ptrace - process trace
#include <sys/ptrace.h>
long ptrace(enum __ptrace_request request, pid_t pid,
void *addr, void *data);
The
ptrace() system call provides a means by which one process (the
"tracer") may observe and control the execution of another process
(the "tracee"), and examine and change the tracee's memory and
registers. It is primarily used to implement breakpoint debugging and system
call tracing.
A tracee first needs to be attached to the tracer. Attachment and subsequent
commands are per thread: in a multithreaded process, every thread can be
individually attached to a (potentially different) tracer, or left not
attached and thus not debugged. Therefore, "tracee" always means
"(one) thread", never "a (possibly multithreaded)
process". Ptrace commands are always sent to a specific tracee using a
call of the form
ptrace(PTRACE_foo, pid, ...)
where
pid is the thread ID of the corresponding Linux thread.
(Note that in this page, a "multithreaded process" means a thread
group consisting of threads created using the
clone(2)
CLONE_THREAD flag.)
A process can initiate a trace by calling
fork(2) and having the
resulting child do a
PTRACE_TRACEME, followed (typically) by an
execve(2). Alternatively, one process may commence tracing another
process using
PTRACE_ATTACH or
PTRACE_SEIZE.
While being traced, the tracee will stop each time a signal is delivered, even
if the signal is being ignored. (An exception is
SIGKILL, which has its
usual effect.) The tracer will be notified at its next call to
waitpid(2) (or one of the related "wait" system calls); that
call will return a
status value containing information that indicates
the cause of the stop in the tracee. While the tracee is stopped, the tracer
can use various ptrace requests to inspect and modify the tracee. The tracer
then causes the tracee to continue, optionally ignoring the delivered signal
(or even delivering a different signal instead).
If the
PTRACE_O_TRACEEXEC option is not in effect, all successful calls
to
execve(2) by the traced process will cause it to be sent a
SIGTRAP signal, giving the parent a chance to gain control before the
new program begins execution.
When the tracer is finished tracing, it can cause the tracee to continue
executing in a normal, untraced mode via
PTRACE_DETACH.
The value of
request determines the action to be performed:
- PTRACE_TRACEME
- Indicate that this process is to be traced by its parent. A process
probably shouldn't make this request if its parent isn't expecting to
trace it. (pid, addr, and data are ignored.)
- The PTRACE_TRACEME request is used only by the tracee; the
remaining requests are used only by the tracer. In the following requests,
pid specifies the thread ID of the tracee to be acted on. For
requests other than PTRACE_ATTACH, PTRACE_SEIZE,
PTRACE_INTERRUPT, and PTRACE_KILL, the tracee must be
stopped.
- PTRACE_PEEKTEXT, PTRACE_PEEKDATA
- Read a word at the address addr in the tracee's memory, returning
the word as the result of the ptrace() call. Linux does not have
separate text and data address spaces, so these two requests are currently
equivalent. (data is ignored; but see NOTES.)
- PTRACE_PEEKUSER
- Read a word at offset addr in the tracee's USER area, which holds
the registers and other information about the process (see
<sys/user.h>). The word is returned as the result of the
ptrace() call. Typically, the offset must be word-aligned, though
this might vary by architecture. See NOTES. (data is ignored; but
see NOTES.)
- PTRACE_POKETEXT, PTRACE_POKEDATA
- Copy the word data to the address addr in the tracee's
memory. As for PTRACE_PEEKTEXT and PTRACE_PEEKDATA, these
two requests are currently equivalent.
- PTRACE_POKEUSER
- Copy the word data to offset addr in the tracee's USER area.
As for PTRACE_PEEKUSER, the offset must typically be word-aligned.
In order to maintain the integrity of the kernel, some modifications to
the USER area are disallowed.
- PTRACE_GETREGS, PTRACE_GETFPREGS
- Copy the tracee's general-purpose or floating-point registers,
respectively, to the address data in the tracer. See
<sys/user.h> for information on the format of this data.
(addr is ignored.) Note that SPARC systems have the meaning of
data and addr reversed; that is, data is ignored and
the registers are copied to the address addr. PTRACE_GETREGS
and PTRACE_GETFPREGS are not present on all architectures.
- PTRACE_GETREGSET (since Linux 2.6.34)
- Read the tracee's registers. addr specifies, in an
architecture-dependent way, the type of registers to be read.
NT_PRSTATUS (with numerical value 1) usually results in reading of
general-purpose registers. If the CPU has, for example, floating-point
and/or vector registers, they can be retrieved by setting addr to
the corresponding NT_foo constant. data points to a
struct iovec, which describes the destination buffer's location and
length. On return, the kernel modifies iov.len to indicate the
actual number of bytes returned.
- PTRACE_SETREGS, PTRACE_SETFPREGS
- Modify the tracee's general-purpose or floating-point registers,
respectively, from the address data in the tracer. As for
PTRACE_POKEUSER, some general-purpose register modifications may be
disallowed. (addr is ignored.) Note that SPARC systems have the
meaning of data and addr reversed; that is, data is
ignored and the registers are copied from the address addr.
PTRACE_SETREGS and PTRACE_SETFPREGS are not present on all
architectures.
- PTRACE_SETREGSET (since Linux 2.6.34)
- Modify the tracee's registers. The meaning of addr and data
is analogous to PTRACE_GETREGSET.
- PTRACE_GETSIGINFO (since Linux 2.3.99-pre6)
- Retrieve information about the signal that caused the stop. Copy a
siginfo_t structure (see sigaction(2)) from the tracee to
the address data in the tracer. (addr is ignored.)
