GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


man:ptrace

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

NAME

     ptrace - process trace

SYNOPSIS

     #include <sys/ptrace.h>
     long ptrace(enum __ptrace_request request, pid_t pid,
                 void *addr, void *data);

DESCRIPTION

     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 sub-
     sequent commands are per thread:  in  a  multithreaded  process,  every
     thread  can  be  individually  attached  to  a  (potentially different)
     tracer, or  left  not  attached  and  thus  not  debugged.   Therefore,
     "tracee" always means "(one) thread", never "a (possibly multithreaded)
     process".  Ptrace commands are always sent to a specific tracee using a
     call of the form
         ptrace(PTRACE_foo, pid, ...)
     where pid is the thread ID of the corresponding Linux thread.
     (Note that in this page, a "multithreaded process" means a thread group
     consisting of threads created using the clone(2) CLONE_THREAD flag.)
     A process can initiate a  trace  by  calling  fork(2)  and  having  the
     resulting  child  do  a  PTRACE_TRACEME,  followed  (typically)  by  an
     execve(2).  Alternatively, one process  may  commence  tracing  another
     process using PTRACE_ATTACH or PTRACE_SEIZE.
     While  being  traced, the tracee will stop each time a signal is deliv-
     ered, even if the signal is being ignored.  (An exception  is  SIGKILL,
     which  has  its usual effect.)  The tracer will be notified at its next
     call to waitpid(2) (or one of the related "wait"  system  calls);  that
     call  will  return a status value containing information that indicates
     the cause of the stop in the tracee.  While the tracee is stopped,  the
     tracer  can  use  various  ptrace  requests  to  inspect and modify the
     tracee.  The tracer then causes  the  tracee  to  continue,  optionally
     ignoring  the  delivered  signal (or even delivering a different signal
     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  pro-
     gram begins execution.
     When  the  tracer  is finished tracing, it can cause the tracee to con-
     tinue executing in a normal, untraced mode via PTRACE_DETACH.
     The value of request determines the action to be performed:
     PTRACE_TRACEME
            Indicate that this process is to be traced  by  its  parent.   A
            process probably shouldn't make this request if its parent isn't
            expecting to trace it.  (pid, addr, and data are ignored.)
            The PTRACE_TRACEME request is  used  only  by  the  tracee;  the
            remaining  requests are used only by the tracer.  In the follow-
            ing requests, pid specifies the thread ID of the  tracee  to  be
            acted  on.  For requests other than PTRACE_ATTACH, PTRACE_SEIZE,
            PTRACE_INTERRUPT, and PTRACE_KILL, the tracee must be stopped.
     PTRACE_PEEKTEXT, PTRACE_PEEKDATA
            Read a word at the address addr in the tracee's memory,  return-
            ing the word as the result of the ptrace() call.  Linux does not
            have separate  text  and  data  address  spaces,  so  these  two
            requests  are  currently  equivalent.  (data is ignored; but see
            NOTES.)
     PTRACE_PEEKUSER
            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.)
     PTRACE_POKETEXT, PTRACE_POKEDATA
            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.
     PTRACE_POKEUSER
            Copy the word data to offset addr in the tracee's USER area.  As
            for PTRACE_PEEKUSER, the offset must typically be  word-aligned.
            In order to maintain the integrity of the kernel, some modifica-
            tions to the USER area are disallowed.
     PTRACE_GETREGS, PTRACE_GETFPREGS
            Copy the tracee's general-purpose or  floating-point  registers,
            respectively,   to   the   address  data  in  the  tracer.   See
            <sys/user.h> for information on the format of this data.   (addr
            is  ignored.)   Note that SPARC systems have the meaning of data
            and addr reversed; that is, data is ignored  and  the  registers
            are copied to the address addr.  PTRACE_GETREGS and PTRACE_GETF-
            PREGS are not present on all architectures.
     PTRACE_GETREGSET (since Linux 2.6.34)
            Read the tracee's registers.  addr specifies,  in  an  architec-
            ture-dependent way, the type of registers to be read.  NT_PRSTA-
            TUS (with numerical value 1) usually results in reading of  gen-
            eral-purpose  registers.  If the CPU has, for example, floating-
            point and/or vector registers, they can be retrieved by  setting
            addr  to  the  corresponding  NT_foo constant.  data points to a
            struct iovec, which describes the destination buffer's  location
            and  length.  On return, the kernel modifies iov.len to indicate
            the actual number of bytes returned.
     PTRACE_SETREGS, PTRACE_SETFPREGS
            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_GETREGSET.
     PTRACE_GETSIGINFO (since Linux 2.3.99-pre6)
            Retrieve information about the  signal  that  caused  the  stop.
            Copy a siginfo_t structure (see sigaction(2)) from the tracee to
            the address data in the tracer.  (addr is ignored.)
     PTRACE_SETSIGINFO (since Linux 2.3.99-pre6)
            Set signal information: copy  a  siginfo_t  structure  from  the
            address data in the tracer to the tracee.  This will affect only
            signals that would normally be delivered to the tracee and  were
            caught  by the tracer.  It may be difficult to tell these normal
            signals from synthetic signals  generated  by  ptrace()  itself.
            (addr is ignored.)
     PTRACE_PEEKSIGINFO (since Linux 3.10)
            Retrieve  siginfo_t  structures  without removing signals from a
            queue.  addr points to a ptrace_peeksiginfo_args structure  that
            specifies  the  ordinal  position  from which copying of signals
            should start, and the number  of  signals  to  copy.   siginfo_t
            structures  are  copied into the buffer pointed to by data.  The
            return value contains the number of copied signals  (zero  indi-
            cates  that  there  is  no signal corresponding to the specified
            ordinal position).  Within the returned siginfo structures,  the
            si_code field includes information (__SI_CHLD, __SI_FAULT, etc.)
            that are not otherwise exposed to user space.
         struct ptrace_peeksiginfo_args {
             u64 off;    /* Ordinal position in queue at which
                            to start copying signals */
             u32 flags;  /* PTRACE_PEEKSIGINFO_SHARED or 0 */
             s32 nr;     /* Number of signals to copy */ };
            Currently, there is only  one  flag,  PTRACE_PEEKSIGINFO_SHARED,
            for dumping signals from the process-wide signal queue.  If this
            flag is not set, signals are read from the per-thread  queue  of
            the specified thread.
     PTRACE_GETSIGMASK (since Linux 3.11)
            Place a copy of the mask of blocked signals (see sigprocmask(2))
