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
       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:

       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
              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.

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

       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
              modifications 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_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.

       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
              indicates that there is no signal corresponding to the specified
              ordinal position).  Within the returned siginfo structures, the
              si_code field includes information (__SI_CHLD, __SI_FAULT, etc.)
              that are not otherwise exposed to user space.

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

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

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

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

              PTRACE_O_TRACESECCOMP (since Linux 3.5)
                     Stop the tracee when a seccomp(2) SECCOMP_RET_TRACE rule
                     is triggered.  A waitpid(2) by the tracer will return a
                     status value such that

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

                     While this triggers a PTRACE_EVENT stop, it is similar to
                     a syscall-enter-stop.  For details, see the note on
                     PTRACE_EVENT_SECCOMP below.  The seccomp event message
                     data (from the SECCOMP_RET_DATA portion of the seccomp
                     filter rule) can be retrieved with PTRACE_GETEVENTMSG.

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

       PTRACE_GETEVENTMSG (since Linux 2.5.46)
              Retrieve a message (as an unsigned long) about the ptrace event
              that just happened, placing it at the address data in the
              tracer.  For PTRACE_EVENT_EXIT, this is the tracee's exit
              status.  For PTRACE_EVENT_FORK, PTRACE_EVENT_VFORK,
              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.)

       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,
              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_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_LISTEN 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
              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.

       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
              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 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
       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 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
       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.

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

           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

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

   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
              execution (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
              initial 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,
       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 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
       (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 expected.

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

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

       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.

   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_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);

       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
       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

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

       EINVAL An attempt was made to set an invalid option.

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

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

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

CONFORMING TO
       SVr4, 4.3BSD.

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

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

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

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

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

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

   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:

       PTRACE_MODE_READ
              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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

             Note that the commoncap LSM does not distinguish between
             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
       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 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
       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 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
       compiled with 2.6 kernel headers failing when run on 2.4 kernels.  This
       can be worked around by redefining PTRACE_SETOPTIONS to
       PTRACE_OLDSETOPTIONS, if that is defined.

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

       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.)

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

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 5.02 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)