- PTRACE_SETSIGINFO (since Linux 2.3.99-pre6)
- Set signal information: copy a siginfo_t structure from the address
data in the tracer to the tracee. This will affect only signals
that would normally be delivered to the tracee and were caught by the
tracer. It may be difficult to tell these normal signals from synthetic
signals generated by ptrace() itself. (addr is
ignored.)
- PTRACE_PEEKSIGINFO (since Linux 3.10)
- Retrieve siginfo_t structures without removing signals from a
queue. addr points to a ptrace_peeksiginfo_args structure
that specifies the ordinal position from which copying of signals should
start, and the number of signals to copy. siginfo_t structures are
copied into the buffer pointed to by data. The return value
contains the number of copied signals (zero indicates that there is no
signal corresponding to the specified ordinal position). Within the
returned siginfo structures, the si_code field includes
information (__SI_CHLD, __SI_FAULT, etc.) that are not
otherwise exposed to user space.
struct ptrace_peeksiginfo_args {
u64 off; /* Ordinal position in queue at which
to start copying signals */
u32 flags; /* PTRACE_PEEKSIGINFO_SHARED or 0 */
s32 nr; /* Number of signals to copy */
};
- Currently, there is only one flag, PTRACE_PEEKSIGINFO_SHARED, for
dumping signals from the process-wide signal queue. If this flag is not
set, signals are read from the per-thread queue of the specified thread.
- PTRACE_GETSIGMASK (since Linux 3.11)
- Place a copy of the mask of blocked signals (see sigprocmask(2)) in
the buffer pointed to by data, which should be a pointer to a
buffer of type sigset_t. The addr argument contains the size
of the buffer pointed to by data (i.e.,
sizeof(sigset_t)).
- PTRACE_SETSIGMASK (since Linux 3.11)
- Change the mask of blocked signals (see sigprocmask(2)) to the
value specified in the buffer pointed to by data, which should be a
pointer to a buffer of type sigset_t. The addr argument
contains the size of the buffer pointed to by data (i.e.,
sizeof(sigset_t)).
- PTRACE_SETOPTIONS (since Linux 2.4.6; see BUGS for caveats)
- Set ptrace options from data. (addr is ignored.) data
is interpreted as a bit mask of options, which are specified by the
following flags:
- PTRACE_O_EXITKILL (since Linux 3.8)
- Send a SIGKILL signal to the tracee if the tracer exits. This
option is useful for ptrace jailers that want to ensure that tracees can
never escape the tracer's control.
- PTRACE_O_TRACECLONE (since Linux 2.5.46)
- Stop the tracee at the next clone(2) and automatically start
tracing the newly cloned process, which will start with a SIGSTOP,
or PTRACE_EVENT_STOP if PTRACE_SEIZE was used. A
waitpid(2) by the tracer will return a status value such
that
-
status>>8 == (SIGTRAP | (PTRACE_EVENT_CLONE<<8))
- The PID of the new process can be retrieved with
PTRACE_GETEVENTMSG.
- This option may not catch clone(2) calls in all cases. If the
tracee calls clone(2) with the CLONE_VFORK flag,
PTRACE_EVENT_VFORK will be delivered instead if
PTRACE_O_TRACEVFORK is set; otherwise if the tracee calls
clone(2) with the exit signal set to SIGCHLD,
PTRACE_EVENT_FORK will be delivered if PTRACE_O_TRACEFORK is
set.
- PTRACE_O_TRACEEXEC (since Linux 2.5.46)
- Stop the tracee at the next execve(2). A waitpid(2) by the
tracer will return a status value such that
-
status>>8 == (SIGTRAP | (PTRACE_EVENT_EXEC<<8))
- If the execing thread is not a thread group leader, the thread ID is reset
to thread group leader's ID before this stop. Since Linux 3.0, the former
thread ID can be retrieved with PTRACE_GETEVENTMSG.
- PTRACE_O_TRACEEXIT (since Linux 2.5.60)
- Stop the tracee at exit. A waitpid(2) by the tracer will return a
status value such that
-
status>>8 == (SIGTRAP | (PTRACE_EVENT_EXIT<<8))
- The tracee's exit status can be retrieved with
PTRACE_GETEVENTMSG.
- The tracee is stopped early during process exit, when registers are still
available, allowing the tracer to see where the exit occurred, whereas the
normal exit notification is done after the process is finished exiting.
Even though context is available, the tracer cannot prevent the exit from
happening at this point.
- PTRACE_O_TRACEFORK (since Linux 2.5.46)
- Stop the tracee at the next fork(2) and automatically start tracing
the newly forked process, which will start with a SIGSTOP, or
PTRACE_EVENT_STOP if PTRACE_SEIZE was used. A
waitpid(2) by the tracer will return a status value such
that
-
status>>8 == (SIGTRAP | (PTRACE_EVENT_FORK<<8))
- The PID of the new process can be retrieved with
PTRACE_GETEVENTMSG.
- PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
- When delivering system call traps, set bit 7 in the signal number (i.e.,
deliver SIGTRAP|0x80). This makes it easy for the tracer to
distinguish normal traps from those caused by a system call.
- PTRACE_O_TRACEVFORK (since Linux 2.5.46)
- Stop the tracee at the next vfork(2) and automatically start
tracing the newly vforked process, which will start with a SIGSTOP,
or PTRACE_EVENT_STOP if PTRACE_SEIZE was used. A
waitpid(2) by the tracer will return a status value such
that
-
status>>8 == (SIGTRAP | (PTRACE_EVENT_VFORK<<8))
- The PID of the new process can be retrieved with
PTRACE_GETEVENTMSG.