            in the buffer pointed to by data, which should be a pointer to a
            buffer of type sigset_t.  The addr argument contains the size of
            the buffer pointed to by data (i.e., sizeof(sigset_t)).
     PTRACE_SETSIGMASK (since Linux 3.11)
            Change the mask of blocked signals (see sigprocmask(2))  to  the
            value  specified  in the buffer pointed to by data, which should
            be a pointer to a buffer of type sigset_t.   The  addr  argument
            contains  the  size  of  the  buffer  pointed  to by data (i.e.,
            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  con-
                   trol.
            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  notifi-
                   cation  is  done  after  the process is finished exiting.
                   Even though context is available, the tracer cannot  pre-
                   vent the exit from happening at this point.
            PTRACE_O_TRACEFORK (since Linux 2.5.46)
                   Stop  the  tracee  at  the next fork(2) and automatically
                   start tracing the newly forked process, which will  start
                   with  a SIGSTOP, or PTRACE_EVENT_STOP if PTRACE_SEIZE was
                   used.  A waitpid(2) by the tracer will  return  a  status
                   value such that
                     status>>8 == (SIGTRAP | (PTRACE_EVENT_FORK<<8))
                   The  PID  of  the  new  process  can  be  retrieved  with
                   PTRACE_GETEVENTMSG.
            PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
                   When delivering system call traps, set bit 7 in the  sig-
                   nal  number  (i.e., deliver SIGTRAP|0x80).  This makes it
                   easy for the tracer  to  distinguish  normal  traps  from
                   those  caused  by  a system call.  (PTRACE_O_TRACESYSGOOD
                   may not work on all architectures.)
            PTRACE_O_TRACEVFORK (since Linux 2.5.46)
                   Stop the tracee at the next  vfork(2)  and  automatically
                   start tracing the newly vforked process, which will start
                   with a SIGSTOP, or PTRACE_EVENT_STOP if PTRACE_SEIZE  was
                   used.   A  waitpid(2)  by the tracer will return a status
                   value such that
                     status>>8 == (SIGTRAP | (PTRACE_EVENT_VFORK<<8))
                   The  PID  of  the  new  process  can  be  retrieved  with
                   PTRACE_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_GETEVENTMSG.
            PTRACE_O_TRACESECCOMP (since Linux 3.5)
                   Stop the tracee when a seccomp(2) SECCOMP_RET_TRACE  rule
                   is  triggered.   A waitpid(2) by the tracer will return a
                   status value such that
                     status>>8 == (SIGTRAP | (PTRACE_EVENT_SECCOMP<<8))
                   While this triggers a PTRACE_EVENT stop, it is similar to
                   a  syscall-enter-stop.   For  details,  see  the  note on
                   PTRACE_EVENT_SECCOMP below.  The  seccomp  event  message
                   data  (from  the  SECCOMP_RET_DATA portion of the seccomp
                   filter rule) can be retrieved with PTRACE_GETEVENTMSG.
            PTRACE_O_SUSPEND_SECCOMP (since Linux 4.3)
                   Suspend the tracee's seccomp protections.   This  applies
                   regardless  of  mode, and can be used when the tracee has
                   not yet installed seccomp filters.  That is, a valid  use
                   case  is to suspend a tracee's seccomp protections before
                   they are installed by the tracee, let the tracee  install
                   the  filters,  and  then clear this flag when the filters
                   should be resumed.  Setting this option requires that the
                   tracer  have  the  CAP_SYS_ADMIN capability, not have any
                   seccomp protections installed, and not have PTRACE_O_SUS-
                   PEND_SECCOMP set on itself.
     PTRACE_GETEVENTMSG (since Linux 2.5.46)
            Retrieve  a message (as an unsigned long) about the ptrace event
            that just happened, placing  it  at  the  address  data  in  the
            tracer.   For  PTRACE_EVENT_EXIT, this is the tracee's exit sta-
            tus.       For      PTRACE_EVENT_FORK,       PTRACE_EVENT_VFORK,
            PTRACE_EVENT_VFORK_DONE, and PTRACE_EVENT_CLONE, this is the PID
            of the new process.  For PTRACE_EVENT_SECCOMP, this is the  sec-
            comp(2)  filter's SECCOMP_RET_DATA associated with the triggered
            rule.  (addr is ignored.)
     PTRACE_CONT
            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.)
     PTRACE_SYSCALL, PTRACE_SINGLESTEP
            Restart the stopped tracee as for PTRACE_CONT, but  arrange  for
            the  tracee  to  be  stopped at the next entry to or exit from a
            system call, or after execution of a single instruction, respec-
            tively.   (The  tracee  will  also,  as  usual,  be stopped upon
            receipt of a signal.)  From the tracer's perspective, the tracee
            will  appear  to have been stopped by receipt of a SIGTRAP.  So,
            for PTRACE_SYSCALL, for example, the  idea  is  to  inspect  the
            arguments  to the system call at the first stop, then do another
            PTRACE_SYSCALL and inspect the return value of the  system  call
            at  the  second  stop.   The  data  argument  is  treated as for
            PTRACE_CONT.  (addr is ignored.)
     PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP (since Linux 2.6.14)
            For PTRACE_SYSEMU, continue and stop on entry to the next system
            call,  which  will  not  be  executed.  See the documentation on
            syscall-stops below.  For PTRACE_SYSEMU_SINGLESTEP, do the  same
            but  also singlestep if not a system call.  This call is used by
            programs like User Mode Linux  that  want  to  emulate  all  the
            tracee's  system  calls.   The  data  argument is treated as for
            PTRACE_CONT.  The addr argument is ignored.  These requests  are
            currently supported only on x86.
     PTRACE_LISTEN (since Linux 3.4)
            Restart  the stopped tracee, but prevent it from executing.  The
            resulting state of the tracee is similar to a process which  has
            been  stopped  by a SIGSTOP (or other stopping signal).  See the
            "group-stop" subsection for additional information.  PTRACE_LIS-
            TEN works only on tracees attached by PTRACE_SEIZE.
     PTRACE_KILL
            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 inter-
            rupted and syscall-exit-stop is reported.  (The interrupted sys-
            tem call is restarted when the tracee  is  restarted.)   If  the
            tracee  was  already  stopped  by a signal and PTRACE_LISTEN was
            sent to it, the tracee stops with PTRACE_EVENT_STOP  and  WSTOP-
            SIG(status)  returns  the stop signal.  If any other ptrace-stop
            is generated at the same time (for example, if a signal is  sent
            to  the tracee), this ptrace-stop happens.  If none of the above
            applies (for example, if the tracee is running in  user  space),
