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

       ptrace - process trace

       #include <sys/ptrace.h>

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

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

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

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

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

       The value of request determines the action to be performed:

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

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

              Read a word at the address addr in the tracee's memory,  return-
              ing 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                     The  PID  of  the  new  process  can  be  retrieved  with

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

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

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

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

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

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

                     The   tracee's   exit   status   can  be  retrieved  with

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

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

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

                     The  PID  of  the  new  process  can  be  retrieved  with

              PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
                     When delivering system call traps, set bit 7 in the  sig-
                     nal  number  (i.e., deliver SIGTRAP|0x80).  This makes it
                     easy for the tracer  to  distinguish  normal  traps  from
                     those  caused  by  a system call.  (PTRACE_O_TRACESYSGOOD
                     may not work on all architectures.)

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

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

                     The  PID  of  the  new  process  can  be  retrieved  with

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

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

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

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

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

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

              PTRACE_O_SUSPEND_SECCOMP (since Linux 4.3)
                     Suspend the tracee's seccomp protections.   This  applies
                     regardless  of  mode, and can be used when the tracee has
                     not yet installed seccomp filters.  That is, a valid  use
                     case  is to suspend a tracee's seccomp protections before
                     they are installed by the tracee, let the tracee  install
                     the  filters,  and  then clear this flag when the filters
                     should be resumed.  Setting this option requires that the
                     tracer  have  the  CAP_SYS_ADMIN capability, not have any
                     seccomp protections installed, and not have PTRACE_O_SUS-
                     PEND_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 sta-
              tus.       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  sec-
              comp(2)  filter's SECCOMP_RET_DATA associated with the triggered
              rule.  (addr is ignored.)

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

              Restart the stopped tracee as for PTRACE_CONT, but  arrange  for
              the  tracee  to  be  stopped at the next entry to or exit from a
              system call, or after execution of a single instruction, respec-
              tively.   (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.)

              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_LIS-
              TEN works only on tracees attached by PTRACE_SEIZE.

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

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

       PTRACE_INTERRUPT (since Linux 3.4)
              Stop  a  tracee.  If the tracee is running or sleeping in kernel
              space and PTRACE_SYSCALL is in effect, the system call is inter-
              rupted and syscall-exit-stop is reported.  (The interrupted sys-
              tem 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  WSTOP-
              SIG(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) == SIG-
              TRAP.   PTRACE_INTERRUPT  only  works  on  tracees  attached  by

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

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

       PTRACE_SEIZE (since Linux 3.4)
              Attach  to  the  process specified in pid, making it a tracee of
              the calling process.  Unlike  PTRACE_ATTACH,  PTRACE_SEIZE  does
              not   stop   the   process.    Group-stops   are   reported   as
              PTRACE_EVENT_STOP and WSTOPSIG(status) returns the stop  signal.
              Automatically  attached children stop with PTRACE_EVENT_STOP and
              WSTOPSIG(status) returns SIGTRAP instead of having SIGSTOP  sig-
              nal delivered to them.  execve(2) does not deliver an extra SIG-
              TRAP.  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 opera-
              tion 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

              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

   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  multi-
       threaded 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  wait-
       pid(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 refus-
       ing ptrace requests.

       The tracer can't assume that the tracee always ends its life by report-
       ing  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) noti-
       fications),  but  it also may be considered "stopped" because it is not
       executing instructions (is not scheduled), and if it was in  group-stop
       before  PTRACE_LISTEN,  it will not respond to signals until SIGCONT is

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

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

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

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

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

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

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

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

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

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

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

       Signal-delivery-stop  is observed by the tracer as waitpid(2) returning
       with WIFSTOPPED(status) true, with the signal returned by WSTOPSIG(sta-
       tus).   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 pre-
       maturely.  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  dif-
       ferent  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-deliv-
       ery-stop.  The tracer can't suppress this side effect (it can only sup-
       press signal injection, which only causes the SIGCONT handler to not be
       executed in the tracee, if such a handler is installed).  In fact, wak-
       ing up from group-stop may be followed by signal-delivery-stop for sig-
       nal(s) other than SIGCONT, if they were pending when SIGCONT was deliv-
       ered.   In other words, SIGCONT may be not the first signal observed by
       the tracee after it was sent.

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

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

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

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

       Group-stop  is observed by the tracer as waitpid(2) returning with WIF-
       STOPPED(status) true, with the stopping  signal  available  via  WSTOP-
       SIG(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

       If tracee was attached using PTRACE_SEIZE, group-stop is  indicated  by
       PTRACE_EVENT_STOP: status>>16 == PTRACE_EVENT_STOP.  This allows detec-
       tion 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 prob-
       lem with transparent handling  of  stopping  signals.   If  the  tracer
       restarts  the  tracee  after  group-stop, the stopping signal is effec-
       tively ignored--the tracee doesn't remain stopped,  it  runs.   If  the
       tracer  doesn't  restart the tracee before entering into the next wait-
       pid(2), future SIGCONT signals will not be reported to the tracer; this
       would cause the SIGCONT signals to have no effect on the tracee.

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

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

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

           (SIGTRAP | PTRACE_EVENT_foo << 8).

       The following events exist:

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

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

              Stop before return from clone(2).