- PTRACE_O_TRACEVFORKDONE (since Linux 2.5.60)
- Stop the tracee at the completion of the next vfork(2). A
waitpid(2) by the tracer will return a status value such
that
-
status>>8 == (SIGTRAP | (PTRACE_EVENT_VFORK_DONE<<8))
- The PID of the new process can (since Linux 2.6.18) be retrieved with
PTRACE_GETEVENTMSG.
- PTRACE_O_TRACESECCOMP (since Linux 3.5)
- Stop the tracee when a seccomp(2) SECCOMP_RET_TRACE rule is
triggered. A waitpid(2) by the tracer will return a status
value such that
-
status>>8 == (SIGTRAP | (PTRACE_EVENT_SECCOMP<<8))
- While this triggers a PTRACE_EVENT stop, it is similar to a
syscall-enter-stop. For details, see the note on
PTRACE_EVENT_SECCOMP below. The seccomp event message data (from
the SECCOMP_RET_DATA portion of the seccomp filter rule) can be
retrieved with PTRACE_GETEVENTMSG.
- PTRACE_O_SUSPEND_SECCOMP (since Linux 4.3)
- Suspend the tracee's seccomp protections. This applies regardless of mode,
and can be used when the tracee has not yet installed seccomp filters.
That is, a valid use case is to suspend a tracee's seccomp protections
before they are installed by the tracee, let the tracee install the
filters, and then clear this flag when the filters should be resumed.
Setting this option requires that the tracer have the CAP_SYS_ADMIN
capability, not have any seccomp protections installed, and not have
PTRACE_O_SUSPEND_SECCOMP set on itself.
- PTRACE_GETEVENTMSG (since Linux 2.5.46)
- Retrieve a message (as an unsigned long) about the ptrace event
that just happened, placing it at the address data in the tracer.
For PTRACE_EVENT_EXIT, this is the tracee's exit status. For
PTRACE_EVENT_FORK, PTRACE_EVENT_VFORK,
PTRACE_EVENT_VFORK_DONE, and PTRACE_EVENT_CLONE, this is the
PID of the new process. For PTRACE_EVENT_SECCOMP, this is the
seccomp(2) filter's SECCOMP_RET_DATA associated with the
triggered rule. (addr is ignored.)
- PTRACE_CONT
- Restart the stopped tracee process. If data is nonzero, it is
interpreted as the number of a signal to be delivered to the tracee;
otherwise, no signal is delivered. Thus, for example, the tracer can
control whether a signal sent to the tracee is delivered or not.
(addr is ignored.)
- PTRACE_SYSCALL, PTRACE_SINGLESTEP
- Restart the stopped tracee as for PTRACE_CONT, but arrange for the
tracee to be stopped at the next entry to or exit from a system call, or
after execution of a single instruction, respectively. (The tracee will
also, as usual, be stopped upon receipt of a signal.) From the tracer's
perspective, the tracee will appear to have been stopped by receipt of a
SIGTRAP. So, for PTRACE_SYSCALL, for example, the idea is to
inspect the arguments to the system call at the first stop, then do
another PTRACE_SYSCALL and inspect the return value of the system
call at the second stop. The data argument is treated as for
PTRACE_CONT. (addr is ignored.)
- PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP (since Linux
2.6.14)
- For PTRACE_SYSEMU, continue and stop on entry to the next system
call, which will not be executed. See the documentation on syscall-stops
below. For PTRACE_SYSEMU_SINGLESTEP, do the same but also
singlestep if not a system call. This call is used by programs like User
Mode Linux that want to emulate all the tracee's system calls. The
data argument is treated as for PTRACE_CONT. The addr
argument is ignored. These requests are currently supported only on
x86.
- PTRACE_LISTEN (since Linux 3.4)
- Restart the stopped tracee, but prevent it from executing. The resulting
state of the tracee is similar to a process which has been stopped by a
SIGSTOP (or other stopping signal). See the "group-stop"
subsection for additional information. PTRACE_LISTEN works only on
tracees attached by PTRACE_SEIZE.
- PTRACE_KILL
- Send the tracee a SIGKILL to terminate it. (addr and
data are ignored.)
- This operation is deprecated; do not use it! Instead, send a
SIGKILL directly using kill(2) or tgkill(2). The
problem with PTRACE_KILL is that it requires the tracee to be in
signal-delivery-stop, otherwise it may not work (i.e., may complete
successfully but won't kill the tracee). By contrast, sending a
SIGKILL directly has no such limitation.
- PTRACE_INTERRUPT (since Linux 3.4)
- Stop a tracee. If the tracee is running or sleeping in kernel space and
PTRACE_SYSCALL is in effect, the system call is interrupted and
syscall-exit-stop is reported. (The interrupted system call is restarted
when the tracee is restarted.) If the tracee was already stopped by a
signal and PTRACE_LISTEN was sent to it, the tracee stops with
PTRACE_EVENT_STOP and WSTOPSIG(status) returns the stop
signal. If any other ptrace-stop is generated at the same time (for
example, if a signal is sent to the tracee), this ptrace-stop happens. If
none of the above applies (for example, if the tracee is running in user
space), it stops with PTRACE_EVENT_STOP with
WSTOPSIG(status) == SIGTRAP. PTRACE_INTERRUPT only
works on tracees attached by PTRACE_SEIZE.
