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

NAME
       ptrace - process trace

SYNOPSIS
       #include <sys/ptrace.h>

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

DESCRIPTION
       The  ptrace() system call provides a means by which one process (the "tracer") may observe and control the execution of another process (the "tracee"), and examā€
       ine 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  individuā€
       ally attached to a (potentially different) tracer, or left not attached and thus not debugged.  Therefore, "tracee" always means "(one) thread", never "a (possiā€
       bly 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 efā€
       fect.)  The tracer will be notified at its next call to waitpid(2) (or one of the related "wait" system calls); that call will return a status  value  containing
       information  that  indicates  the cause of the stop in the tracee.  While the tracee is stopped, the tracer can use various ptrace requests to inspect and modify
       the tracee.  The tracer then causes the tracee to continue, optionally ignoring the delivered signal (or even delivering a different signal instead).

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

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

       The value of request determines the action to be performed:

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

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

       PTRACE_PEEKTEXT, PTRACE_PEEKDATA
              Read a word at the address addr in the tracee's memory, returning the word as the result of the ptrace() call.  Linux does not have separate text and data
              address spaces, so these two requests are currently equivalent.  (data is ignored; but see NOTES.)

       PTRACE_PEEKUSER
              Read  a  word at offset addr in the tracee's USER area, which holds the registers and other information about the process (see <sys/user.h>).  The word is
              returned as the result of the ptrace() call.  Typically, the offset must be word-aligned, though this might vary by architecture.  See  NOTES.   (data  is
              ignored; but see NOTES.)

       PTRACE_POKETEXT, PTRACE_POKEDATA
              Copy the word data to the address addr in the tracee's memory.  As for PTRACE_PEEKTEXT and PTRACE_PEEKDATA, these two requests are currently equivalent.

       PTRACE_POKEUSER
              Copy the word data to offset addr in the tracee's USER area.  As for PTRACE_PEEKUSER, the offset must typically be word-aligned.  In order to maintain the
              integrity of the kernel, some modifications to the USER area are disallowed.

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

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

       PTRACE_SETREGS, PTRACE_SETFPREGS
              Modify the tracee's general-purpose or floating-point registers, respectively, from the address data in the tracer.  As for PTRACE_POKEUSER, some general-
              purpose register modifications may be disallowed.  (addr is ignored.)  Note that SPARC systems have the meaning of data and addr reversed; that  is,  data
              is ignored and the registers are copied from the address addr.  PTRACE_SETREGS and PTRACE_SETFPREGS are not present on all architectures.

       PTRACE_SETREGSET (since Linux 2.6.34)
              Modify the tracee's registers.  The meaning of addr and data is analogous to PTRACE_GETREGSET.

       PTRACE_GETSIGINFO (since Linux 2.3.99-pre6)
              Retrieve  information  about  the  signal  that caused the stop.  Copy a siginfo_t structure (see sigaction(2)) from the tracee to the address data in the
              tracer.  (addr is ignored.)

       PTRACE_SETSIGINFO (since Linux 2.3.99-pre6)
              Set signal information: copy a siginfo_t structure from the address data in the tracer to the tracee.  This will affect only signals that  would  normally
              be  delivered  to the tracee and were caught by the tracer.  It may be difficult to tell these normal signals from synthetic signals generated by ptrace()
              itself.  (addr is ignored.)

       PTRACE_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  posiā€
              tion  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  esā€
                     cape the tracer's control.

              PTRACE_O_TRACECLONE (since Linux 2.5.46)
                     Stop  the  tracee  at  the  next  clone(2)  and  automatically  start  tracing  the  newly  cloned  process,  which  will  start with a SIGSTOP, or
                     PTRACE_EVENT_STOP if PTRACE_SEIZE was used.  A waitpid(2) by the tracer will return a status value such that

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

                     The PID of the new process can be retrieved with PTRACE_GETEVENTMSG.

                     This option may not catch clone(2) calls in all cases.  If the tracee calls clone(2) with the CLONE_VFORK flag, PTRACE_EVENT_VFORK will  be  delivā€
                     ered  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 forā€
                     mer thread ID can be retrieved with PTRACE_GETEVENTMSG.

              PTRACE_O_TRACEEXIT (since Linux 2.5.60)
                     Stop the tracee at exit.  A waitpid(2) by the tracer will return a status value such that

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

                     The tracee's exit status can be retrieved with PTRACE_GETEVENTMSG.

                     The  tracee  is  stopped early during process exit, when registers are still available, allowing the tracer to see where the exit occurred, whereas
                     the normal exit notification is done after the process is finished exiting.  Even though context is available, the tracer cannot prevent  the  exit
                     from happening at this point.