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

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

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

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

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

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

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

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

       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

       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  SIG-
       TRAP by querying PTRACE_GETSIGINFO for the following cases:

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

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

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

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

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

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

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

       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 (inclu-
       sive).  The behavior in later kernel versions is documented in the next

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

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

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

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

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

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

   Informational and restarting ptrace commands
       Most  ptrace  commands   (all   except   PTRACE_ATTACH,   PTRACE_SEIZE,
       PTRACE_TRACEME,  PTRACE_INTERRUPT,  and PTRACE_KILL) require the tracee
       to be in a ptrace-stop, otherwise they fail with ESRCH.

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

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

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

       The call

           ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

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

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

           ptrace(cmd, pid, 0, sig);

       tracee is in signal-delivery-stop, sig is the signal to be injected (if
       it  is  nonzero).   Otherwise,  sig may be ignored.  (When restarting a
       tracee from a ptrace-stop other than signal-delivery-stop,  recommended
       practice is to always pass 0 in sig.)

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

           ptrace(PTRACE_ATTACH, pid, 0, 0);


           ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);

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

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

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

       The request

           ptrace(PTRACE_TRACEME, 0, 0, 0);

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


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

       options are in effect, then children created by, respectively, vfork(2)
       or  clone(2)  with  the  CLONE_VFORK flag, fork(2) or clone(2) with the
       exit signal set to SIGCHLD, and other kinds of clone(2), are  automati-
       cally  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

       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 wait-
          pid(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.

       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.

       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,  dif-
              ferent  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 pro-
              cesses that they cannot send signals to or  those  running  set-
              user-ID/set-group-ID  programs,  for  obvious reasons.  Alterna-
              tively, the process may already be being traced, or (on  kernels
              before 2.6.26) be init(1) (PID 1).

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

       SVr4, 4.3BSD.

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

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

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

       The  layout of the contents of memory and the USER area are quite oper-
       ating-system- and architecture-specific.  The offset supplied, and  the
       data  returned,  might not entirely match with the definition of struct

       The size of a "word" is  determined  by  the  operating-system  variant
       (e.g., for 32-bit Linux it is 32 bits).

       This page documents the way the ptrace() call works currently in Linux.
       Its behavior differs significantly on other flavors of  UNIX.   In  any
       case,  use  of  ptrace() is highly specific to the operating system and

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

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

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

              For "write" operations, or other operations that are  more  dan-
              gerous,  such  as:  ptrace  attaching (PTRACE_ATTACH) to another
              process or  calling  process_vm_writev(2).   (PTRACE_MODE_ATTACH
              was effectively the default before Linux 2.6.27.)

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

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

              Use  the caller's real UID and GID or permitted capabilities for
              LSM checks.  This was effectively the default before Linux 4.5.

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

              Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.




       One further modifier can be ORed with the access mode:

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

       Note  that all of the PTRACE_MODE_* constants described in this subsec-
       tion 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 algo-
       rithm 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:

          o 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.

          o 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 capa-
          bility 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;  other-
             wise  (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:

             o The caller and the target process are in the same  user  names-
               pace,  and  the  caller's capabilities are a proper superset of
               the target process's permitted capabilities.

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

             Note   that  the  commoncap  LSM  does  not  distinguish  between

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

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

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

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

       *  ptrace() PTRACE_TRACEME.

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

       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 opera-
              tion 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  Documen-
              tation/security/Yama.txt before Linux 4.13) for further details.

              The use of PTRACE_TRACEME is unchanged.

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

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

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

       With respect to values 1 and 2, note that creating a new user namespace
       effectively removes the protection offered by Yama.  This is because  a
       process  in  the  parent user namespace whose effective UID matches the
       UID of the creator of a child namespace has all capabilities (including
       CAP_SYS_PTRACE) when performing operations within the child user names-
       pace (and  further-removed  descendants  of  that  namespace).   Conse-
       quently, 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.

       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  com-
       piled  with  2.6  kernel headers failing when run on 2.4 kernels.  This
       can be worked around by redefining PTRACE_SETOPTIONS  to  PTRACE_OLDSE-
       TOPTIONS, if that is defined.

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

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

       A SIGKILL signal may still cause a PTRACE_EVENT_EXIT stop before actual
       signal  death.   This may be changed in the future; SIGKILL is meant to
       always immediately kill tasks even under  ptrace.   Last  confirmed  on
       Linux 3.13.

       Some  system  calls return with EINTR if a signal was sent to a tracee,
       but delivery was suppressed by the tracer.  (This is very typical oper-
       ation: it is usually done by debuggers on every attach, in order to not
       introduce a bogus SIGSTOP).  As of Linux 3.2.9,  the  following  system
       calls are affected (this list is likely incomplete): epoll_wait(2), and
       read(2) from an inotify(7) file descriptor.  The usual symptom of  this
       bug is that when you attach to a quiescent process with the command

           strace -p <process-ID>

       then, instead of the usual and expected one-line output such as

           restart_syscall(<... resuming interrupted call ...>_


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

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

               clock_gettime(CLOCK_MONOTONIC, {15370, 690928118}) = 0

       What  is  not  visible  here  is  that  the  process  was  blocked   in
       epoll_wait(2)  before  strace(1)  has attached to it.  Attaching caused
       epoll_wait(2) to return to user space with the error  EINTR.   In  this
       particular  case,  the program reacted to EINTR by checking the current
       time, and then executing epoll_wait(2) again.  (Programs which  do  not
       expect  such  "stray" EINTR errors may behave in an unintended way upon
       an strace(1) attach.)

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

       This page is part of release 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

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