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          ptrace - process trace

          #include <sys/ptrace.h>

          long ptrace(enum __ptrace_request request, pid_t pid,
                      void *addr, void *data);

          The ptrace() system call provides a means by which one pro-
          cess (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.  Attach-
          ment and subsequent commands are per thread: in a multi-
          threaded 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

          (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 (typi-
          cally) by an execve(2).  Alternatively, one process may com-
          mence tracing another process using PTRACE_ATTACH or

          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

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          (or even delivering a different signal instead).

          If the PTRACE_O_TRACEEXEC option is not in effect, all suc-
          cessful 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

          The value of request determines the action to be performed:

               Indicate that this process is to be traced by its par-
               ent.  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_KILL, the tracee must be stopped.

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

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

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

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

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               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 sys-
               tems 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 loca-
               tion and length.  On return, the kernel modifies
               iov.len to indicate the actual number of bytes

               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 deliv-
               ered to the tracee and were caught by the tracer.  It
               may be difficult to tell these normal signals from syn-
               thetic signals generated by ptrace() itself.  (addr is

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          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 struc-
               tures, 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 spec-
               ified 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.

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

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

                    The PID of the new process can be retrieved with

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

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

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

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

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

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

                    The tracee's exit status can be retrieved with

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

               PTRACE_O_TRACEFORK (since Linux 2.5.46)

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

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

                    The PID of the new process can be retrieved with

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

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

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

                    The PID of the new process can be retrieved with

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

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

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

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

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

                    While this triggers a PTRACE_EVENT stop, it is
                    similar to a syscall-enter-stop.  For details, see
                    the note on PTRACE_EVENT_SECCOMP below.  The sec-
                    comp event message data (from the SECCOMP_RET_DATA

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                    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
               is the PID of the new process.  For
               PTRACE_EVENT_SECCOMP, this is the seccomp(2) filter's
               SECCOMP_RET_DATA associated with the triggered rule.
               (addr is ignored.)

               Restart the stopped tracee process.  If data is
               nonzero, it is interpreted as the number of a signal to
               be delivered to the tracee; otherwise, no signal is
               delivered.  Thus, for example, the tracer can control
               whether a signal sent to the tracee is delivered or
               not.  (addr is ignored.)

               Restart the stopped tracee as for PTRACE_CONT, but
               arrange for the tracee to be stopped at the next entry
               to or exit from a system call, or after execution of a
               single instruction, respectively.  (The tracee will
               also, as usual, be stopped upon receipt of a signal.)
               From the tracer's perspective, the tracee will appear
               to have been stopped by receipt of a SIGTRAP.  So, for
               PTRACE_SYSCALL, for example, the idea is to inspect the
               arguments to the system call at the first stop, then do
               another PTRACE_SYSCALL and inspect the return value of
               the system call at the second stop.  The data argument
               is treated as for PTRACE_CONT.  (addr is ignored.)

          PTRACE_SET_SYSCALL (since Linux 2.6.16)
               When in syscall-enter-stop, change the number of the

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

          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 sin-
               glestep 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 execut-
               ing.  The resulting state of the tracee is similar to a
               process which has been stopped by a SIGSTOP (or other
               stopping signal).  See the "group-stop" subsection for
               additional information.  PTRACE_LISTEN works only on
               tracees attached by PTRACE_SEIZE.

               Send the tracee a SIGKILL to terminate it.  (addr and
               data are ignored.)

               This operation is deprecated; do not Instead, send a
               SIGKILL directly using kill(2) or tgkill(2).  The prob-
               lem 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 sys-
               tem 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

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               happens.  If none of the above applies (for example, if
               the tracee is running in user space), it stops with
               PTRACE_EVENT_STOP with WSTOPSIG(status) == SIGTRAP.
               PTRACE_INTERRUPT only works on tracees attached by

               Attach to the process specified in pid, making it a
               tracee of the calling process.  The tracee is sent a
               SIGSTOP, but will not necessarily have stopped by the
               completion of this call; use waitpid(2) to wait for the
               tracee to stop.  See the "Attaching and detaching" sub-
               section for additional information.  (addr and data are

               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 chil-
               dren stop with PTRACE_EVENT_STOP and WSTOPSIG(status)
               returns SIGTRAP instead of having SIGSTOP signal deliv-
               ered 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 chil-
               dren that are automatically attached using
               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

               data is either a pointer to a struct sock_filter array
               that is large enough to store the BPF program, or NULL

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               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

               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 config-
               ured with both the CONFIG_SECCOMP_FILTER and the
               CONFIG_CHECKPOINT_RESTORE options.

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

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

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

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

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               exceeds the size specified by the addr argument, the
               output data is truncated.

               The ptrace_syscall_info structure contains the follow-
               ing fields:

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

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

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

                    The entry component of the union contains informa-
                    tion relating to a system call entry stop.