              PTRACE_O_TRACEFORK (since Linux 2.5.46)
                     Stop the tracee at the next fork(2) and automatically start tracing the newly forked process, which will start with a SIGSTOP, or PTRACE_EVENT_STOP
                     if PTRACE_SEIZE was used.  A waitpid(2) by the tracer will return a status value such that

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

                     The PID of the new process can be retrieved with PTRACE_GETEVENTMSG.

              PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
                     When delivering system call traps, set bit 7 in the signal number (i.e., deliver SIGTRAP|0x80).  This makes it easy for the tracer  to  distinguish
                     normal traps from those caused by a system call.

              PTRACE_O_TRACEVFORK (since Linux 2.5.46)
                     Stop  the  tracee  at  the  next  vfork(2)  and  automatically  start  tracing  the  newly  vforked  process,  which  will start with a SIGSTOP, or
                     PTRACE_EVENT_STOP if PTRACE_SEIZE was used.  A waitpid(2) by the tracer will return a status value such that

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

                     The PID of the new process can be retrieved with PTRACE_GETEVENTMSG.

              PTRACE_O_TRACEVFORKDONE (since Linux 2.5.60)
                     Stop the tracee at the completion of the next vfork(2).  A waitpid(2) by the tracer will return a status value such that

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

                     The PID of the new process can (since Linux 2.6.18) be retrieved with PTRACE_GETEVENTMSG.

              PTRACE_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 secā€
                     comp 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_VFORK, PTRACE_EVENT_VFORK_DONE, and PTRACE_EVENT_CLONE, this is the PID of the new
              process.  For PTRACE_EVENT_SECCOMP, this is the seccomp(2) filter's SECCOMP_RET_DATA associated with the triggered rule.  (addr is ignored.)

       PTRACE_CONT
              Restart the stopped tracee process.  If data is nonzero, it is interpreted as the number of a signal to be delivered to the tracee; otherwise,  no  signal
              is delivered.  Thus, for example, the tracer can control whether a signal sent to the tracee is delivered or not.  (addr is ignored.)

       PTRACE_SYSCALL, PTRACE_SINGLESTEP
              Restart  the  stopped tracee as for PTRACE_CONT, but arrange for the tracee to be stopped at the next entry to or exit from a system call, or after execuā€
              tion 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 arā€
              gument 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).

       PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP (since Linux 2.6.14)
              For  PTRACE_SYSEMU,  continue  and  stop on entry to the next system call, which will not be executed.  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.

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

              This  operation  is  deprecated; do not use it!  Instead, send a SIGKILL directly using kill(2) or tgkill(2).  The problem with PTRACE_KILL is that it reā€
              quires 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.

       PTRACE_ATTACH
              Attach to the process specified in pid, making it a tracee of the calling process.  The tracee is sent a SIGSTOP, but will not necessarily have stopped by
              the completion of this call; use waitpid(2) to wait for the tracee to stop.  See the "Attaching and  detaching"  subsection  for  additional  information.
              (addr and data are ignored.)

              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  atā€
              tached 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
              immediately.

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

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

              addr is an integer specifying the index of the filter to be dumped.  The most recently installed filter has the index 0.  If addr is greater than the numā€
              ber 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.

       PTRACE_DETACH
              Restart the stopped tracee as for PTRACE_CONT, but first detach from it.  Under Linux, a tracee can be detached in this way regardless of which method was
              used to initiate tracing.  (addr is ignored.)

       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 supā€
              plied 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:

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

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

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

              PTRACE_SYSCALL_INFO_NONE
                     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 genā€
       erated 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 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 WIFā€
       STOPPED(status) true.  They may be differentiated by examining the value status>>8, and if there is ambiguity  in  that  value,  by  querying  PTRACE_GETSIGINFO.
       (Note: the WSTOPSIG(status) macro can't be used to perform this examination, because it returns the value (status>>8) & 0xff.)

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

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

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

           ptrace(PTRACE_restart, pid, 0, sig)

       where PTRACE_restart is one of the restarting ptrace requests.  If sig is 0, then a signal is not delivered.  Otherwise, the signal sig is delivered.  This operā€
       ation 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 reā€
       ported; 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  undeā€
       fined.

