(PTRACE_PEEKTEXT/PEEKDATA/PEEKUSER, pid, addr, 0);
           ptrace(PTRACE_POKETEXT/POKEDATA/POKEUSER, pid, addr, long_val);
           ptrace(PTRACE_GETREGS/GETFPREGS, pid, 0, &struct);
           ptrace(PTRACE_SETREGS/SETFPREGS, pid, 0, &struct);
           ptrace(PTRACE_GETREGSET, pid, NT_foo, &iov);
           ptrace(PTRACE_SETREGSET, pid, NT_foo, &iov);
           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo);
           ptrace(PTRACE_SETSIGINFO, pid, 0, &siginfo);
           ptrace(PTRACE_GETEVENTMSG, pid, 0, &long_var);
           ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

       Note that some errors are not reported.  For example, setting signal
       information (siginfo) may have no effect in some ptrace-stops, yet the
       call may succeed (return 0 and not set errno); querying
       PTRACE_GETEVENTMSG may succeed and return some random value if current
       ptrace-stop is not documented as returning a meaningful event message.

       The call

           ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

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

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

           ptrace(cmd, pid, 0, sig);

       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"

       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 signal-

       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

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

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

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

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

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

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

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

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

   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.

       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 zero.

       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 occurred.

       EBUSY  (i386 only) There was an error with allocating or freeing a debug

       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).

       SVr4, 4.3BSD.

       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

       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

   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 checks.

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

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

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

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

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

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

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

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

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

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

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

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

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

             Note that the commoncap LSM does not distinguish between

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

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

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

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

       *  ptrace() PTRACE_TRACEME.

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

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

              The use of PTRACE_TRACEME is unchanged.

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

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

              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

       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

       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

               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.

       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)

       This page is part of release 5.13 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                              2021-03-22                          PTRACE(2)