            it  stops  with  PTRACE_EVENT_STOP with WSTOPSIG(status) == SIG-
            TRAP.   PTRACE_INTERRUPT  only  works  on  tracees  attached  by
            PTRACE_SEIZE.
     PTRACE_ATTACH
            Attach  to  the  process specified in pid, making it a tracee of
            the calling process.  The tracee is sent a SIGSTOP, but will not
            necessarily  have  stopped  by  the completion of this call; use
            waitpid(2) to wait for the tracee to stop.  See  the  "Attaching
            and detaching" subsection for additional information.  (addr and
            data are ignored.)
            Permission to perform a PTRACE_ATTACH is governed  by  a  ptrace
            access mode PTRACE_MODE_ATTACH_REALCREDS check; see below.
     PTRACE_SEIZE (since Linux 3.4)
            Attach  to  the  process specified in pid, making it a tracee of
            the calling process.  Unlike  PTRACE_ATTACH,  PTRACE_SEIZE  does
            not   stop   the   process.    Group-stops   are   reported   as
            PTRACE_EVENT_STOP and WSTOPSIG(status) returns the stop  signal.
            Automatically  attached children stop with PTRACE_EVENT_STOP and
            WSTOPSIG(status) returns SIGTRAP instead of having SIGSTOP  sig-
            nal delivered to them.  execve(2) does not deliver an extra SIG-
            TRAP.  Only a PTRACE_SEIZEd process can accept  PTRACE_INTERRUPT
            and   PTRACE_LISTEN   commands.    The  "seized"  behavior  just
            described  is  inherited  by  children  that  are  automatically
            attached   using  PTRACE_O_TRACEFORK,  PTRACE_O_TRACEVFORK,  and
            PTRACE_O_TRACECLONE.  addr must be zero.  data  contains  a  bit
            mask of ptrace options to activate immediately.
            Permission  to  perform  a  PTRACE_SEIZE is governed by a ptrace
            access mode PTRACE_MODE_ATTACH_REALCREDS check; see below.
     PTRACE_SECCOMP_GET_FILTER (since Linux 4.4)
            This operation allows the tracer to dump  the  tracee's  classic
            BPF filters.
            addr  is  an  integer  specifying  the index of the filter to be
            dumped.  The most recently installed filter has the index 0.  If
            addr is greater than the number of installed filters, the opera-
            tion fails with the error ENOENT.
            data is either a pointer to a struct sock_filter array  that  is
            large enough to store the BPF program, or NULL if the program is
            not to be stored.
            Upon success, the return value is the number of instructions  in
            the  BPF  program.  If data was NULL, then this return value can
            be used to correctly size the struct sock_filter array passed in
            a subsequent call.
            This  operation  fails with the error EACCESS if the caller does
            not have the CAP_SYS_ADMIN capability or if  the  caller  is  in
            strict  or  filter  seccomp  mode.  If the filter referred to by
            addr is not a classic BPF filter, the operation fails  with  the
            error EMEDIUMTYPE.
            This  operation  is  available if the kernel was configured with
            both the CONFIG_SECCOMP_FILTER and the CONFIG_CHECKPOINT_RESTORE
            options.
     PTRACE_DETACH
            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.)
 Death under ptrace
     When a (possibly multithreaded) process receives a killing signal  (one
     whose disposition is set to SIG_DFL and whose default action is to kill
     the process), all threads exit.  Tracees report their  death  to  their
     tracer(s).  Notification of this event is delivered via waitpid(2).
     Note  that the killing signal will first cause signal-delivery-stop (on
     one tracee only), and only after it is injected by the tracer (or after
     it  was dispatched to a thread which isn't traced), will death from the
     signal happen on all tracees within a multithreaded process.  (The term
     "signal-delivery-stop" is explained below.)
     SIGKILL does not generate signal-delivery-stop and therefore the tracer
     can't suppress it.  SIGKILL kills even within  system  calls  (syscall-
     exit-stop  is not generated prior to death by SIGKILL).  The net effect
     is that SIGKILL always kills the process (all  its  threads),  even  if
     some threads of the process are ptraced.
     When  the  tracee  calls  _exit(2), it reports its death to its tracer.
     Other threads are not affected.
     When any thread executes exit_group(2),  every  tracee  in  its  thread
     group reports its death to its tracer.
     If  the  PTRACE_O_TRACEEXIT option is on, PTRACE_EVENT_EXIT will happen
     before actual death.  This applies to exits via exit(2), exit_group(2),
     and signal deaths (except SIGKILL, depending on the kernel version; see
     BUGS below), and when threads are torn down on execve(2)  in  a  multi-
     threaded process.
     The  tracer cannot assume that the ptrace-stopped tracee exists.  There
     are many scenarios when the tracee  may  die  while  stopped  (such  as
     SIGKILL).   Therefore,  the  tracer must be prepared to handle an ESRCH
     error on any  ptrace  operation.   Unfortunately,  the  same  error  is
     returned  if  the tracee exists but is not ptrace-stopped (for commands
     which require a stopped tracee), or if it is not traced by the  process
     which  issued  the  ptrace call.  The tracer needs to keep track of the
     stopped/running state of the tracee, and  interpret  ESRCH  as  "tracee
     died  unexpectedly"  only if it knows that the tracee has been observed
     to enter ptrace-stop.  Note that  there  is  no  guarantee  that  wait-
     pid(WNOHANG) will reliably report the tracee's death status if a ptrace
     operation returned ESRCH.  waitpid(WNOHANG) may return 0  instead.   In
     other words, the tracee may be "not yet fully dead", but already refus-
     ing ptrace requests.
     The tracer can't assume that the tracee always ends its life by report-
     ing  WIFEXITED(status)  or  WIFSIGNALED(status);  there are cases where
     this does not occur.  For example, if a thread other than thread  group
     leader  does  an  execve(2),  it disappears; its PID will never be seen
     again, and any subsequent ptrace  stops  will  be  reported  under  the
     thread group leader's PID.
 Stopped states
     A tracee can be in two states: running or stopped.  For the purposes of
     ptrace, a tracee which is blocked in a system call  (such  as  read(2),
     pause(2),  etc.)  is nevertheless considered to be running, even if the
     tracee is blocked for a long time.   The  state  of  the  tracee  after
     PTRACE_LISTEN  is somewhat of a gray area: it is not in any ptrace-stop
     (ptrace commands won't work on it, and it will deliver waitpid(2) noti-
     fications),  but  it also may be considered "stopped" because it is not
     executing instructions (is not scheduled), and if it was in  group-stop
     before  PTRACE_LISTEN,  it will not respond to signals until SIGCONT is
     received.
     There are many kinds of states when  the  tracee  is  stopped,  and  in
     ptrace  discussions  they are often conflated.  Therefore, it is impor-
     tant to use precise terms.