   Group-stop
       When  a (possibly multithreaded) process receives a stopping signal, all threads stop.  If some threads are traced, they enter a group-stop.  Note that the stopā€
       ping 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 reā€
       sult is returned by some other classes of ptrace-stops, therefore the recommended practice is to perform the call

           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

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

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

       As of Linux 2.6.38, after the tracer sees the tracee ptrace-stop and until it restarts or kills it, the tracee will not run, and will not send notifications (exā€
       cept 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 ignoredā€”the tracee doesn't remain stopped, it runs.  If the tracer doesn't restart the tracee before entering into
       the next waitpid(2), future SIGCONT signals will not be reported to the tracer; this would cause the SIGCONT signals to have no effect on the tracee.

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

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

       PTRACE_EVENT stops are observed by the tracer as waitpid(2) returning with WIFSTOPPED(status), and WSTOPSIG(status) returns SIGTRAP  (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:

       PTRACE_EVENT_VFORK
              Stop before return from vfork(2) or clone(2) with the CLONE_VFORK flag.  When the tracee is continued after this stop, it will wait for child to exit/exec
              before continuing its execution (in other words, the usual behavior on vfork(2)).

       PTRACE_EVENT_FORK
              Stop before return from fork(2) or clone(2) with the exit signal set to SIGCHLD.

       PTRACE_EVENT_CLONE
              Stop before return from clone(2).

       PTRACE_EVENT_VFORK_DONE
              Stop before return from vfork(2) or clone(2) with the CLONE_VFORK flag, but after the child unblocked this tracee by exiting or execing.

       For all four stops described above, the stop occurs in the parent (i.e., the tracee), not in the newly created thread.  PTRACE_GETEVENTMSG can  be  used  to  reā€
       trieve the new thread's ID.

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

       PTRACE_EVENT_EXIT
              Stop  before  exit (including death from exit_group(2)), signal death, or exit caused by execve(2) in a multithreaded process.  PTRACE_GETEVENTMSG returns
              the exit status.  Registers can be examined (unlike when "real" exit happens).  The tracee is still alive; it needs to be PTRACE_CONTed or PTRACE_DETACHed
              to finish exiting.

       PTRACE_EVENT_STOP
              Stop induced by PTRACE_INTERRUPT command, or group-stop, or initial ptrace-stop when a new child is attached (only if attached using PTRACE_SEIZE).

       PTRACE_EVENT_SECCOMP
              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.

   Syscall-stops
       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 PTRACE_SYSEMU_SINGLESTEP.

       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 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 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 behavā€
       ior 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.

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

   Informational and restarting ptrace commands
       Most  ptrace commands (all except PTRACE_ATTACH, PTRACE_SEIZE, PTRACE_TRACEME, PTRACE_INTERRUPT, and PTRACE_KILL) require the tracee to be in a ptrace-stop, othā€
       erwise 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 (reā€
       turn 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  meaningā€
       ful event message.

       The call

           ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

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

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

           ptrace(cmd, pid, 0, sig);

       where cmd is PTRACE_CONT, PTRACE_LISTEN, PTRACE_DETACH, PTRACE_SYSCALL, PTRACE_SINGLESTEP, PTRACE_SYSEMU, or PTRACE_SYSEMU_SINGLESTEP.  If the tracee is in  sigā€
       nal-delivery-stop,  sig is the signal to be injected (if it is nonzero).  Otherwise, sig may be ignored.  (When restarting a tracee from a ptrace-stop other than
       signal-delivery-stop, recommended practice is to always pass 0 in sig.)

   Attaching and detaching
       A thread can be attached to the tracer using the call

           ptrace(PTRACE_ATTACH, pid, 0, 0);

       or

           ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);

       PTRACE_ATTACH sends SIGSTOP to this thread.  If the tracer wants this SIGSTOP to have no effect, it needs to suppress it.  Note that if other signals are concurā€
       rently sent to this thread during attach, the tracer may see the tracee enter signal-delivery-stop with other signal(s) first!  The usual practice is to reinject
       these signals until SIGSTOP is seen, then suppress SIGSTOP injection.  The design bug here is that a ptrace attach and a concurrently delivered SIGSTOP may  race
       and the concurrent SIGSTOP may be lost.

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

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

       The request

           ptrace(PTRACE_TRACEME, 0, 0, 0);

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

           raise(SIGSTOP);

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

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

       Detaching of the tracee is performed by:

           ptrace(PTRACE_DETACH, pid, 0, sig);

       PTRACE_DETACH  is  a  restarting operation; therefore it requires the tracee to be in ptrace-stop.  If the tracee is in signal-delivery-stop, a signal can be inā€
       jected.  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 conā€
       current 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 isā€”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  sigā€
       nal 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  reā€
          port 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 posā€
          sibility 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 exā€
          ecing tracee.

