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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 tracing.

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

           ptrace(PTRACE_foo, pid, ...)

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

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

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

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

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

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

       The value of request determines the action to be performed:

              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

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

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

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

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

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

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

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

       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

       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.,

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

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

              PTRACE_O_EXITKILL (since Linux 3.8)
                     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 PTRACE_GETEVENTMSG.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              PTRACE_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 ignored.)

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

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

              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 immediately.

              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

       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 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

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

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

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

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

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

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

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

           ptrace(PTRACE_restart, pid, 0, sig)

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

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

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

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

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

           ptrace(PTRACE_restart, pid, 0, stopsig)

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

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

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

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

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

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

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

           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

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

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

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

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

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

       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

              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 exiting.

              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 syscall-exit-stop.)

       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 expensive.

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

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

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

       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 PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, or
       PTRACE_O_TRACECLONE options.

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

           ptrace(cmd, pid, 0, sig);

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

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

           ptrace(PTRACE_ATTACH, pid, 0, 0);


           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-delivery-stop.

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

       Detaching of the tracee is performed by:

           ptrace(PTRACE_DETACH, pid, 0, sig);

       PTRACE_DETACH is a restarting operation; therefore it requires the tracee to be in ptrace-
       stop.  If the tracee is in signal-delivery-stop, a signal can be injected.  Otherwise, the
       sig parameter may be silently ignored.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   Real parent
       The  ptrace  API  (ab)uses the standard UNIX parent/child signaling over waitpid(2).  This
       used to cause the real parent of the process to stop receiving several kinds of waitpid(2)
       notifications when the child process is traced by some other process.

       Many  of  these bugs have been fixed, but as of Linux 2.6.38 several still exist; see BUGS

       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


       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 execve(2).

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

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

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

       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 details.)

              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 PTRACE_TRACEME.

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

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


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

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

       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 example:

           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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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at