     In this manual page, any stopped state in which the tracee is ready  to
     accept  ptrace commands from the tracer is called ptrace-stop.  Ptrace-
     stops can be further subdivided into signal-delivery-stop,  group-stop,
     syscall-stop,  PTRACE_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 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.)
 Signal-delivery-stop
     When a (possibly multithreaded)  process  receives  any  signal  except
     SIGKILL,  the kernel selects an arbitrary thread which handles the sig-
     nal.  (If the signal is generated with tgkill(2), the target thread can
     be  explicitly  selected  by  the  caller.)   If the selected thread is
     traced, it enters signal-delivery-stop.  At this point, the  signal  is
     not  yet delivered to the process, and can be suppressed by the tracer.
     If the tracer doesn't suppress the signal, it passes the signal to  the
     tracee  in the next ptrace restart request.  This second step of signal
     delivery is called signal injection in this manual page.  Note that  if
     the  signal  is  blocked, signal-delivery-stop doesn't happen until the
     signal is unblocked, with the usual exception  that  SIGSTOP  can't  be
     blocked.
     Signal-delivery-stop  is observed by the tracer as waitpid(2) returning
     with WIFSTOPPED(status) true, with the signal returned by WSTOPSIG(sta-
     tus).   If  the  signal  is  SIGTRAP,  this  may be a different kind of
     ptrace-stop; see the "Syscall-stops" and "execve"  sections  below  for
     details.   If WSTOPSIG(status) returns a stopping signal, this may be a
     group-stop; see below.
 Signal injection and suppression
     After signal-delivery-stop is observed by the tracer, the tracer should
     restart the tracee with the call
         ptrace(PTRACE_restart, pid, 0, sig)
     where  PTRACE_restart is one of the restarting ptrace requests.  If sig
     is 0, then a signal is not delivered.  Otherwise,  the  signal  sig  is
     delivered.   This  operation  is called signal injection in this manual
     page, to distinguish it from signal-delivery-stop.
     The sig value may be different from  the  WSTOPSIG(status)  value:  the
     tracer can cause a different signal to be injected.
     Note  that a suppressed signal still causes system calls to return pre-
     maturely.  In this case, system calls will  be  restarted:  the  tracer
     will  observe  the  tracee to reexecute the interrupted system call (or
     restart_syscall(2) system call for a few system calls which use a  dif-
     ferent  mechanism  for  restarting)  if the tracer uses PTRACE_SYSCALL.
     Even system calls (such as poll(2)) which  are  not  restartable  after
     signal  are  restarted after signal is suppressed; however, kernel bugs
     exist which cause some system calls to fail with EINTR even  though  no
     observable signal is injected to the tracee.
     Restarting  ptrace  commands  issued in ptrace-stops other than signal-
     delivery-stop are not guaranteed to inject a signal,  even  if  sig  is
     nonzero.   No  error  is reported; a nonzero sig may simply be ignored.
     Ptrace users should not try to "create a  new  signal"  this  way:  use
     tgkill(2) instead.
     The  fact that signal injection requests may be ignored when restarting
     the tracee after ptrace stops that are not signal-delivery-stops  is  a
     cause  of  confusion  among ptrace users.  One typical scenario is that
     the tracer observes group-stop, mistakes it  for  signal-delivery-stop,
     restarts the tracee with
         ptrace(PTRACE_restart, pid, 0, stopsig)
     with  the  intention of injecting stopsig, but stopsig gets ignored and
     the tracee continues to run.
     The SIGCONT signal has a side effect of waking up (all  threads  of)  a
     group-stopped  process.   This side effect happens before signal-deliv-
     ery-stop.  The tracer can't suppress this side effect (it can only sup-
     press signal injection, which only causes the SIGCONT handler to not be
     executed in the tracee, if such a handler is installed).  In fact, wak-
     ing up from group-stop may be followed by signal-delivery-stop for sig-
     nal(s) other than SIGCONT, if they were pending when SIGCONT was deliv-
     ered.   In other words, SIGCONT may be not the first signal observed by
     the tracee after it was sent.
     Stopping signals cause (all threads of) a process to enter  group-stop.
     This  side  effect happens after signal injection, and therefore can be
     suppressed by the tracer.
     In Linux 2.4 and earlier, the SIGSTOP signal can't be injected.
     PTRACE_GETSIGINFO can be used to retrieve a siginfo_t  structure  which
     corresponds  to the delivered signal.  PTRACE_SETSIGINFO may be used to
     modify it.  If PTRACE_SETSIGINFO has been used to alter siginfo_t,  the
     si_signo  field  and  the  sig parameter in the restarting command must
     match, otherwise the result is undefined.
 Group-stop
     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 WIF-
     STOPPED(status) true, with the stopping  signal  available  via  WSTOP-
     SIG(status).   The  same  result  is  returned by some other classes of
     ptrace-stops, therefore the recommended practice is to perform the call
         ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)
     The call can be avoided if the signal is not SIGSTOP, SIGTSTP, SIGTTIN,
     or SIGTTOU; only these four  signals  are  stopping  signals.   If  the
     tracer  sees  something else, it can't be a group-stop.  Otherwise, the
     tracer needs to call  PTRACE_GETSIGINFO.   If  PTRACE_GETSIGINFO  fails
     with  EINVAL, then it is definitely a group-stop.  (Other failure codes
     are possible, such as ESRCH ("no such process") if a SIGKILL killed the
     tracee.)
     If  tracee  was attached using PTRACE_SEIZE, group-stop is indicated by
     PTRACE_EVENT_STOP: status>>16 == PTRACE_EVENT_STOP.  This allows detec-
     tion  of group-stops without requiring an extra PTRACE_GETSIGINFO call.
     As of Linux 2.6.38, after the tracer sees the  tracee  ptrace-stop  and
     until  it  restarts  or kills it, the tracee will not run, and will not
     send notifications (except SIGKILL death) to the tracer,  even  if  the
     tracer enters into another waitpid(2) call.
     The  kernel behavior described in the previous paragraph causes a prob-
     lem with transparent handling  of  stopping  signals.   If  the  tracer
     restarts  the  tracee  after  group-stop, the stopping signal is effec-
     tively ignored--the tracee doesn't remain stopped,  it  runs.   If  the
     tracer  doesn't  restart the tracee before entering into the next wait-
     pid(2), future SIGCONT signals will not be reported to the tracer; this
     would cause the SIGCONT signals to have no effect on the tracee.
     Since Linux 3.4, there is a method to overcome this problem: instead of
     PTRACE_CONT, a PTRACE_LISTEN command can be used to restart a tracee in
     a way where it does not execute, but waits for a new event which it can
     report via waitpid(2) (such as when it is restarted by a SIGCONT).
 PTRACE_EVENT stops
     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:
     PTRACE_EVENT_VFORK
            Stop  before  return  from  vfork(2)  or   clone(2)   with   the