- PTRACE_ATTACH
- Attach to the process specified in pid, making it a tracee of the
calling process. The tracee is sent a SIGSTOP, but will not
necessarily have stopped by the completion of this call; use
waitpid(2) to wait for the tracee to stop. See the "Attaching
and detaching" subsection for additional information. (addr
and data are ignored.)
- Permission to perform a PTRACE_ATTACH is governed by a ptrace
access mode PTRACE_MODE_ATTACH_REALCREDS check; see below.
- PTRACE_SEIZE (since Linux 3.4)
- Attach to the process specified in pid, making it a tracee of the
calling process. Unlike PTRACE_ATTACH, PTRACE_SEIZE does not
stop the process. Group-stops are reported as PTRACE_EVENT_STOP and
WSTOPSIG(status) returns the stop signal. Automatically attached
children stop with PTRACE_EVENT_STOP and WSTOPSIG(status)
returns SIGTRAP instead of having SIGSTOP signal delivered
to them. execve(2) does not deliver an extra SIGTRAP. Only a
PTRACE_SEIZEd process can accept PTRACE_INTERRUPT and
PTRACE_LISTEN commands. The "seized" behavior just
described is inherited by children that are automatically attached using
PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, and
PTRACE_O_TRACECLONE. addr must be zero. data contains
a bit mask of ptrace options to activate immediately.
- Permission to perform a PTRACE_SEIZE is governed by a ptrace access
mode PTRACE_MODE_ATTACH_REALCREDS check; see below.
- PTRACE_SECCOMP_GET_FILTER (since Linux 4.4)
- This operation allows the tracer to dump the tracee's classic BPF
filters.
- addr is an integer specifying the index of the filter to be dumped.
The most recently installed filter has the index 0. If addr is
greater than the number of installed filters, the operation fails with the
error ENOENT.
- data is either a pointer to a struct sock_filter array that
is large enough to store the BPF program, or NULL if the program is not to
be stored.
- Upon success, the return value is the number of instructions in the BPF
program. If data was NULL, then this return value can be used to
correctly size the struct sock_filter array passed in a subsequent
call.
- This operation fails with the error EACCES if the caller does not
have the CAP_SYS_ADMIN capability or if the caller is in strict or
filter seccomp mode. If the filter referred to by addr is not a
classic BPF filter, the operation fails with the error
EMEDIUMTYPE.
- This operation is available if the kernel was configured with both the
CONFIG_SECCOMP_FILTER and the CONFIG_CHECKPOINT_RESTORE
options.
- PTRACE_DETACH
- Restart the stopped tracee as for PTRACE_CONT, but first detach
from it. Under Linux, a tracee can be detached in this way regardless of
which method was used to initiate tracing. (addr is ignored.)
- PTRACE_GET_THREAD_AREA (since Linux 2.6.0)
- This operation performs a similar task to get_thread_area(2). It
reads the TLS entry in the GDT whose index is given in addr,
placing a copy of the entry into the struct user_desc pointed to by
data. (By contrast with get_thread_area(2), the
entry_number of the struct user_desc is ignored.)
- PTRACE_SET_THREAD_AREA (since Linux 2.6.0)
- This operation performs a similar task to set_thread_area(2). It
sets the TLS entry in the GDT whose index is given in addr,
assigning it the data supplied in the struct user_desc pointed to
by data. (By contrast with set_thread_area(2), the
entry_number of the struct user_desc is ignored; in other
words, this ptrace operation can't be used to allocate a free TLS
entry.)
- 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.
When a (possibly multithreaded) process receives a killing signal (one whose
disposition is set to
SIG_DFL and whose default action is to kill the
process), all threads exit. Tracees report their death to their tracer(s).
Notification of this event is delivered via
waitpid(2).
Note that the killing signal will first cause signal-delivery-stop (on one
tracee only), and only after it is injected by the tracer (or after it was
dispatched to a thread which isn't traced), will death from the signal happen
on
all tracees within a multithreaded process. (The term
"signal-delivery-stop" is explained below.)
SIGKILL does not generate signal-delivery-stop and therefore the tracer
can't suppress it.
SIGKILL kills even within system calls
(syscall-exit-stop is not generated prior to death by
SIGKILL). The net
effect is that
SIGKILL always kills the process (all its threads), even
if some threads of the process are ptraced.
When the tracee calls
_exit(2), it reports its death to its tracer. Other
threads are not affected.
When any thread executes
exit_group(2), every tracee in its thread group
reports its death to its tracer.
If the
PTRACE_O_TRACEEXIT option is on,
PTRACE_EVENT_EXIT will
happen before actual death. This applies to exits via
exit(2),
exit_group(2), and signal deaths (except
SIGKILL, depending on
the kernel version; see BUGS below), and when threads are torn down on
execve(2) in a multithreaded process.
The tracer cannot assume that the ptrace-stopped tracee exists. There are many
scenarios when the tracee may die while stopped (such as
SIGKILL).
Therefore, the tracer must be prepared to handle an
ESRCH error on any
ptrace operation. Unfortunately, the same error is returned if the tracee
exists but is not ptrace-stopped (for commands which require a stopped
tracee), or if it is not traced by the process which issued the ptrace call.
The tracer needs to keep track of the stopped/running state of the tracee, and
interpret
ESRCH as "tracee died unexpectedly" only if it
knows that the tracee has been observed to enter ptrace-stop. Note that there
is no guarantee that
waitpid(WNOHANG) will reliably report the tracee's
death status if a ptrace operation returned
ESRCH.
waitpid(WNOHANG) may return 0 instead. In other words, the tracee may
be "not yet fully dead", but already refusing ptrace requests.