                    The exit component of the union contains informa-
                    tion relating to a system call exit stop.


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                    The seccomp component of the union contains infor-
                    mation relating to a PTRACE_EVENT_SECCOMP stop.

                    No component of the union contains relevant infor-

        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 hap-
          pen 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 pro-

          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 opera-
          tion.  Unfortunately, the same error is returned if the tra-
          cee 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-

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          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 subse-
          quent 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 con-
          sidered 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.  There-
          fore, 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.

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

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

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

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

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

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              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

          Note that a suppressed signal still causes system calls to
          return prematurely.  In this case, system calls will be res-
          tarted: 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 sig-
          nal are restarted after signal is suppressed; however, ker-
          nel bugs exist which cause some system calls to fail with
          EINTR even though no observable signal is injected to the

          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 "cre-
          ate 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 hap-
          pens 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, wak-
          ing 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

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

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

          PTRACE_GETSIGINFO can be used to retrieve a siginfo_t struc-
          ture 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 com-
          mand must match, otherwise the result is undefined.

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

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

              ((PTRACE_EVENT_foo<<8) | SIGTRAP).

          The following events exist:

               Stop before return from vfork(2) or clone(2) with the
               CLONE_VFORK flag.  When the tracee is continued after
               this stop, it will wait for child to exit/exec before
               continuing its execution (in other words, the usual
               behavior on vfork(2)).

               Stop before return from fork(2) or clone(2) with the
               exit signal set to SIGCHLD.

               Stop before return from clone(2).

               Stop before return from vfork(2) or clone(2) with the
               CLONE_VFORK flag, but after the child unblocked this
               tracee by exiting or execing.

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          For all four stops described above, the stop occurs in the
          parent (i.e., the tracee), not in the newly created thread.
          PTRACE_GETEVENTMSG can be used to retrieve the new thread's

               Stop before return from execve(2).  Since Linux 3.0,
               PTRACE_GETEVENTMSG returns the former thread ID.

               Stop before exit (including death from exit_group(2)),
               signal death, or exit caused by execve(2) in a multi-
               threaded process.  PTRACE_GETEVENTMSG returns the exit
               status.  Registers can be examined (unlike when "real"
               exit happens).  The tracee is still alive; it needs to
               be PTRACE_CONTed or PTRACE_DETACHed to finish exiting.

               Stop induced by PTRACE_INTERRUPT command, or group-
               stop, or initial ptrace-stop when a new child is
               attached (only if attached using PTRACE_SEIZE).

               Stop triggered by a seccomp(2) rule on tracee syscall
               entry when PTRACE_O_TRACESECCOMP has been set by the
               tracer.  The seccomp event message data (from the
               SECCOMP_RET_DATA portion of the seccomp filter rule)
               can be retrieved with PTRACE_GETEVENTMSG.  The seman-
               tics of this stop are described in detail in a separate
               section below.

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

          If the tracee was restarted by PTRACE_SYSCALL or
          PTRACE_SYSEMU, the tracee enters syscall-enter-stop just
          prior to entering any system call (which will not be exe-
          cuted 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 inter-
          rupted 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 contin-
          ued using any other method (including PTRACE_SYSEMU), no
          syscall-exit-stop occurs.  Note that all mentions

          However, even if the tracee was continued using

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          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

               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.

               SIGTRAP was sent by the kernel.

              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 per-
          formance penalty.

          Syscall-enter-stop and syscall-exit-stop are

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          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

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

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

          Restarts from this stop will behave as if the stop had
          occurred right before the system call in question.  In par-
          ticular, 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 res-
          tarted 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

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

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

          [Details of these kinds of stops are yet to be documented.]

        Informational and restarting ptrace commands
          Most ptrace commands (all except PTRACE_ATTACH,
          PTRACE_KILL) require the tracee to be in a ptrace-stop, oth-
          erwise 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, set-
          ting 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,

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

              ptrace(cmd, pid, 0, sig);

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

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

              ptrace(PTRACE_ATTACH, pid, 0, 0);


              ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);

          PTRACE_ATTACH sends SIGSTOP to this thread.  If the tracer
          wants this SIGSTOP to have no effect, it needs to suppress
          it.  Note that if other signals are concurrently sent to
          this thread during attach, the tracer may see the tracee
          enter signal-delivery-stop with other signal(s) first!  The
          usual practice is to reinject these signals until SIGSTOP is
          seen, then suppress SIGSTOP injection.  The design bug here
          is that a ptrace attach and a concurrently delivered SIGSTOP
          may race and the concurrent SIGSTOP may be lost.

          Since attaching sends SIGSTOP and the tracer usually sup-
          presses 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 pro-
          cess.  If you need to stop it after attach (or at any other
          time) without sending it any signals, use PTRACE_INTERRUPT

          The request

              ptrace(PTRACE_TRACEME, 0, 0, 0);

          turns the calling thread into a tracee.  The thread contin-
          ues to run (doesn't enter ptrace-stop).  A common practice
          is to follow the PTRACE_TRACEME with


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          and allow the parent (which is our tracer now) to observe
          our signal-delivery-stop.