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

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

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

       On receiving the PTRACE_EVENT_EXEC stop notification, the tracer should clean up all its internal data structures describing the threads of this process, and reā€
       tain only one data structureā€”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  delivā€
       ered (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  opā€
       tion 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.

RETURN VALUE
       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.

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

       EFAULT There  was an attempt to read from or write to an invalid area in the tracer's or the tracee's memory, probably because the area wasn't mapped or accessiā€
              ble.  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); unprivā€
              ileged processes cannot trace processes that they cannot send signals to or those running set-user-ID/set-group-ID programs, for obvious reasons.   Alterā€
              natively, the process may already be being traced, or (on kernels before 2.6.26) be init(1) (PID 1).

       ESRCH  The specified process does not exist, or is not currently being traced by the caller, or is not stopped (for requests that require a stopped tracee).

CONFORMING TO
       SVr4, 4.3BSD.

NOTES
       Although  arguments  to ptrace() are interpreted according to the prototype given, glibc currently declares ptrace() as a variadic function with only the request
       argument fixed.  It is recommended to always supply four arguments, even if the requested operation does not use them, setting unused/ignored arguments to 0L  or
       (void *) 0.

       In Linux kernels before 2.6.26, init(1), the process with PID 1, may not be traced.

       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 opā€
       eration 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)ā€”for  example,
       SELinux, Yama, or Smackā€”and by the commoncap LSM (which is always invoked).

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

       PTRACE_MODE_READ
              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.

       PTRACE_MODE_ATTACH
              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:

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

       PTRACE_MODE_REALCREDS
              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:

       PTRACE_MODE_READ_FSCREDS
              Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.

       PTRACE_MODE_READ_REALCREDS
              Defined as PTRACE_MODE_READ | PTRACE_MODE_REALCREDS.

       PTRACE_MODE_ATTACH_FSCREDS
              Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS.

       PTRACE_MODE_ATTACH_REALCREDS
              Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS.

       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 seā€
              curity violation and there is no reason to generate a security audit record.  This modifier suppresses the generation of such an audit record for the parā€
              ticular 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.,  unā€
       der /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 credenā€
          tials(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:

          ā€¢ 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.

          ā€¢ 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 implementaā€
          tion 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  speciā€
             fies PTRACE_MODE_REALCREDS, so) use the caller's permitted capability set.

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

             ā€¢ 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 capabilā€
               ities.

             ā€¢ 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.

   /proc/sys/kernel/yama/ptrace_scope
       On  systems  with  the  Yama  Linux  Security  Module  (LSM)  installed  (i.e.,  the  kernel  was  configured  with  CONFIG_SECURITY_YAMA),  the   /proc/sys/kerā€
       nel/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 senā€
       sitive  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 checkā€”for example, ptrace() PTRACE_ATTACH.  (See the "Ptrace access mode checking" discusā€
          sion 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 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 changed.

       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 opā€
       erations within the child user namespace (and further-removed descendants of that namespace).  Consequently, when a process tries to use user namespaces to sandā€
       box 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 reā€
       turned via the function return value.

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

       Group-stop notifications are sent to the tracer, but not to real parent.  Last confirmed on 2.6.38.6.

       If  a thread group leader is traced and exits by calling _exit(2), a PTRACE_EVENT_EXIT stop will happen for it (if requested), but the subsequent WIFEXITED notiā€
       fication will not be delivered until all other threads exit.  As explained above, if one of other threads calls execve(2), the death of the thread  group  leader
       will  never  be  reported.  If the execed thread is not traced by this tracer, the tracer will never know that execve(2) happened.  One possible workaround is to
       PTRACE_DETACH the thread group leader instead of restarting it in this case.  Last confirmed on 2.6.38.6.

       A SIGKILL signal may still cause a PTRACE_EVENT_EXIT stop before actual signal death.  This may be changed in the future; SIGKILL is meant to always  immediately
       kill tasks even under ptrace.  Last confirmed on 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 ...>_

       or

           select(6, [5], NULL, [5], NULL_

       ('_' denotes the cursor position), you observe more than one line.  For example:

               clock_gettime(CLOCK_MONOTONIC, {15370, 690928118}) = 0
               epoll_wait(4,_

       What is not visible here is that the process was blocked in epoll_wait(2) before strace(1) has attached to it.  Attaching caused epoll_wait(2) to return to  user
       space  with  the error EINTR.  In this particular case, the program reacted to EINTR by checking the current time, and then executing epoll_wait(2) again.  (Proā€
       grams 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
       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),  capabiliā€
       ties(7), signal(7)

Linux                                                                          2021-03-22                                                                      PTRACE(2)