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.   For  details,  see  the  note  on
                 PTRACE_EVENT_SECCOMP  below.   The  seccomp event message
                 data (from the SECCOMP_RET_DATA portion  of  the  seccomp
                 filter rule) can be retrieved with PTRACE_GETEVENTMSG.

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

   PTRACE_GETEVENTMSG (since Linux 2.5.46)
          Retrieve a message (as an unsigned long) about the ptrace  event
          that  just  happened,  placing  it  at  the  address data in the
          tracer.   For  PTRACE_EVENT_EXIT,  this  is  the  tracee's  exit
          status.       For     PTRACE_EVENT_FORK,     PTRACE_EVENT_VFORK,
          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.   See  the  documentation  on
          syscall-stops  below.  For PTRACE_SYSEMU_SINGLESTEP, do the same
          but also singlestep if not a system call.  This call is used  by
          programs  like  User  Mode  Linux  that  want to emulate all the
          tracee's system calls.  The data  argument  is  treated  as  for
          PTRACE_CONT.   The addr argument is ignored.  These requests are
          currently supported only on x86.

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

          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.)

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

   PTRACE_SEIZE (since Linux 3.4)
          Attach to the process specified in pid, making it  a  tracee  of
          the  calling  process.   Unlike PTRACE_ATTACH, PTRACE_SEIZE does
          not   stop   the   process.    Group-stops   are   reported   as
          PTRACE_EVENT_STOP  and WSTOPSIG(status) returns the stop signal.
          Automatically attached children stop with PTRACE_EVENT_STOP  and
          WSTOPSIG(status)  returns  SIGTRAP  instead  of  having  SIGSTOP
          signal delivered to them.  execve(2) does not deliver  an  extra
          SIGTRAP.     Only    a    PTRACE_SEIZEd   process   can   accept
          PTRACE_INTERRUPT  and  PTRACE_LISTEN  commands.   The   "seized"
          behavior  just  described  is  inherited  by  children  that are
          automatically      attached      using       PTRACE_O_TRACEFORK,
          PTRACE_O_TRACEVFORK,  and  PTRACE_O_TRACECLONE.   addr  must  be
          zero.  data contains a bit mask of ptrace  options  to  activate

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

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

          addr  is  an  integer  specifying  the index of the filter to be
          dumped.  The most recently installed filter has the index 0.  If
          addr  is  greater  than  the  number  of  installed filters, the
          operation fails with the error ENOENT.

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

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

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

          This  operation  is  available if the kernel was configured with

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

   PTRACE_GET_THREAD_AREA (since Linux 2.6.0)
          This operation performs a similar  task  to  get_thread_area(2).
          It  reads the TLS entry in the GDT whose index is given in addr,
          placing a copy of the entry into the struct user_desc pointed to
          by data.  (By contrast with get_thread_area(2), the entry_number
          of the struct user_desc is ignored.)

   PTRACE_SET_THREAD_AREA (since Linux 2.6.0)
          This operation performs a similar  task  to  set_thread_area(2).
          It  sets  the TLS entry in the GDT whose index is given in addr,
          assigning it the data supplied in the struct  user_desc  pointed
          to   by   data.    (By  contrast  with  set_thread_area(2),  the
          entry_number of the struct user_desc is ignored; in other words,
          this  ptrace  operation  can't  be  used  to allocate a free TLS

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   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.
          The semantics of this stop are described in detail in a separate
          section below.

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

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

   However,  even if the tracee was continued using PTRACE_SYSCALL , it is
   not guaranteed that the next stop will be a  syscall-exit-stop.   Other
   possibilities  are  that  the  tracee  may  stop in a PTRACE_EVENT stop
   (including  seccomp  stops),  exit   (if   it   entered   _exit(2)   or
   exit_group(2)),  be  killed  by  SIGKILL,  or  die silently (if it is a
   thread group leader, the execve(2) happened in another thread, and that
   thread  is  not  traced by the same tracer; this situation is discussed

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

   Syscall-stops  can  be  distinguished  from  signal-delivery-stop  with
   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.  In general, a  syscall-enter-
   stop is always followed by syscall-exit-stop, PTRACE_EVENT stop, or the
   tracee's death; no other kinds of ptrace-stop  can  occur  in  between.
   However,  note  that  seccomp stops (see below) can cause syscall-exit-
   stops, without preceding syscall-entry-stops.  If seccomp  is  in  use,
   care needs to be taken not to misinterpret such stops as syscall-entry-

   If after syscall-enter-stop, the tracer uses a restarting command other
   than PTRACE_SYSCALL, syscall-exit-stop is not generated.