            CLONE_VFORK flag.  When the tracee is continued after this stop,
            it will wait for child to exit/exec before continuing its execu-
            tion (in other words, the usual behavior on vfork(2)).
     PTRACE_EVENT_FORK
            Stop before return from fork(2) or clone(2) with the exit signal
            set to SIGCHLD.
     PTRACE_EVENT_CLONE
            Stop before return from clone(2).
     PTRACE_EVENT_VFORK_DONE
            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.
     PTRACE_EVENT_EXEC
            Stop  before  return   from   execve(2).    Since   Linux   3.0,
            PTRACE_GETEVENTMSG returns the former thread ID.
     PTRACE_EVENT_EXIT
            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.
     PTRACE_EVENT_STOP
            Stop induced by PTRACE_INTERRUPT command, or group-stop, or ini-
            tial  ptrace-stop when a new child is attached (only if attached
            using PTRACE_SEIZE).
     PTRACE_EVENT_SECCOMP
            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.
 Syscall-stops
     If the tracee was restarted by  PTRACE_SYSCALL  or  PTRACE_SYSEMU,  the
     tracee enters syscall-enter-stop just prior to entering any system call
     (which will not be executed if the  restart  was  using  PTRACE_SYSEMU,
     regardless  of  any  change  made to registers at this point or how the
     tracee is restarted after this stop).  No matter  which  method  caused
     the   syscall-entry-stop,  if  the  tracer  restarts  the  tracee  with
     PTRACE_SYSCALL, the tracee enters  syscall-exit-stop  when  the  system
     call  is finished, or if it is interrupted by a signal.  (That is, sig-
     nal-delivery-stop never happens between syscall-enter-stop and syscall-
     exit-stop; it happens after syscall-exit-stop.).  If the tracee is con-
     tinued using any other method (including  PTRACE_SYSEMU),  no  syscall-
     exit-stop  occurs.   Note that all mentions PTRACE_SYSEMU apply equally
     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  SIG-
     TRAP by querying PTRACE_GETSIGINFO for the following cases:
     si_code <= 0
            SIGTRAP  was  delivered  as a result of a user-space action, for
            example, a system call (tgkill(2), kill(2), sigqueue(3),  etc.),
            expiration  of a POSIX timer, change of state on a POSIX message
            queue, or completion of an asynchronous I/O request.
     si_code == SI_KERNEL (0x80)
            SIGTRAP was sent by the kernel.
     si_code == SIGTRAP or si_code == (SIGTRAP|0x80)
            This is a syscall-stop.
     However, syscall-stops happen very often (twice per system  call),  and
     performing  PTRACE_GETSIGINFO  for  every  syscall-stop may be somewhat
     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 syscall-entry-
     stops.
     If after syscall-enter-stop, the tracer uses a restarting command other
     than PTRACE_SYSCALL, syscall-exit-stop is not generated.
     PTRACE_GETSIGINFO  on  syscall-stops  returns SIGTRAP in si_signo, with
     si_code set to SIGTRAP or (SIGTRAP|0x80).
 PTRACE_EVENT_SECCOMP stops (Linux 3.5 to 4.7)
     The behavior of PTRACE_EVENT_SECCOMP stops and their  interaction  with
     other  kinds of ptrace stops has changed between kernel versions.  This
     documents the behavior from their introduction until Linux 4.7  (inclu-
     sive).  The behavior in later kernel versions is documented in the next
     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 expected.
 PTRACE_EVENT_SECCOMP stops (since Linux 4.8)
     Starting with Linux 4.8, the PTRACE_EVENT_SECCOMP stop was reordered to
     occur between syscall-entry-stop and syscall-exit-stop.  Note that sec-
     comp no longer runs (and no PTRACE_EVENT_SECCOMP will be  reported)  if
     the system call is skipped due to PTRACE_SYSEMU.
     Functionally,  a  PTRACE_EVENT_SECCOMP  stop  functions comparably to a
     syscall-entry-stop (i.e., continuations using PTRACE_SYSCALL will cause
     syscall-exit-stops, the system call number may be changed and any other
     modified registers are visible to the  to-be-executed  system  call  as
     well).   Note  that  there  may  be, but need not have been a preceding
     syscall-entry-stop.
     After a PTRACE_EVENT_SECCOMP stop, seccomp will be rerun, with  a  SEC-
     COMP_RET_TRACE  rule  now  functioning the same as a SECCOMP_RET_ALLOW.
     Specifically, this means that if registers are not modified during  the
     PTRACE_EVENT_SECCOMP stop, the system call will then be allowed.
 PTRACE_SINGLESTEP stops
     [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_GETSIG-
         INFO,  pid,  0,  &siginfo); ptrace(PTRACE_SETSIGINFO, pid, 0, &sig-
         info);    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);
     where cmd is PTRACE_CONT, PTRACE_LISTEN, PTRACE_DETACH, PTRACE_SYSCALL,
     PTRACE_SINGLESTEP,  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);
     or
         ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);
     PTRACE_ATTACH  sends  SIGSTOP to this thread.  If the tracer wants this
     SIGSTOP to have no effect, it needs to suppress it.  Note that if other
     signals  are concurrently sent to this thread during attach, the tracer
     may see the tracee  enter  signal-delivery-stop  with  other  signal(s)
     first!   The  usual practice is to reinject these signals until SIGSTOP
     is seen, then suppress SIGSTOP injection.  The design bug here is  that
     a  ptrace  attach and a concurrently delivered SIGSTOP may race and the
     concurrent SIGSTOP may be lost.
     Since attaching sends SIGSTOP and the  tracer  usually  suppresses  it,
     this may cause a stray EINTR return from the currently executing system
     call in the tracee, as described in the "Signal injection and  suppres-
     sion" section.
     Since  Linux  3.4,  PTRACE_SEIZE  can be used instead of PTRACE_ATTACH.
     PTRACE_SEIZE does not stop the attached process.  If you need  to  stop
     it  after attach (or at any other time) without sending it any signals,
     use PTRACE_INTERRUPT command.
     The request
         ptrace(PTRACE_TRACEME, 0, 0, 0);
     turns the calling thread into a tracee.  The thread  continues  to  run
     (doesn't  enter  ptrace-stop).   A  common  practice  is  to follow the
     PTRACE_TRACEME with
         raise(SIGSTOP);
     and allow the parent (which is our tracer now) to observe  our  signal-
     delivery-stop.
     If  the PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, or PTRACE_O_TRACECLONE
     options are in effect, then children created by, respectively, vfork(2)
     or  clone(2)  with  the  CLONE_VFORK flag, fork(2) or clone(2) with the
     exit signal set to SIGCHLD, and other kinds of clone(2), are  automati-
     cally  attached  to the same tracer which traced their parent.  SIGSTOP
     is delivered to the children, causing them  to  enter  signal-delivery-
     stop after they exit the system call which created them.
     Detaching of the tracee is performed by:
         ptrace(PTRACE_DETACH, pid, 0, sig);
     PTRACE_DETACH  is  a  restarting  operation;  therefore it requires the
     tracee to be in ptrace-stop.  If the tracee is in signal-delivery-stop,
     a signal can be injected.  Otherwise, the sig parameter may be silently
     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 wait-

