# ptrace

## 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 size.  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_SEIZE** d 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_CONT** ed or **PTRACE_DETACH** ed 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 **SIGSTOP** s.  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_ATTACH** ed rather that
           **PTRACE_SEIZE** d, 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.
    


## 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.
    


* * *