The tracer can't assume that the tracee
always ends its life by reporting
WIFEXITED(status) or
WIFSIGNALED(status); there are cases where
this does not occur. For example, if a thread other than thread group leader
does an
execve(2), it disappears; its PID will never be seen again, and
any subsequent ptrace stops will be reported under the thread group leader's
PID.
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.)
When a (possibly multithreaded) process receives any signal except
SIGKILL, the kernel selects an arbitrary thread which handles the
signal. (If the signal is generated with
tgkill(2), the target thread
can be explicitly selected by the caller.) If the selected thread is traced,
it enters signal-delivery-stop. At this point, the signal is not yet delivered
to the process, and can be suppressed by the tracer. If the tracer doesn't
suppress the signal, it passes the signal to the tracee in the next ptrace
restart request. This second step of signal delivery is called
signal
injection in this manual page. Note that if the signal is blocked,
signal-delivery-stop doesn't happen until the signal is unblocked, with the
usual exception that
SIGSTOP can't be blocked.
Signal-delivery-stop is observed by the tracer as
waitpid(2) returning
with
WIFSTOPPED(status) true, with the signal returned by
WSTOPSIG(status). If the signal is
SIGTRAP, this may be a
different kind of ptrace-stop; see the "Syscall-stops" and
"execve" sections below for details. If
WSTOPSIG(status)
returns a stopping signal, this may be a group-stop; see below.
After signal-delivery-stop is observed by the tracer, the tracer should restart
the tracee with the call
ptrace(PTRACE_restart, pid, 0, sig)
where
PTRACE_restart is one of the restarting ptrace requests. If
sig is 0, then a signal is not delivered. Otherwise, the signal
sig is delivered. This operation is called
signal injection in
this manual page, to distinguish it from signal-delivery-stop.
The
sig value may be different from the
WSTOPSIG(status) value:
the tracer can cause a different signal to be injected.
Note that a suppressed signal still causes system calls to return prematurely.
In this case, system calls will be restarted: the tracer will observe the
tracee to reexecute the interrupted system call (or
restart_syscall(2)
system call for a few system calls which use a different mechanism for
restarting) if the tracer uses
PTRACE_SYSCALL. Even system calls (such
as
poll(2)) which are not restartable after signal are restarted after
signal is suppressed; however, kernel bugs exist which cause some system calls
to fail with
EINTR even though no observable signal is injected to the
tracee.
Restarting ptrace commands issued in ptrace-stops other than
signal-delivery-stop are not guaranteed to inject a signal, even if
sig
is nonzero. No error is reported; a nonzero
sig may simply be ignored.
Ptrace users should not try to "create a new signal" this way: use
tgkill(2) instead.
The fact that signal injection requests may be ignored when restarting the
tracee after ptrace stops that are not signal-delivery-stops is a cause of
confusion among ptrace users. One typical scenario is that the tracer observes
group-stop, mistakes it for signal-delivery-stop, restarts the tracee with
ptrace(PTRACE_restart, pid, 0, stopsig)
with the intention of injecting
stopsig, but
stopsig gets ignored
and the tracee continues to run.
The
SIGCONT signal has a side effect of waking up (all threads of) a
group-stopped process. This side effect happens before signal-delivery-stop.
The tracer can't suppress this side effect (it can only suppress signal
injection, which only causes the
SIGCONT handler to not be executed in
the tracee, if such a handler is installed). In fact, waking up from
group-stop may be followed by signal-delivery-stop for signal(s)
other
than SIGCONT, if they were pending when
SIGCONT was
delivered. In other words,
SIGCONT may be not the first signal observed
by the tracee after it was sent.
Stopping signals cause (all threads of) a process to enter group-stop. This side
effect happens after signal injection, and therefore can be suppressed by the
tracer.
In Linux 2.4 and earlier, the
SIGSTOP signal can't be injected.
PTRACE_GETSIGINFO can be used to retrieve a
siginfo_t structure
which corresponds to the delivered signal.
PTRACE_SETSIGINFO may be
used to modify it. If
PTRACE_SETSIGINFO has been used to alter
siginfo_t, the
si_signo field and the
sig parameter in
the restarting command must match, otherwise the result is undefined.
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).
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.
If the tracee was restarted by
PTRACE_SYSCALL or
PTRACE_SYSEMU,
the tracee enters syscall-enter-stop just prior to entering any system call
(which will not be executed if the restart was using
PTRACE_SYSEMU,
regardless of any change made to registers at this point or how the tracee is
restarted after this stop). No matter which method caused the
syscall-entry-stop, if the tracer restarts the tracee with
PTRACE_SYSCALL, the tracee enters syscall-exit-stop when the system
call is finished, or if it is interrupted by a signal. (That is,
signal-delivery-stop never happens between syscall-enter-stop and
syscall-exit-stop; it happens
after syscall-exit-stop.). If the tracee
is continued using any other method (including
PTRACE_SYSEMU), no
syscall-exit-stop occurs. Note that all mentions
PTRACE_SYSEMU apply
equally to
PTRACE_SYSEMU_SINGLESTEP.
However, even if the tracee was continued using
PTRACE_SYSCALL, it is not
guaranteed that the next stop will be a syscall-exit-stop. Other possibilities
are that the tracee may stop in a
PTRACE_EVENT stop (including seccomp
stops), exit (if it entered
_exit(2) or
exit_group(2)), be
killed by
SIGKILL, or die silently (if it is a thread group leader, the
execve(2) happened in another thread, and that thread is not traced by
the same tracer; this situation is discussed later).
