oracular (2) ptrace.2.gz

Provided by: manpages-dev_6.8-2_all bug

NAME

       ptrace - process trace

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/ptrace.h>

       long ptrace(enum __ptrace_request op, 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 operations 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 op determines the operation to be performed:

       PTRACE_TRACEME
              Indicate  that this process is to be traced by its parent.  A process probably shouldn't make this
              operation if its parent isn't expecting to trace it.  (pid, addr, and data are ignored.)

              The PTRACE_TRACEME operation is used only by the tracee; the remaining operations are used only by
              the  tracer.   In  the following operations, pid specifies the thread ID of the tracee to be acted
              on.  For operations 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 operations
              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 operations 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_SET_SYSCALL (since Linux 2.6.16)
              When  in  syscall-enter-stop, change the number of the system call that is about to be executed to
              the number specified in the data argument.  The addr  argument  is  ignored.   This  operation  is
              currently  supported  only  on  arm (and arm64, though only for backwards compatibility), but most
              other architectures have other means of accomplishing this (usually by changing the register  that
              the userland code passed the system call number in).

       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 operations 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.)

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

              The ptrace_syscall_info structure contains the following fields:

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

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

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

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

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

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

              PTRACE_SYSCALL_INFO_NONE
                     No component of the union contains relevant information.

              In  case  of  system  call  entry  or  exit stops, the data returned by PTRACE_GET_SYSCALL_INFO is
              limited to type PTRACE_SYSCALL_INFO_NONE unless PTRACE_O_TRACESYSGOOD option  is  set  before  the
              corresponding system call stop has occurred.

   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 operations.

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

           ((PTRACE_EVENT_foo<<8) | SIGTRAP).

       The following events exist:

       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 operation.

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

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

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

           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*   operations   return   the   requested  data  (but  see  NOTES),  the
       PTRACE_SECCOMP_GET_FILTER  operation  returns  the  number  of  instructions  in  the  BPF  program,  the
       PTRACE_GET_SYSCALL_INFO  operation returns the number of bytes available to be written by the kernel, and
       other operations return zero.

       On error, all operations return -1, and errno is set to indicate the error.  Since the value returned  by
       a  successful  PTRACE_PEEK*  operation  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    op 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 operation.

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

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

STANDARDS

       None.

HISTORY

       SVr4, 4.3BSD.

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

NOTES

       Although arguments to ptrace() are interpreted according to the prototype given, glibc currently declares
       ptrace() as a variadic function with only the op 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.

       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:

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

            (5.2)  Deny access if neither of the following is true:

                   •  The  caller  and  the  target  process  are  in  the same user namespace, and the caller's
                      capabilities are a superset of the target process's permitted 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  operations  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 Linux 2.6 kernel headers, PTRACE_SETOPTIONS is declared with a different value than the one
       for Linux 2.4.  This leads to applications compiled with Linux 2.6 kernel headers  failing  when  run  on
       Linux 2.4.  This can be worked around by redefining 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)