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       ptrace - process trace


       Standard C library (libc, -lc)


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
                     When delivering system call traps, set bit 7 in  the  signal  number  (i.e.,
                     deliver  SIGTRAP|0x80).   This  makes  it easy for the tracer to distinguish
                     normal traps from those caused by a system call.

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

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

       PTRACE_GETEVENTMSG (since Linux 2.5.46)
              Retrieve a message (as an unsigned long) about the ptrace event that just happened,
              placing  it  at the address data in the tracer.  For PTRACE_EVENT_EXIT, this is the
              tracee's    exit    status.      For     PTRACE_EVENT_FORK,     PTRACE_EVENT_VFORK,
              PTRACE_EVENT_VFORK_DONE,  and  PTRACE_EVENT_CLONE,  this  is  the  PID  of  the new
              process.    For   PTRACE_EVENT_SECCOMP,   this   is   the    seccomp(2)    filter's
              SECCOMP_RET_DATA associated with the triggered rule.  (addr is ignored.)

              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.)

       PTRACE_SET_SYSCALL (since Linux 2.6.16)
              When  in  syscall-enter-stop, change the number of the system call that is about to
              be executed to the number specified in the data argument.   The  addr  argument  is
              ignored.   This  request is currently supported only on arm (and arm64, though only
              for backwards compatibility), but most other  architectures  have  other  means  of
              accomplishing  this (usually by changing the register that the userland code passed
              the system call number in).

              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 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.)

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

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

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

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

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

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

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

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

              This  operation  is  available  if  the  kernel  was  configured  with   both   the

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

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

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

       PTRACE_GET_SYSCALL_INFO (since Linux 5.3)
              Retrieve information about the system call that caused the stop.   The  information
              is  placed  into the buffer pointed by the data argument, which should be a pointer
              to a buffer of type struct ptrace_syscall_info.  The  addr  argument  contains  the
              size   of  the  buffer  pointed  to  by  the  data  argument  (i.e.,  sizeof(struct
              ptrace_syscall_info)).  The return value contains the number of bytes available  to
              be  written  by  the  kernel.   If the size of the data to be written by the kernel
              exceeds the size specified by the addr argument, the output data is truncated.

              The ptrace_syscall_info structure contains the following fields:

                  struct ptrace_syscall_info {
                      __u8 op;        /* Type of system call stop */
                      __u32 arch;     /* AUDIT_ARCH_* value; see seccomp(2) */
                      __u64 instruction_pointer; /* CPU instruction pointer */
                      __u64 stack_pointer;    /* CPU stack pointer */
                      union {
                          struct {    /* op == PTRACE_SYSCALL_INFO_ENTRY */
                              __u64 nr;       /* System call number */
                              __u64 args[6];  /* System call arguments */
                          } entry;
                          struct {    /* op == PTRACE_SYSCALL_INFO_EXIT */
                              __s64 rval;     /* System call return value */
                              __u8 is_error;  /* System call error flag;
                                                 Boolean: does rval contain
                                                 an error value (-ERRCODE) or
                                                 a nonerror return value? */
                          } exit;
                          struct {    /* op == PTRACE_SYSCALL_INFO_SECCOMP */
                              __u64 nr;       /* System call number */
                              __u64 args[6];  /* System call arguments */
                              __u32 ret_data; /* SECCOMP_RET_DATA portion
                                                 of SECCOMP_RET_TRACE
                                                 return value */
                          } seccomp;

              The op, arch, instruction_pointer, and stack_pointer fields  are  defined  for  all
              kinds  of  ptrace  system  call  stops.   The rest of the structure is a union; one
              should read only those fields that are meaningful for the kind of system call  stop
              specified by the op field.

              The  op  field  has  one  of  the  following  values  (defined in <linux/ptrace.h>)
              indicating what type of stop occurred and which part of the union is filled:

                     The entry component of the union contains information relating to  a  system
                     call entry stop.

                     The  exit  component  of the union contains information relating to a system
                     call exit stop.

                     The seccomp component of  the  union  contains  information  relating  to  a
                     PTRACE_EVENT_SECCOMP stop.

                     No component of the union contains relevant information.

   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, PTRACE_EVENT stops, 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  (or  for  PTRACE_EVENT_STOP,
       returns  the  stopping  signal if tracee is in a group-stop).  An additional bit is set in
       the higher byte of the status word: the value status>>8 will be

           ((PTRACE_EVENT_foo<<8) | SIGTRAP).

       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.   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 equally to PTRACE_SYSEMU_SINGLESTEP.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       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,   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),  the
       PTRACE_SECCOMP_GET_FILTER  request  returns the number of instructions in the BPF program,
       the PTRACE_GET_SYSCALL_INFO request returns the number of bytes available to be written by
       the kernel, and other requests return zero.

       On error, all requests return -1, and errno is set to indicate the error.  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 (before Linux 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.

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

       A tracees parent continues to be the tracer even if that tracer calls execve(2).

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

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

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

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

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

              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

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

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

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

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

              Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.




       One further modifier can be ORed with the access mode:

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

       Note  that  all  of  the  PTRACE_MODE_* constants described in this 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

       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

       (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

       (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:

            (5.1)  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

            (5.2)  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 superset  of  the  target  process's  permitted

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

                   Note that the commoncap LSM does not distinguish between PTRACE_MODE_READ  and

       (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 values:

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

              The use of PTRACE_TRACEME is unchanged.

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

              A  target  process  can  employ the prctl(2) PR_SET_PTRACER operation to declare an
              additional PID that is allowed to  perform  PTRACE_MODE_ATTACH  operations  on  the
              target.   See  the  kernel  source  file Documentation/admin-guide/LSM/Yama.rst (or
              Documentation/security/Yama.txt before Linux 4.13) for further details.

              The use of PTRACE_TRACEME is unchanged.

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

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

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

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

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


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

       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.)

       Contrary to the normal rules, the glibc wrapper for ptrace() can set errno to zero.


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