        pid(2), or fed into ptrace calls, is the tracee's thread ID.)   That
        is,  the  tracee's  thread ID is reset to be the same as its process
        ID, which is the same as the thread group leader's thread ID.
  • Then a PTRACE_EVENT_EXEC stop happens, if the PTRACE_O_TRACEEXEC

option was turned on.

  • If the thread group leader has reported its PTRACE_EVENT_EXIT stop

by this time, it appears to the tracer that the dead thread leader

        "reappears  from  nowhere".  (Note: the thread group leader does not
        report death via WIFEXITED(status) until there is at least one other
        live  thread.   This eliminates the possibility that the tracer will
        see it dying and then reappearing.)  If the thread group leader  was
        still  alive, for the tracer this may look as if thread group leader
        returns from a different  system  call  than  it  entered,  or  even
        "returned  from  a  system call even though it was not in any system
        call".  If the thread group leader was not traced (or was traced  by
        a  different  tracer), then during execve(2) it will appear as if it
        has become a tracee of the tracer of the execing tracee.
     All of the above effects are the artifacts of the thread ID  change  in
     the tracee.
     The  PTRACE_O_TRACEEXEC option is the recommended tool for dealing with
     this situation.  First, it enables PTRACE_EVENT_EXEC stop, which occurs
     before   execve(2)   returns.    In  this  stop,  the  tracer  can  use
     PTRACE_GETEVENTMSG to retrieve the tracee's former  thread  ID.   (This
     feature  was  introduced in Linux 3.0.)  Second, the PTRACE_O_TRACEEXEC
     option disables legacy SIGTRAP generation on execve(2).
     When the tracer receives PTRACE_EVENT_EXEC  stop  notification,  it  is
     guaranteed  that  except  this  tracee  and the thread group leader, no
     other threads from the process are alive.
     On receiving the PTRACE_EVENT_EXEC stop notification, the tracer should
     clean  up  all  its  internal data structures describing the threads of
     this process, and retain only one data structure--one  which  describes
     the single still running tracee, with
         thread ID == thread group ID == process ID.
     Example: two threads call execve(2) at the same time:
  • we get syscall-enter-stop in thread 1:

PID1 execve("/bin/foo", "foo" <unfinished …>

  • we issue PTRACE_SYSCALL for thread 1
  • we get syscall-enter-stop in thread 2:

PID2 execve("/bin/bar", "bar" <unfinished …>

  • we issue PTRACE_SYSCALL for thread 2
  • we get PTRACE_EVENT_EXEC for PID0, we issue PTRACE_SYSCALL
  • we get syscall-exit-stop for PID0:

PID0 <… execve resumed> ) = 0

     If  the  PTRACE_O_TRACEEXEC  option  is  not  in effect for the execing
     tracee,  and  if   the   tracee   was   PTRACE_ATTACHed   rather   that
     PTRACE_SEIZEd, the kernel delivers an extra SIGTRAP to the tracee after
     execve(2) returns.  This is an ordinary signal (similar  to  one  which
     can  be  generated  by  kill -TRAP), not a special kind of ptrace-stop.
     Employing PTRACE_GETSIGINFO for this signal returns si_code  set  to  0
     (SI_USER).   This signal may be blocked by signal mask, and thus may be
     delivered (much) later.
     Usually, the tracer (for example, strace(1)) would  not  want  to  show
     this  extra  post-execve SIGTRAP signal to the user, and would suppress
     its delivery to the tracee (if SIGTRAP is  set  to  SIG_DFL,  it  is  a
     killing signal).  However, determining which SIGTRAP to suppress is not
     easy.  Setting the PTRACE_O_TRACEEXEC option or using PTRACE_SEIZE  and
     thus suppressing this extra SIGTRAP is the recommended approach.
 Real parent
     The  ptrace  API (ab)uses the standard UNIX parent/child signaling over
     waitpid(2).  This used to cause the real parent of the process to  stop
     receiving  several  kinds  of  waitpid(2)  notifications when the child
     process is traced by some other process.
     Many of these bugs have been fixed, but  as  of  Linux  2.6.38  several
     still exist; see BUGS below.
     As of Linux 2.6.38, the following is believed to work correctly:
  • exit/death by signal is reported first to the tracer, then, when the

tracer consumes the waitpid(2) result, to the real parent (to the

        real  parent  only  when the whole multithreaded process exits).  If
        the tracer and the real parent are the same process, the  report  is
        sent only once.

RETURN VALUE

     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, and 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.