Syscall-enter-stop and syscall-exit-stop are observed by the tracer as
waitpid(2) returning with
WIFSTOPPED(status) true, and
WSTOPSIG(status) giving
SIGTRAP. If the
PTRACE_O_TRACESYSGOOD option was set by the tracer, then
WSTOPSIG(status) will give the value
(SIGTRAP | 0x80).
Syscall-stops can be distinguished from signal-delivery-stop with
SIGTRAP
by querying
PTRACE_GETSIGINFO for the following cases:
- si_code <= 0
- SIGTRAP was delivered as a result of a user-space action, for
example, a system call (tgkill(2), kill(2),
sigqueue(3), etc.), expiration of a POSIX timer, change of state on
a POSIX message queue, or completion of an asynchronous I/O request.
- si_code == SI_KERNEL (0x80)
- SIGTRAP was sent by the kernel.
- si_code == SIGTRAP or si_code == (SIGTRAP|0x80)
- This is a syscall-stop.
However, syscall-stops happen very often (twice per system call), and performing
PTRACE_GETSIGINFO for every syscall-stop may be somewhat expensive.
Some architectures allow the cases to be distinguished by examining registers.
For example, on x86,
rax == -
ENOSYS in syscall-enter-stop. Since
SIGTRAP (like any other signal) always happens
after
syscall-exit-stop, and at this point
rax almost never contains
-
ENOSYS, the
SIGTRAP looks like "syscall-stop which is not
syscall-enter-stop"; in other words, it looks like a "stray
syscall-exit-stop" and can be detected this way. But such detection is
fragile and is best avoided.
Using the
PTRACE_O_TRACESYSGOOD option is the recommended method to
distinguish syscall-stops from other kinds of ptrace-stops, since it is
reliable and does not incur a performance penalty.
Syscall-enter-stop and syscall-exit-stop are indistinguishable from each other
by the tracer. The tracer needs to keep track of the sequence of ptrace-stops
in order to not misinterpret syscall-enter-stop as syscall-exit-stop or vice
versa. In general, a syscall-enter-stop is always followed by
syscall-exit-stop,
PTRACE_EVENT stop, or the tracee's death; no other
kinds of ptrace-stop can occur in between. However, note that seccomp stops
(see below) can cause syscall-exit-stops, without preceding
syscall-entry-stops. If seccomp is in use, care needs to be taken not to
misinterpret such stops as syscall-entry-stops.
If after syscall-enter-stop, the tracer uses a restarting command other than
PTRACE_SYSCALL, syscall-exit-stop is not generated.
PTRACE_GETSIGINFO on syscall-stops returns
SIGTRAP in
si_signo, with
si_code set to
SIGTRAP or
(SIGTRAP|0x80).
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.
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.
[Details of these kinds of stops are yet to be documented.]
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.)
A thread can be attached to the tracer using the call
ptrace(PTRACE_ATTACH, pid, 0, 0);
or
ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);
PTRACE_ATTACH sends
SIGSTOP to this thread. If the tracer wants
this
SIGSTOP to have no effect, it needs to suppress it. Note that if
other signals are concurrently sent to this thread during attach, the tracer
may see the tracee enter signal-delivery-stop with other signal(s) first! The
usual practice is to reinject these signals until
SIGSTOP is seen, then
suppress
SIGSTOP injection. The design bug here is that a ptrace attach
and a concurrently delivered
SIGSTOP may race and the concurrent
SIGSTOP may be lost.
Since attaching sends
SIGSTOP and the tracer usually suppresses it, this
may cause a stray
EINTR return from the currently executing system call
in the tracee, as described in the "Signal injection and
suppression" section.
Since Linux 3.4,
PTRACE_SEIZE can be used instead of
PTRACE_ATTACH.
PTRACE_SEIZE does not stop the attached process.
If you need to stop it after attach (or at any other time) without sending it
any signals, use
PTRACE_INTERRUPT command.
The request
ptrace(PTRACE_TRACEME, 0, 0, 0);
turns the calling thread into a tracee. The thread continues to run (doesn't
enter ptrace-stop). A common practice is to follow the
PTRACE_TRACEME
with
raise(SIGSTOP);
and allow the parent (which is our tracer now) to observe our
signal-delivery-stop.
If the
PTRACE_O_TRACEFORK,
PTRACE_O_TRACEVFORK, or
PTRACE_O_TRACECLONE options are in effect, then children created by,
respectively,
vfork(2) or
clone(2) with the
CLONE_VFORK
flag,
fork(2) or
clone(2) with the exit signal set to
SIGCHLD, and other kinds of
clone(2), are automatically attached
to the same tracer which traced their parent.
SIGSTOP is delivered to
the children, causing them to enter signal-delivery-stop after they exit the
system call which created them.
Detaching of the tracee is performed by:
ptrace(PTRACE_DETACH, pid, 0, sig);
PTRACE_DETACH is a restarting operation; therefore it requires the tracee
to be in ptrace-stop. If the tracee is in signal-delivery-stop, a signal can
be injected. Otherwise, the
sig parameter may be silently ignored.
