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NAME

       ptrace - process trace

SYNOPSIS

       #include <sys/ptrace.h>

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

DESCRIPTION

       The ptrace() system call provides a means by which one process (the "tracer") may observe and control the
       execution of another process (the "tracee"), and examine and change the tracee's  memory  and  registers.
       It is primarily used to implement breakpoint debugging and system call tracing.

       A tracee first needs to be attached to the tracer.  Attachment and 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:

       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 return, the kernel modifies
              iov.len to indicate the actual number of bytes returned.

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

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

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

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

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

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

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

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

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

       PTRACE_SETOPTIONS (since Linux 2.4.6; see BUGS for caveats)
              Set ptrace options from data.  (addr is ignored.)  data is interpreted as a bit mask  of  options,
              which are specified by the following flags:

              PTRACE_O_EXITKILL (since Linux 3.8)
                     Send  a SIGKILL signal to the tracee if the tracer exits.  This option is useful for ptrace
                     jailers that want to ensure that tracees can never escape the tracer's control.

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

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

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

                     This option may not catch clone(2) calls in all cases.  If the tracee calls  clone(2)  with
                     the  CLONE_VFORK  flag, PTRACE_EVENT_VFORK will be delivered instead if PTRACE_O_TRACEVFORK
                     is set; otherwise if the tracee calls  clone(2)  with  the  exit  signal  set  to  SIGCHLD,
                     PTRACE_EVENT_FORK will be delivered if PTRACE_O_TRACEFORK is set.

              PTRACE_O_TRACEEXEC (since Linux 2.5.46)
                     Stop  the  tracee  at  the next execve(2).  A waitpid(2) by the tracer will return a status
                     value such that

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

                     If the execing thread is not a thread group leader, the thread ID is reset to thread  group
                     leader's  ID before this stop.  Since Linux 3.0, the former thread ID can be retrieved with
                     PTRACE_GETEVENTMSG.

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

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

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

                     The tracee is stopped early during  process  exit,  when  registers  are  still  available,
                     allowing the tracer to see where the exit occurred, whereas the normal exit 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_TRACESYSGOOD may not work on all architectures.)

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

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

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

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

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

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

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

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

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

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

       PTRACE_GETEVENTMSG (since Linux 2.5.46)
              Retrieve  a message (as an unsigned long) about the ptrace event that just happened, placing it at
              the address data in the tracer.  For PTRACE_EVENT_EXIT, this is the  tracee's  exit  status.   For
              PTRACE_EVENT_FORK,  PTRACE_EVENT_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  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_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 requires the tracee to be in signal-delivery-
              stop, otherwise it may not work (i.e., may complete successfully but won't kill the  tracee).   By
              contrast, sending a SIGKILL directly has no such limitation.

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

       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    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 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 number of installed  filters,  the  operation
              fails with the error ENOENT.

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

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

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

              This operation is available if the kernel was configured with both the  CONFIG_SECCOMP_FILTER  and
              the CONFIG_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 supplied in the struct user_desc pointed to by
              data.   (By contrast with set_thread_area(2), the entry_number of the struct user_desc is ignored;
              in other words, this ptrace operation can't be used to allocate a free TLS entry.)

   Death under ptrace
       When a (possibly multithreaded) process receives a killing  signal  (one  whose  disposition  is  set  to
       SIG_DFL  and  whose default action is to kill the process), all threads exit.  Tracees report their death
       to their tracer(s).  Notification of this event is delivered via waitpid(2).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   Signal-delivery-stop
       When  a  (possibly  multithreaded)  process  receives  any  signal  except SIGKILL, the kernel selects an
       arbitrary thread which handles the 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.

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

       Group-stop is observed by the tracer as waitpid(2)  returning  with  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
       call.

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

       The kernel behavior described in the previous paragraph causes a 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).

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

       PTRACE_EVENT  stops  are  observed  by  the  tracer  as waitpid(2) returning with WIFSTOPPED(status), and
       WSTOPSIG(status) returns SIGTRAP.  An additional bit is set in the higher byte of the  status  word:  the
       value status>>8 will be

           (SIGTRAP | PTRACE_EVENT_foo << 8).