ERRORS

     EBUSY  (i386 only) There was an error  with  allocating  or  freeing  a
            debug register.
     EFAULT There was an attempt to read from or write to an invalid area in
            the tracer's or the tracee's memory, probably because  the  area
            wasn't  mapped  or accessible.  Unfortunately, under Linux, dif-
            ferent variations of this fault will return EIO or  EFAULT  more
            or less arbitrarily.
     EINVAL An attempt was made to set an invalid option.
     EIO    request is invalid, or an attempt was made to read from or write
            to an invalid area in the tracer's or the  tracee's  memory,  or
            there  was  a word-alignment violation, or an invalid signal was
            specified during a restart request.
     EPERM  The specified process cannot be traced.  This could  be  because
            the  tracer has insufficient privileges (the required capability
            is CAP_SYS_PTRACE); unprivileged  processes  cannot  trace  pro-
            cesses  that  they  cannot send signals to or those running set-
            user-ID/set-group-ID programs, for  obvious  reasons.   Alterna-
            tively,  the process may already be being traced, or (on kernels
            before 2.6.26) be init(1) (PID 1).
     ESRCH  The specified process does not exist, or is not currently  being
            traced  by  the  caller,  or  is  not stopped (for requests that
            require a stopped tracee).

CONFORMING TO

     SVr4, 4.3BSD.

NOTES

     Although arguments to ptrace() are interpreted according to the  proto-
     type  given,  glibc  currently declares ptrace() as a variadic function
     with only the request argument fixed.  It is recommended to always sup-
     ply  four arguments, even if the requested operation does not use them,
     setting unused/ignored arguments to 0L or (void *) 0.
     In Linux kernels before 2.6.26, init(1), the process with  PID  1,  may
     not be traced.
     A  tracees  parent continues to be the tracer even if that tracer calls
     execve(2).
     The layout of the contents of memory and the USER area are quite  oper-
     ating-system-  and architecture-specific.  The offset supplied, and the
     data returned, might not entirely match with the definition  of  struct
     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() opera-
     tions), require so-called "ptrace access mode"  checks,  whose  outcome
     determines  whether  an  operation  is  permitted  (or, in a few cases,
     causes a "read" operation to return sanitized data).  These checks  are
     performed  in cases where one process can inspect sensitive information
     about, or in some cases modify the  state  of,  another  process.   The
     checks are based on factors such as the credentials and capabilities of
     the two processes, whether or not the "target" process is dumpable, and
     the  results  of  checks performed by any enabled Linux Security Module
     (LSM)--for example, SELinux, Yama, or Smack--and by the  commoncap  LSM
     (which is always invoked).
     Prior  to Linux 2.6.27, all access checks were of a single type.  Since
     Linux 2.6.27, two access mode levels are distinguished:
     PTRACE_MODE_READ
            For "read" operations or other operations that are less  danger-
            ous,    such    as:    get_robust_list(2);    kcmp(2);   reading
            /proc/[pid]/auxv, /proc/[pid]/environ, or  /proc/[pid]/stat;  or
            readlink(2) of a /proc/[pid]/ns/* file.
     PTRACE_MODE_ATTACH
            For  "write"  operations, or other operations that are more dan-
            gerous, such as: ptrace  attaching  (PTRACE_ATTACH)  to  another
            process  or  calling  process_vm_writev(2).  (PTRACE_MODE_ATTACH
            was effectively the default before Linux 2.6.27.)
     Since Linux 4.5, the above access mode checks are combined (ORed)  with
     one of the following modifiers:
     PTRACE_MODE_FSCREDS
            Use  the caller's filesystem UID and GID (see credentials(7)) or
            effective capabilities for LSM checks.
     PTRACE_MODE_REALCREDS
            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:
     PTRACE_MODE_READ_FSCREDS
            Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.
     PTRACE_MODE_READ_REALCREDS
            Defined as PTRACE_MODE_READ | PTRACE_MODE_REALCREDS.
     PTRACE_MODE_ATTACH_FSCREDS
            Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS.
     PTRACE_MODE_ATTACH_REALCREDS
            Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS.
     One further modifier can be ORed with the access mode:
     PTRACE_MODE_NOAUDIT (since Linux 3.3)
            Don't audit this access mode check.  This modifier  is  employed
            for  ptrace  access  mode  checks  (such  as checks when reading
            /proc/[pid]/stat) that merely cause the output to be filtered or
            sanitized,  rather  than  causing an error to be returned to the
            caller.  In these cases, accessing the file is  not  a  security
            violation  and  there  is no reason to generate a security audit
            record.  This modifier suppresses  the  generation  of  such  an
            audit record for the particular access check.
     Note  that all of the PTRACE_MODE_* constants described in this subsec-
     tion are kernel-internal, and not visible to user space.  The  constant
     names  are mentioned here in order to label the various kinds of ptrace
     access mode checks that are performed  for  various  system  calls  and
     accesses  to  various pseudofiles (e.g., under /proc).  These names are
     used in other manual pages to provide a simple shorthand  for  labeling
     the different kernel checks.
     The  algorithm  employed  for  ptrace  access  mode checking determines
     whether the calling process is allowed  to  perform  the  corresponding
     action  on  the  target  process.   (In the case of opening /proc/[pid]
     files, the "calling process" is the  one  opening  the  file,  and  the
     process with the corresponding PID is the "target process".)  The algo-
     rithm is as follows:
     1. If the calling thread and the target thread are in the  same  thread
        group, access is always allowed.
     2. If  the  access  mode  specifies  PTRACE_MODE_FSCREDS, then, for the
        check in the next step, employ the caller's filesystem UID and  GID.
        (As  noted  in  credentials(7),  the  filesystem  UID and GID almost
        always have the same values as the corresponding effective IDs.)
        Otherwise, the access mode specifies PTRACE_MODE_REALCREDS,  so  use
        the  caller's  real  UID  and  GID  for the checks in the next step.
        (Most APIs that check the caller's UID and  GID  use  the  effective
        IDs.   For  historical reasons, the PTRACE_MODE_REALCREDS check uses
        the real IDs instead.)
     3. Deny access if neither of the following is true:
        o The real, effective, and saved-set user IDs of  the  target  match
          the caller's user ID, and the real, effective, and saved-set group
          IDs of the target match the caller's group ID.
        o The caller has the CAP_SYS_PTRACE capability in the user namespace
          of the target.
     4. Deny  access  if the target process "dumpable" attribute has a value
        other than 1 (SUID_DUMP_USER; see the discussion of  PR_SET_DUMPABLE
        in  prctl(2)), and the caller does not have the CAP_SYS_PTRACE capa-
        bility in the user namespace of the target process.
     5. The kernel LSM security_ptrace_access_check() interface  is  invoked
        to  see  if  ptrace  access is permitted.  The results depend on the
        LSM(s).  The implementation of this interface in the  commoncap  LSM
        performs the following steps:
        a) If  the  access  mode  includes PTRACE_MODE_FSCREDS, then use the
           caller's effective capability set in the following check;  other-
           wise  (the  access  mode specifies PTRACE_MODE_REALCREDS, so) use
           the caller's permitted capability set.
        b) Deny access if neither of the following is true:
           o The caller and the target process are in the same  user  names-
             pace,  and  the  caller's capabilities are a proper superset of
             the target process's permitted capabilities.
           o The caller has the  CAP_SYS_PTRACE  capability  in  the  target
             process's user namespace.
           Note   that  the  commoncap  LSM  does  not  distinguish  between
           PTRACE_MODE_READ and PTRACE_MODE_ATTACH.
     6. If access has not been denied by any of the  preceding  steps,  then
        access is allowed.
 /proc/sys/kernel/yama/ptrace_scope
     On  systems  with the Yama Linux Security Module (LSM) installed (i.e.,
     the   kernel   was   configured   with    CONFIG_SECURITY_YAMA),    the
     /proc/sys/kernel/yama/ptrace_scope file (available since Linux 3.4) can
     be used to restrict the ability to trace a process with  ptrace()  (and
     thus  also the ability to use tools such as strace(1) and gdb(1)).  The
     goal of such restrictions is to prevent  attack  escalation  whereby  a
     compromised  process  can  ptrace-attach  to  other sensitive processes
     (e.g., a GPG agent or an SSH session) owned by the  user  in  order  to
     gain  additional  credentials  that may exist in memory and thus expand
     the scope of the attack.
     More precisely, the Yama LSM limits two types of operations:
  • Any operation that performs a ptrace access mode PTRACE_MODE_ATTACH

