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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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ptrace(2) System Calls Manual ptrace(2)
ptrace - process trace
Standard C library (libc, -lc)
#include <sys/ptrace.h>
long ptrace(enum __ptrace_request op, 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
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_SEIZEd process can accept PTRACE_INTERRUPT and
PTRACE_LISTEN commands. The "seized" behavior just
described is inherited by children that are automatically
attached using PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, and
PTRACE_O_TRACECLONE. addr must be zero. data contains a
bit mask of ptrace options to activate immediately.
Permission to perform a PTRACE_SEIZE is governed by a
ptrace access mode PTRACE_MODE_ATTACH_REALCREDS check; see
below.
PTRACE_SECCOMP_GET_FILTER (since Linux 4.4)
This operation allows the tracer to dump the tracee's
classic BPF filters.
addr is an integer specifying the index of the filter to be
dumped. The most recently installed filter has the index
0. If addr is greater than the number of installed
filters, the operation fails with the error ENOENT.
data is either a pointer to a struct sock_filter array that
is large enough to store the BPF program, or NULL if the
program is not to be stored.
Upon success, the return value is the number of
instructions in the BPF program. If data was NULL, then
this return value can be used to correctly size the struct
sock_filter array passed in a subsequent call.
This operation fails with the error EACCES if the caller
does not have the CAP_SYS_ADMIN capability or if the caller
is in strict or filter seccomp mode. If the filter
referred to by addr is not a classic BPF filter, the
operation fails with the error EMEDIUMTYPE.
This operation is available if the kernel was configured
with both the CONFIG_SECCOMP_FILTER and the
CONFIG_CHECKPOINT_RESTORE options.
PTRACE_DETACH
Restart the stopped tracee as for PTRACE_CONT, but first
detach from it. Under Linux, a tracee can be detached in
this way regardless of which method was used to initiate
tracing. (addr is ignored.)
PTRACE_GET_THREAD_AREA (since Linux 2.6.0)
This operation performs a similar task to
get_thread_area(2). It reads the TLS entry in the GDT
whose index is given in addr, placing a copy of the entry
into the struct user_desc pointed to by data. (By contrast
with get_thread_area(2), the entry_number of the struct
user_desc is ignored.)
PTRACE_SET_THREAD_AREA (since Linux 2.6.0)
This operation performs a similar task to
set_thread_area(2). It sets the TLS entry in the GDT whose
index is given in addr, assigning it the data supplied in
the struct user_desc pointed to by data. (By contrast with
set_thread_area(2), the entry_number of the struct
user_desc is ignored; in other words, this ptrace operation
can't be used to allocate a free TLS entry.)
PTRACE_GET_SYSCALL_INFO (since Linux 5.3)
Retrieve information about the system call that caused the
stop. The information is placed into the buffer pointed by
the data argument, which should be a pointer to a buffer of
type struct ptrace_syscall_info. The addr argument
contains the size of the buffer pointed to by the data
argument (i.e., sizeof(struct ptrace_syscall_info)). The
return value contains the number of bytes available to be
written by the kernel. If the size of the data to be
written by the kernel exceeds the size specified by the
addr argument, the output data is truncated.
The ptrace_syscall_info structure contains the following
fields:
struct ptrace_syscall_info {
__u8 op; /* Type of system call stop */
__u32 arch; /* AUDIT_ARCH_* value; see seccomp(2) */
__u64 instruction_pointer; /* CPU instruction pointer */
__u64 stack_pointer; /* CPU stack pointer */
union {
struct { /* op == PTRACE_SYSCALL_INFO_ENTRY */
__u64 nr; /* System call number */
__u64 args[6]; /* System call arguments */
} entry;
struct { /* op == PTRACE_SYSCALL_INFO_EXIT */
__s64 rval; /* System call return value */
__u8 is_error; /* System call error flag;
Boolean: does rval contain
an error value (-ERRCODE) or
a nonerror return value? */
} exit;
struct { /* op == PTRACE_SYSCALL_INFO_SECCOMP */
__u64 nr; /* System call number */
__u64 args[6]; /* System call arguments */
__u32 ret_data; /* SECCOMP_RET_DATA portion
of SECCOMP_RET_TRACE
return value */
} seccomp;
};
};
The op, arch, instruction_pointer, and stack_pointer fields
are defined for all kinds of ptrace system call stops. The
rest of the structure is a union; one should read only
those fields that are meaningful for the kind of system
call stop specified by the op field.
The op field has one of the following values (defined in
<linux/ptrace.h>) indicating what type of stop occurred and
which part of the union is filled:
PTRACE_SYSCALL_INFO_ENTRY
The entry component of the union contains
information relating to a system call entry stop.
PTRACE_SYSCALL_INFO_EXIT
The exit component of the union contains information
relating to a system call exit stop.
PTRACE_SYSCALL_INFO_SECCOMP
The seccomp component of the union contains
information relating to a PTRACE_EVENT_SECCOMP stop.
PTRACE_SYSCALL_INFO_NONE
No component of the union contains relevant
information.
In case of system call entry or exit stops, the data
returned by PTRACE_GET_SYSCALL_INFO is limited to type
PTRACE_SYSCALL_INFO_NONE unless PTRACE_O_TRACESYSGOOD
option is set before the corresponding system call stop has
occurred.
Death under ptrace
When a (possibly multithreaded) process receives a killing signal
(one whose disposition is set to SIG_DFL and whose default action
is to kill the process), all threads exit. Tracees report their
death to their tracer(s). Notification of this event is delivered
via waitpid(2).