          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.  Other-
          wise, 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 tra-
          cee 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 com-
          plication 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 con-
          fusing to tracers:

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          *  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 gener-
          ation on execve(2).

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

          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

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          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, determin-
          ing 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 sig-
          naling 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

          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 cor-

          *  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

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          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 appropri-
          ately.  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.

               (i386 only) There was an error with allocating or free-
               ing a debug register.

               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 arbi-

               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 viola-
               tion, or an invalid signal was specified during a res-
               tart request.

               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 pro-
               cess may already be being traced, or (on kernels before
               2.6.26) be init(1) (PID 1).

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


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          SVr4, 4.3BSD.

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

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

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

          The layout of the contents of memory and the USER area are
          quite operating-system- and architecture-specific.  The off-
          set 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 cur-
          rently 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 "tar-
          get" process is dumpable, and the results of checks per-
          formed by any enabled Linux Security Module (LSM)-for exam-
          ple, 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 dis-

               For "read" operations or other operations that are less
               dangerous, such as: get_robust_list(2); kcmp(2); read-
               ing /proc/[pid]/auxv, /proc/[pid]/environ, or

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               /proc/[pid]/stat; or readlink(2) of a /proc/[pid]/ns/*

               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 effec-
               tively the default before Linux 2.6.27.)

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

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

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

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

               Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.




          One further modifier can be ORed with the access mode:

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

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          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 vari-
          ous pseudofiles (e.g., under /proc). These names are used in
          other manual pages to provide a simple shorthand for label-
          ing 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 corre-
          sponding PID is the "target process".)  The algorithm is as

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

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

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

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

             +o The real, effective, and saved-set user IDs of the tar-
               get match the caller's user ID, and the real, effec-
               tive, 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 discus-
             sion of PR_SET_DUMPABLE in prctl(2)), and the caller does
             not have the CAP_SYS_PTRACE capability in the user names-
             pace 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

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             interface in the commoncap LSM performs the following

             a) If the access mode includes PTRACE_MODE_FSCREDS, then
                use the caller's effective capability set in the fol-
                lowing 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:

                +o 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 capabili-

                +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.

          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 com-
          promised process can ptrace-attach to other sensitive pro-
          cesses (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:

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          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 pre-
               defined 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 names-
          pace 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,

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          PTRACE_PEEKDATA, and PTRACE_PEEKUSER requests have a differ-
          ent API: they store the result at the address specified by
          the data parameter, and the return value is the error flag.
          The glibc wrapper function provides the API given in
          DESCRIPTION above, with the result being returned via the
          function return value.

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

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

          If a thread group leader is traced and exits by calling
          _exit(2), a PTRACE_EVENT_EXIT stop will happen for it (if
          requested), but the subsequent WIFEXITED notification will
          not be delivered until all other threads exit.  As explained
          above, if one of other threads calls execve(2), the death of
          the thread group leader will never be reported.  If the
          execed thread is not traced by this tracer, the tracer will
          never know that execve(2) happened.  One possible workaround
          is to PTRACE_DETACH the thread group leader instead of
          restarting it in this case.  Last confirmed on

          A SIGKILL signal may still cause a PTRACE_EVENT_EXIT stop
          before actual signal death.  This may be changed in the
          future; SIGKILL is meant to always immediately kill tasks
          even under ptrace.  Last confirmed on Linux 3.13.

          Some system calls return with EINTR if a signal was sent to
          a tracee, but delivery was suppressed by the tracer.  (This
          is very typical operation: it is usually done by debuggers
          on every attach, in order to not introduce a bogus SIGSTOP).
          As of Linux 3.2.9, the following system calls are affected
          (this list is likely incomplete): epoll_wait(2), and read(2)
          from an inotify(7) file descriptor.  The usual symptom of
          this bug is that when you attach to a quiescent process with
          the command

              strace -p <process-ID>

          then, instead of the usual and expected one-line output such

              restart_syscall(<... resuming interrupted call ...>_


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              select(6, [5], NULL, [5], NULL_

          ('_' denotes the cursor position), you observe more than one
          line.  For example:

                  clock_gettime(CLOCK_MONOTONIC, {15370, 690928118}) = 0

          What is not visible here is that the process was blocked in
          epoll_wait(2) before strace(1) has attached to it.  Attach-
          ing caused epoll_wait(2) to return to user space with the
          error EINTR.  In this particular case, the program reacted
          to EINTR by checking the current time, and then executing
          epoll_wait(2) again.  (Programs which do not expect such
          "stray" EINTR errors may behave in an unintended way upon an
          strace(1) attach.)

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

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

          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at

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