check–for example, ptrace() PTRACE_ATTACH. (See the "Ptrace access

        mode checking" discussion above.)
  • ptrace() PTRACE_TRACEME.
     A process  that  has  the  CAP_SYS_PTRACE  capability  can  update  the
     /proc/sys/kernel/yama/ptrace_scope  file with one of the following val-
     ues:
     0 ("classic ptrace permissions")
            No  additional   restrictions   on   operations   that   perform
            PTRACE_MODE_ATTACH checks (beyond those imposed by the commoncap
            and other LSMs).
            The use of PTRACE_TRACEME is unchanged.
     1 ("restricted ptrace") [default value]
            When performing an operation that requires a  PTRACE_MODE_ATTACH
            check,  the  calling process must either have the CAP_SYS_PTRACE
            capability in the user namespace of the  target  process  or  it
            must have a predefined relationship with the target process.  By
            default, the predefined relationship is that the target  process
            must be a descendant of the caller.
            A  target  process can employ the prctl(2) PR_SET_PTRACER opera-
            tion to declare an additional PID that  is  allowed  to  perform
            PTRACE_MODE_ATTACH  operations  on  the  target.  See the kernel
            source file Documentation/admin-guide/LSM/Yama.rst (or  Documen-
            tation/security/Yama.txt before Linux 4.13) for further details.
            The use of PTRACE_TRACEME is unchanged.
     2 ("admin-only attach")
            Only processes with the CAP_SYS_PTRACE capability  in  the  user
            namespace  of  the target process may perform PTRACE_MODE_ATTACH
            operations or trace children that employ PTRACE_TRACEME.
     3 ("no attach")
            No process may perform PTRACE_MODE_ATTACH  operations  or  trace
            children that employ PTRACE_TRACEME.
            Once  this  value  has  been  written  to the file, it cannot be
            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 names-
     pace  (and  further-removed  descendants  of  that  namespace).  Conse-
     quently, when a process tries to use user namespaces to sandbox itself,
     it inadvertently weakens the protections offered by the Yama LSM.
 C library/kernel differences
     At  the  system  call  level, the PTRACE_PEEKTEXT, PTRACE_PEEKDATA, and
     PTRACE_PEEKUSER requests have a different API: they store the result at
     the  address  specified  by the data parameter, and the return value is
     the error flag.  The glibc wrapper function provides the API  given  in
     DESCRIPTION  above,  with  the  result  being returned via the function
     return value.

BUGS

     On hosts with 2.6 kernel headers, PTRACE_SETOPTIONS is declared with  a
     different  value than the one for 2.4.  This leads to applications com-
     piled with 2.6 kernel headers failing when run on  2.4  kernels.   This
     can  be  worked around by redefining PTRACE_SETOPTIONS to PTRACE_OLDSE-
     TOPTIONS, if that is defined.
     Group-stop notifications are sent to the tracer, but not to  real  par-
     ent.  Last confirmed on 2.6.38.6.
     If  a  thread  group  leader is traced and exits by calling _exit(2), a
     PTRACE_EVENT_EXIT stop will happen for it (if requested), but the  sub-
     sequent  WIFEXITED  notification  will not be delivered until all other
     threads exit.  As explained  above,  if  one  of  other  threads  calls
     execve(2), the death of the thread group leader will never be reported.
     If the execed thread is not traced by  this  tracer,  the  tracer  will
     never  know  that  execve(2)  happened.   One possible workaround is to
     PTRACE_DETACH the thread group leader instead of restarting it in  this
     case.  Last confirmed on 2.6.38.6.
     A SIGKILL signal may still cause a PTRACE_EVENT_EXIT stop before actual
     signal death.  This may be changed in the future; SIGKILL is  meant  to
     always  immediately  kill  tasks  even under ptrace.  Last confirmed on
     Linux 3.13.
     Some system calls return with EINTR if a signal was sent to  a  tracee,
     but delivery was suppressed by the tracer.  (This is very typical oper-
     ation: it is usually done by debuggers on every attach, in order to not
     introduce  a  bogus  SIGSTOP).  As of Linux 3.2.9, the following system
     calls are affected (this list is likely incomplete): epoll_wait(2), and
     read(2)  from an inotify(7) file descriptor.  The usual symptom of this
     bug is that when you attach to a quiescent process with the command
         strace -p <process-ID>
     then, instead of the usual and expected one-line output such as
         restart_syscall(<... resuming interrupted call ...>_
     or
         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
             epoll_wait(4,_
     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.)

SEE ALSO

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

COLOPHON

     This  page  is  part of release 4.16 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
     https://www.kernel.org/doc/man-pages/.

Linux 2018-04-30 PTRACE(2)

/data/webs/external/dokuwiki/data/pages/man/ptrace.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki