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       ptrace - process trace


       #include <sys/ptrace.h>

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


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

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

           ptrace(PTRACE_foo, pid, ...)

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

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

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

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

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

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

       The value of request determines the action to be performed:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              PTRACE_O_EXITKILL (since Linux 3.8)
                     If a tracer sets this flag, a SIGKILL signal will be sent
                     to every tracee if the  tracer  exits.   This  option  is
                     useful  for  ptrace  jailers  that  want  to  ensure that
                     tracees can never escape the tracer's control.

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

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

                     The  PID  of  the  new  process  can  be  retrieved  with

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

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

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

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

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

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

                     The  tracee's  exit  status   can   be   retrieved   with

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

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

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

                     The  PID  of  the  new  process  can  be  retrieved  with

              PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
                     When  delivering  system  call  traps,  set  bit 7 in the
                     signal number (i.e., deliver SIGTRAP|0x80).   This  makes
                     it  easy  for the tracer to distinguish normal traps from
                     those caused by a  system  call.   (PTRACE_O_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, in that  the  tracee  has  not  yet
                     entered  the  syscall  that  seccomp  triggered  on.  The
                     seccomp event message  data  (from  the  SECCOMP_RET_DATA
                     portion of the seccomp filter rule) can be retrieved with

              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.)

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

              Restart the stopped tracee as for PTRACE_CONT, but  arrange  for
              the  tracee  to  be  stopped at the next entry to or exit from a
              system  call,  or  after  execution  of  a  single  instruction,
              respectively.   (The tracee will also, as usual, be stopped upon
              receipt of a signal.)  From the tracer's perspective, the tracee
              will  appear  to have been stopped by receipt of a SIGTRAP.  So,
              for PTRACE_SYSCALL, for example, the  idea  is  to  inspect  the
              arguments  to the system call at the first stop, then do another
              PTRACE_SYSCALL and inspect the return value of the  system  call
              at  the  second  stop.   The  data  argument  is  treated as for
              PTRACE_CONT.  (addr is ignored.)

              For PTRACE_SYSEMU, continue and stop on entry to the next system
              call, which will not be executed.  For PTRACE_SYSEMU_SINGLESTEP,
              do the same but also singlestep if not a system call.  This call
              is  used  by  programs like User Mode Linux that want to emulate
              all the tracee's system calls.  The data argument is treated  as
              for  PTRACE_CONT.  The addr argument is ignored.  These requests
              are currently supported only on x86.

       PTRACE_LISTEN (since Linux 3.4)
              Restart the stopped tracee, but prevent it from executing.   The
              resulting  state of the tracee is similar to a process which has
              been stopped by a SIGSTOP (or other stopping signal).   See  the
              "group-stop"     subsection    for    additional    information.
              PTRACE_LISTEN works only on tracees attached by PTRACE_SEIZE.

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

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

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

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

              Processes   that  are  not  dumpable  (see  the  description  of
              PR_SET_DUMPABLE  in  prctl(2))  can   not   be   attached    via
              ptrace(2) PTRACE_ATTACH.

       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

              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.)

   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, and so on.  These stopped states are described in  detail

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

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

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

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

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

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

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

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

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

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

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

           ptrace(PTRACE_restart, pid, 0, sig)

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

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

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

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

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

           ptrace(PTRACE_restart, pid, 0, stopsig)

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

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

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

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

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

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

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

           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

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

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

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

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

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

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

       PTRACE_EVENT  stops  are observed by the tracer as waitpid(2) returning
       with WIFSTOPPED(status),  and  WSTOPSIG(status)  returns  SIGTRAP.   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
              execution (in other words, the usual behavior on vfork(2)).

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

              Stop before return from clone(2).

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

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

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

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

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

              Stop triggered by a seccomp(2) rule on tracee syscall entry when
              PTRACE_O_TRACESECCOMP  has  been set by the tracer.  The seccomp
              event message data (from the  SECCOMP_RET_DATA  portion  of  the
              seccomp filter rule) can be retrieved with PTRACE_GETEVENTMSG.

       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,  the  tracee  enters
       syscall-enter-stop  just  prior  to  entering  any system call.  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

       Other possibilities are that the tracee  may  stop  in  a  PTRACE_EVENT
       stop,  exit  (if  it  entered  _exit(2) or exit_group(2)), be killed by
       SIGKILL, or die silently (if it is a thread group leader, the execve(2)
       happened  in  another thread, and that thread is not traced by the same
       tracer; this situation is discussed later).

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

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

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

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

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

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

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

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

       Syscall-enter-stop  and  syscall-exit-stop  are  indistinguishable from
       each other by the tracer.  The  tracer  needs  to  keep  track  of  the
       sequence  of  ptrace-stops  in order to not misinterpret syscall-enter-
       stop as syscall-exit-stop or vice versa.  The  rule  is  that  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

       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).

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

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

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

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

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

       The call

           ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

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

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

           ptrace(cmd, pid, 0, sig);

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

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

           ptrace(PTRACE_ATTACH, pid, 0, 0);


           ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);

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

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

       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

       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

       *  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,
              different  variations  of  this  fault will return EIO or EFAULT
              more or less arbitrarily.

       EINVAL An attempt was made to set an invalid option.

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

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

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


       SVr4, 4.3BSD.


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

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

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

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

       On  systems  with  the  Yama  Linux  Security  Module  installed,   the
       /proc/sys/kernel/yama/ptrace_scope  (available  since Linux 3.4) can be
       used to restrict the ability to trace a  process  with  ptrace(2)  (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 and thus expand the scope of the attack.

       A  process with the CAP_SYS_PTRACE capability can update this file with
       one of the following values:

       0 ("classic ptrace permissions")
              A process can PTRACE_ATTACH to any other process  running  under
              the  same  UID,  as long as the tracee is dumpable (i.e., it did
              did not transition UIDs or start start privileged,  and  it  has
              not  employed  the  prctl(2)  PR_SET_DUMPABLE  operation to make
              itself nondumpable).

              The use of PTRACE_TRACEME is unchanged.

       1 ("restricted ptrace")
              A process must have a predefined relationship with the tracee to
              which  it  wants  to  PTRACE_ATTACH.  By default, the predefined
              relationship is that the tracee is a child of the caller and the
              aforementioned  classic  criteria  are  also  met.  To specify a
              different process that may trace it, a  tracee  can  employ  the
              prctl(2)  PR_SET_PTRACER  operation  to  declare a different PID
              that is allowed to PTRACE_ATTACH to the tracee.  (See the kernel
              source    file   Documentation/security/Yama.txt   for   further

              The use of PTRACE_TRACEME is unchanged.

       2 ("admin-only attach")
              Only  processes  with  the  CAP_SYS_PTRACE  capability  may  use
              PTRACE_ATTACH or trace children that employ PTRACE_TRACEME.

       3 ("no attach")
              No  process  may use PTRACE_ATTACH or trace children that employ

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

   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
       compiled with 2.6 kernel headers failing when run on 2.4 kernels.  This
       can    be    worked   around   by   redefining   PTRACE_SETOPTIONS   to
       PTRACE_OLDSETOPTIONS, if that is defined.

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

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

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

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

           strace -p <process-ID>

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

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


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

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

           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), 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)


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