   PTRACE_GETSIGINFO  on  syscall-stops  returns SIGTRAP in si_signo, with
   si_code set to SIGTRAP or (SIGTRAP|0x80).

   PTRACE_EVENT_SECCOMP stops (Linux 3.5 to 4.7)
   The behavior of PTRACE_EVENT_SECCOMP stops and their  interaction  with
   other  kinds of ptrace stops has changed between kernel versions.  This
   documents  the  behavior  from  their  introduction  until  Linux   4.7
   (inclusive).   The  behavior  in later kernel versions is documented in
   the next section.

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

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

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

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

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

   [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,
   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


   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

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

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

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

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

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

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

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

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





   One further modifier can be ORed with the access mode:

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

   Note  that  all  of  the  PTRACE_MODE_*  constants  described  in  this
   subsection are kernel-internal, and not visible  to  user  space.   The
   constant  names  are mentioned here in order to label the various kinds
   of ptrace access mode checks that  are  performed  for  various  system
   calls  and  accesses to various pseudofiles (e.g., under /proc).  These
   names are used in other manual pages to provide a simple shorthand  for
   labeling the different kernel checks.

   The  algorithm  employed  for  ptrace  access  mode checking determines
   whether the calling process is allowed  to  perform  the  corresponding
   action  on  the  target  process.   (In the case of opening /proc/[pid]
   files, the "calling process" is the  one  opening  the  file,  and  the
   process  with  the  corresponding  PID  is  the "target process".)  The
   algorithm is as follows:

   1.  If the calling thread and the target thread are in the same  thread
       group, access is always allowed.

   2.  If  the  access  mode  specifies PTRACE_MODE_FSCREDS, then, for the
       check in the next step, employ the caller's filesystem UID and GID.
       (As  noted  in  credentials(7),  the  filesystem UID and GID almost
       always have the same values as the corresponding effective IDs.)

       Otherwise, the access mode specifies PTRACE_MODE_REALCREDS, so  use
       the  caller's  real  UID  and  GID for the checks in the next step.
       (Most APIs that check the caller's UID and GID  use  the  effective
       IDs.   For historical reasons, the PTRACE_MODE_REALCREDS check uses
       the real IDs instead.)

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

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

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

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

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

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

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

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

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

          Note  that  the  commoncap  LSM  does  not  distinguish  between

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

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

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

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

   *  ptrace() PTRACE_TRACEME.

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

   0 ("classic ptrace permissions")
          No  additional   restrictions   on   operations   that   perform
          PTRACE_MODE_ATTACH checks (beyond those imposed by the commoncap
          and other LSMs).

          The use of PTRACE_TRACEME is unchanged.

   1 ("restricted ptrace") [default value]
          When performing an operation that requires a  PTRACE_MODE_ATTACH
          check,  the  calling process must either have the CAP_SYS_PTRACE
          capability in the user namespace of the  target  process  or  it
          must have a predefined relationship with the target process.  By
          default, the predefined relationship is that the target  process
          must be a descendant of the caller.

          A   target   process  can  employ  the  prctl(2)  PR_SET_PTRACER
          operation to declare  an  additional  PID  that  is  allowed  to
          perform  PTRACE_MODE_ATTACH  operations  on the target.  See the
          kernel source file Documentation/security/Yama.txt  for  further

          The use of PTRACE_TRACEME is unchanged.

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

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

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

   With respect to values 1 and 2, note that creating a new user namespace
   effectively removes the protection offered by Yama.  This is because  a
   process  in  the  parent user namespace whose effective UID matches the
   UID of the creator of a child namespace has all capabilities (including
   CAP_SYS_PTRACE)  when  performing  operations  within  the  child  user
   namespace  (and  further-removed  descendants   of   that   namespace).
   Consequently,  when  a  process tries to use user namespaces to sandbox
   itself, it inadvertently weakens the protections offered  by  the  Yama

   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)


   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at


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