       The following events exist:

       PTRACE_EVENT_VFORK
              Stop  before  return  from  vfork(2)  or  clone(2)  with the CLONE_VFORK flag.  When the tracee is
              continued after this stop, it will wait for child to exit/exec before continuing its 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 retrieve the new thread's ID.

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

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

       PTRACE_EVENT_STOP
              Stop  induced  by PTRACE_INTERRUPT command, or group-stop, or 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 behavior from their  introduction  until  Linux  4.7
       (inclusive).  The behavior in later kernel versions is documented in the next section.

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

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

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

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

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

   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, otherwise they fail with ESRCH.

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

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

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

       The call

           ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

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

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

           ptrace(cmd, pid, 0, sig);

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

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

           ptrace(PTRACE_ATTACH, pid, 0, 0);

       or

           ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);

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

       Since  attaching  sends SIGSTOP and the tracer usually suppresses it, this may cause a stray EINTR return
       from the currently executing system call in the  tracee,  as  described  in  the  "Signal  injection  and
       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  injected.  Otherwise, the sig parameter may be
       silently ignored.

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

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

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

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

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

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

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

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

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

RETURN VALUE

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

       On  error,  all  requests  return  -1,  and  errno  is  set appropriately.  Since the value returned by a
       successful PTRACE_PEEK* request may be -1, the caller must clear errno before the call, and then check it
       afterward to determine whether or not an error occurred.

ERRORS

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

       EFAULT There  was  an  attempt  to  read from or write to an invalid area in the tracer's or the tracee's
              memory, probably because the area  wasn't  mapped  or  accessible.   Unfortunately,  under  Linux,
              different variations of this fault will return EIO or EFAULT more or less arbitrarily.

       EINVAL An attempt was made to set an invalid option.

       EIO    request  is  invalid,  or  an  attempt  was  made  to read from or write to an invalid area in the
              tracer's or the tracee's memory, or there was a word-alignment violation, or an invalid signal was
              specified during a restart request.

       EPERM  The  specified  process  cannot  be  traced.   This  could  be because the tracer has insufficient
              privileges (the required  capability  is  CAP_SYS_PTRACE);  unprivileged  processes  cannot  trace
              processes that they cannot send signals to or those running set-user-ID/set-group-ID programs, for
              obvious reasons.  Alternatively, the process may already be being traced, or  (on  kernels  before
              2.6.26) be init(1) (PID 1).

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

CONFORMING TO

       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 operation is permitted (or, in a few cases,
       causes a "read" operation to return sanitized data).  These checks  are  performed  in  cases  where  one
       process  can  inspect sensitive information about, or in some cases modify the state of, another process.
       The checks are based on factors such as the credentials and capabilities of the two processes, whether or
       not  the  "target" process is dumpable, and the results of checks performed by any enabled Linux Security
       Module (LSM)—for example, SELinux, Yama, or Smack—and by the commoncap LSM (which is always invoked).

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

       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  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 instead.)

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

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

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

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

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

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

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

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

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

             Note that the commoncap LSM does not distinguish between 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/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 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  operations  within  the  child  user  namespace  (and  further-removed  descendants  of  that
       namespace).   Consequently,  when  a  process  tries  to  use  user  namespaces  to  sandbox  itself,  it
       inadvertently weakens the protections offered by the Yama LSM.

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

BUGS

       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 notification will not be delivered  until  all  other
       threads exit.  As explained above, if one of other threads calls execve(2), the death of the thread group
       leader will never be reported.  If the execed thread is not traced by this tracer, the tracer will  never
       know  that  execve(2)  happened.   One  possible  workaround  is to PTRACE_DETACH the thread group leader
       instead of restarting it in this case.  Last confirmed on 2.6.38.6.

       A SIGKILL signal may still cause a PTRACE_EVENT_EXIT stop  before  actual  signal  death.   This  may  be
       changed  in  the  future;  SIGKILL  is  meant  to  always immediately kill tasks even under ptrace.  Last
       confirmed on 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.
       (Programs which do not expect such "stray" EINTR errors may behave in an unintended way upon an strace(1)
       attach.)

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), capabilities(7), signal(7)

COLOPHON

       This  page  is  part  of  release  4.15  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
       https://www.kernel.org/doc/man-pages/.