If the tracee is running when the tracer wants to detach it, the usual solution
is to send
SIGSTOP (using
tgkill(2), to make sure it goes to the
correct thread), wait for the tracee to stop in signal-delivery-stop for
SIGSTOP and then detach it (suppressing
SIGSTOP injection). A
design bug is that this can race with concurrent
SIGSTOPs. Another
complication is that the tracee may enter other ptrace-stops and needs to be
restarted and waited for again, until
SIGSTOP is seen. Yet another
complication is to be sure that the tracee is not already ptrace-stopped,
because no signal delivery happens while it is—not even
SIGSTOP.
If the tracer dies, all tracees are automatically detached and restarted, unless
they were in group-stop. Handling of restart from group-stop is currently
buggy, but the "as planned" behavior is to leave tracee stopped and
waiting for
SIGCONT. If the tracee is restarted from
signal-delivery-stop, the pending signal is injected.
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.
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.
On success, the
PTRACE_PEEK* requests return the requested data (but see
NOTES), the
PTRACE_SECCOMP_GET_FILTER request returns the number of
instructions in the BPF program, and other requests return zero.
On error, all requests return -1, and
errno is set appropriately. Since
the value returned by a successful
PTRACE_PEEK* request may be -1, the
caller must clear
errno before the call, and then check it afterward to
determine whether or not an error occurred.
- EBUSY
- (i386 only) There was an error with allocating or freeing a debug
register.
- EFAULT
- There was an attempt to read from or write to an invalid area in the
tracer's or the tracee's memory, probably because the area wasn't mapped
or accessible. Unfortunately, under Linux, different variations of this
fault will return EIO or EFAULT more or less
arbitrarily.
- EINVAL
- An attempt was made to set an invalid option.
- EIO
- request is invalid, or an attempt was made to read from or write to
an invalid area in the tracer's or the tracee's memory, or there was a
word-alignment violation, or an invalid signal was specified during a
restart request.
- EPERM
- The specified process cannot be traced. This could be because the tracer
has insufficient privileges (the required capability is
CAP_SYS_PTRACE); unprivileged processes cannot trace processes that
they cannot send signals to or those running set-user-ID/set-group-ID
programs, for obvious reasons. Alternatively, the process may already be
being traced, or (on kernels before 2.6.26) be init(1) (PID
1).
- ESRCH
- The specified process does not exist, or is not currently being traced by
the caller, or is not stopped (for requests that require a stopped
tracee).
SVr4, 4.3BSD.
Although arguments to
ptrace() are interpreted according to the prototype
given, glibc currently declares
ptrace() as a variadic function with
only the
request argument fixed. It is recommended to always supply
four arguments, even if the requested operation does not use them, setting
unused/ignored arguments to
0L or
(void *) 0.
In Linux kernels before 2.6.26,
init(1), the process with PID 1, may not
be traced.
A tracees parent continues to be the tracer even if that tracer calls
execve(2).
The layout of the contents of memory and the USER area are quite
operating-system- and architecture-specific. The 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.
Various parts of the kernel-user-space API (not just
ptrace()
operations), require so-called "ptrace access mode" checks, whose
outcome determines whether an operation is permitted (or, in a few cases,
causes a "read" operation to return sanitized data). These checks
are performed in cases where one process can inspect sensitive information
about, or in some cases modify the state of, another process. The checks are
based on factors such as the credentials and capabilities of the two
processes, whether or not the "target" process is dumpable, and the
results of checks performed by any enabled Linux Security Module
(LSM)—for example, SELinux, Yama, or Smack—and by the commoncap
LSM (which is always invoked).
Prior to Linux 2.6.27, all access checks were of a single type. Since Linux
2.6.27, two access mode levels are distinguished:
- PTRACE_MODE_READ
- For "read" operations or other operations that are less
dangerous, such as: get_robust_list(2); kcmp(2); reading
/proc/[pid]/auxv, /proc/[pid]/environ, or
/proc/[pid]/stat; or readlink(2) of a
/proc/[pid]/ns/* file.
- PTRACE_MODE_ATTACH
- For "write" operations, or other operations that are more
dangerous, such as: ptrace attaching (PTRACE_ATTACH) to another
process or calling process_vm_writev(2). (PTRACE_MODE_ATTACH
was effectively the default before Linux 2.6.27.)
Since Linux 4.5, the above access mode checks are combined (ORed) with one of
the following modifiers:
- PTRACE_MODE_FSCREDS
- Use the caller's filesystem UID and GID (see credentials(7)) or
effective capabilities for LSM checks.
- PTRACE_MODE_REALCREDS
- Use the caller's real UID and GID or permitted capabilities for LSM
checks. This was effectively the default before Linux 4.5.
Because combining one of the credential modifiers with one of the aforementioned
access modes is typical, some macros are defined in the kernel sources for the
combinations:
- PTRACE_MODE_READ_FSCREDS
- Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.
- PTRACE_MODE_READ_REALCREDS
- Defined as PTRACE_MODE_READ | PTRACE_MODE_REALCREDS.
- PTRACE_MODE_ATTACH_FSCREDS
- Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS.
- PTRACE_MODE_ATTACH_REALCREDS
- Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS.
One further modifier can be ORed with the access mode:
- PTRACE_MODE_NOAUDIT (since Linux 3.3)
- Don't audit this access mode check. This modifier is employed for ptrace
access mode checks (such as checks when reading /proc/[pid]/stat)
that merely cause the output to be filtered or sanitized, rather than
causing an error to be returned to the caller. In these cases, accessing
the file is not a security violation and there is no reason to generate a
security audit record. This modifier suppresses the generation of such an
audit record for the particular access check.
Note that all of the
PTRACE_MODE_* constants described in this subsection
are kernel-internal, and not visible to user space. The constant names are
mentioned here in order to label the various kinds of ptrace access mode
checks that are performed for various system calls and accesses to various
pseudofiles (e.g., under
/proc). These names are used in other manual
pages to provide a simple shorthand for labeling the different kernel checks.
The algorithm employed for ptrace access mode checking determines whether the
calling process is allowed to perform the corresponding action on the target
process. (In the case of opening
/proc/[pid] files, the "calling
process" is the one opening the file, and the process with the
corresponding PID is the "target process".) The algorithm is as
follows:
- 1.
- If the calling thread and the target thread are in the same thread group,
access is always allowed.
- 2.
- If the access mode specifies PTRACE_MODE_FSCREDS, then, for the
check in the next step, employ the caller's filesystem UID and GID. (As
noted in credentials(7), the filesystem UID and GID almost always
have the same values as the corresponding effective IDs.)
- Otherwise, the access mode specifies PTRACE_MODE_REALCREDS, so use
the caller's real UID and GID for the checks in the next step. (Most APIs
that check the caller's UID and GID use the effective IDs. For historical
reasons, the PTRACE_MODE_REALCREDS check uses the real IDs
instead.)
- 3.
- Deny access if neither of the following is true:
- •
- The real, effective, and saved-set user IDs of the target match the
caller's user ID, and the real, effective, and saved-set group IDs
of the target match the caller's group ID.
- •
- The caller has the CAP_SYS_PTRACE capability in the user namespace
of the target.
- 4.
- Deny access if the target process "dumpable" attribute has a
value other than 1 (SUID_DUMP_USER; see the discussion of
PR_SET_DUMPABLE in prctl(2)), and the caller does not have
the CAP_SYS_PTRACE capability in the user namespace of the target
process.
- 5.
- The kernel LSM security_ptrace_access_check() interface is invoked
to see if ptrace access is permitted. The results depend on the LSM(s).
The implementation of this interface in the commoncap LSM performs the
following steps:
- a)
- If the access mode includes PTRACE_MODE_FSCREDS, then use the
caller's effective capability set in the following check; otherwise
(the access mode specifies PTRACE_MODE_REALCREDS, so) use the
caller's permitted capability set.
- b)
- Deny access if neither of the following is true:
- •
- The caller and the target process are in the same user namespace, and the
caller's capabilities are a superset of the target process's
permitted capabilities.
- •
- The caller has the CAP_SYS_PTRACE capability in the target
process's user namespace.
- Note that the commoncap LSM does not distinguish between
PTRACE_MODE_READ and PTRACE_MODE_ATTACH.
- 6.
- If access has not been denied by any of the preceding steps, then access
is allowed.
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.
At the system call level, the
PTRACE_PEEKTEXT,
PTRACE_PEEKDATA,
and
PTRACE_PEEKUSER requests have a different API: they store the
result at the address specified by the
data parameter, and the return
value is the error flag. The glibc wrapper function provides the API given in
DESCRIPTION above, with the result being returned via the function return
value.
On hosts with 2.6 kernel headers,
PTRACE_SETOPTIONS is declared with a
different value than the one for 2.4. This leads to applications compiled with
2.6 kernel headers failing when run on 2.4 kernels. This can be worked around
by redefining
PTRACE_SETOPTIONS to
PTRACE_OLDSETOPTIONS, if that
is defined.
Group-stop notifications are sent to the tracer, but not to real parent. Last
confirmed on 2.6.38.6.
If a thread group leader is traced and exits by calling
_exit(2), a
PTRACE_EVENT_EXIT stop will happen for it (if requested), but the
subsequent
WIFEXITED notification will not be delivered until all other
threads exit. As explained above, if one of other threads calls
execve(2), the death of the thread group leader will
never be
reported. If the execed thread is not traced by this tracer, the tracer will
never know that
execve(2) happened. One possible workaround is to
PTRACE_DETACH the thread group leader instead of restarting it in this
case. Last confirmed on 2.6.38.6.
A
SIGKILL signal may still cause a
PTRACE_EVENT_EXIT stop before
actual signal death. This may be changed in the future;
SIGKILL is
meant to always immediately kill tasks even under ptrace. Last confirmed on
Linux 3.13.
Some system calls return with
EINTR if a signal was sent to a tracee, but
delivery was suppressed by the tracer. (This is very typical operation: it is
usually done by debuggers on every attach, in order to not introduce a bogus
SIGSTOP). As of Linux 3.2.9, the following system calls are affected
(this list is likely incomplete):
epoll_wait(2), and
read(2)
from an
inotify(7) file descriptor. The usual symptom of this bug is
that when you attach to a quiescent process with the command
strace -p <process-ID>
then, instead of the usual and expected one-line output such as
restart_syscall(<... resuming interrupted call ...>_
or
select(6, [5], NULL, [5], NULL_
('_' denotes the cursor position), you observe more than one line. For example:
clock_gettime(CLOCK_MONOTONIC, {15370, 690928118}) = 0
epoll_wait(4,_
What is not visible here is that the process was blocked in
epoll_wait(2)
before
strace(1) has attached to it. Attaching caused
epoll_wait(2) to return to user space with the error
EINTR. In
this particular case, the program reacted to
EINTR by checking the
current time, and then executing
epoll_wait(2) again. (Programs which
do not expect such "stray"
EINTR errors may behave in an
unintended way upon an
strace(1) attach.)
Contrary to the normal rules, the glibc wrapper for
ptrace() can set
errno to zero.
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)