Note that the killing signal will first cause signal-delivery-stop
(on one tracee only), and only after it is injected by the tracer
(or after it was dispatched to a thread which isn't traced), will
death from the signal happen on all tracees within a multithreaded
process. (The term "signal-delivery-stop" is explained below.)
SIGKILL does not generate signal-delivery-stop and therefore the
tracer can't suppress it. SIGKILL kills even within system calls
(syscall-exit-stop is not generated prior to death by SIGKILL).
The net effect is that SIGKILL always kills the process (all its
threads), even if some threads of the process are ptraced.
When the tracee calls _exit(2), it reports its death to its
tracer. Other threads are not affected.
When any thread executes exit_group(2), every tracee in its thread
group reports its death to its tracer.
If the PTRACE_O_TRACEEXIT option is on, PTRACE_EVENT_EXIT will
happen before actual death. This applies to exits via exit(2),
exit_group(2), and signal deaths (except SIGKILL, depending on the
kernel version; see BUGS below), and when threads are torn down on
execve(2) in a multithreaded process.
The tracer cannot assume that the ptrace-stopped tracee exists.
There are many scenarios when the tracee may die while stopped
(such as SIGKILL). Therefore, the tracer must be prepared to
handle an ESRCH error on any ptrace operation. Unfortunately, the
same error is returned if the tracee exists but is not ptrace-
stopped (for commands which require a stopped tracee), or if it is
not traced by the process which issued the ptrace call. The
tracer needs to keep track of the stopped/running state of the
tracee, and interpret ESRCH as "tracee died unexpectedly" only if
it knows that the tracee has been observed to enter ptrace-stop.
Note that there is no guarantee that waitpid(WNOHANG) will
reliably report the tracee's death status if a ptrace operation
returned ESRCH. waitpid(WNOHANG) may return 0 instead. In other
words, the tracee may be "not yet fully dead", but already
refusing ptrace operations.
The tracer can't assume that the tracee always ends its life by
reporting WIFEXITED(status) or WIFSIGNALED(status); there are
cases where this does not occur. For example, if a thread other
than thread group leader does an execve(2), it disappears; its PID
will never be seen again, and any subsequent ptrace stops will be
reported under the thread group leader's PID.
Stopped states
A tracee can be in two states: running or stopped. For the
purposes of ptrace, a tracee which is blocked in a system call
(such as read(2), pause(2), etc.) is nevertheless considered to
be running, even if the tracee is blocked for a long time. The
state of the tracee after PTRACE_LISTEN is somewhat of a gray
area: it is not in any ptrace-stop (ptrace commands won't work on
it, and it will deliver waitpid(2) notifications), but it also may
be considered "stopped" because it is not executing instructions
(is not scheduled), and if it was in group-stop before
PTRACE_LISTEN, it will not respond to signals until SIGCONT is
received.
There are many kinds of states when the tracee is stopped, and in
ptrace discussions they are often conflated. Therefore, it is
important to use precise terms.
In this manual page, any stopped state in which the tracee is
ready to accept ptrace commands from the tracer is called ptrace-
stop. Ptrace-stops can be further subdivided into signal-
delivery-stop, group-stop, syscall-stop, PTRACE_EVENT stops, and
so on. These stopped states are described in detail below.
When the running tracee enters ptrace-stop, it notifies its tracer
using waitpid(2) (or one of the other "wait" system calls). Most
of this manual page assumes that the tracer waits with:
pid = waitpid(pid_or_minus_1, &status, __WALL);
Ptrace-stopped tracees are reported as returns with pid greater
than 0 and WIFSTOPPED(status) true.
The __WALL flag does not include the WSTOPPED and WEXITED flags,
but implies their functionality.
Setting the WCONTINUED flag when calling waitpid(2) is not
recommended: the "continued" state is per-process and consuming it
can confuse the real parent of the tracee.
Use of the WNOHANG flag may cause waitpid(2) to return 0 ("no wait
results available yet") even if the tracer knows there should be a
notification. Example:
errno = 0;
ptrace(PTRACE_CONT, pid, 0L, 0L);
if (errno == ESRCH) {
/* tracee is dead */
r = waitpid(tracee, &status, __WALL | WNOHANG);
/* r can still be 0 here! */
}
The following kinds of ptrace-stops exist: signal-delivery-stops,
group-stops, PTRACE_EVENT stops, syscall-stops. They all are
reported by waitpid(2) with WIFSTOPPED(status) true. They may be
differentiated by examining the value status>>8, and if there is
ambiguity in that value, by querying PTRACE_GETSIGINFO. (Note:
the WSTOPSIG(status) macro can't be used to perform this
examination, because it returns the value (status>>8) & 0xff.)
Signal-delivery-stop
When a (possibly multithreaded) process receives any signal except
SIGKILL, the kernel selects an arbitrary thread which handles the
signal. (If the signal is generated with tgkill(2), the target
thread can be explicitly selected by the caller.) If the selected
thread is traced, it enters signal-delivery-stop. At this point,
the signal is not yet delivered to the process, and can be
suppressed by the tracer. If the tracer doesn't suppress the
signal, it passes the signal to the tracee in the next ptrace
restart operation. This second step of signal delivery is called
signal injection in this manual page. Note that if the signal is
blocked, signal-delivery-stop doesn't happen until the signal is
unblocked, with the usual exception that SIGSTOP can't be blocked.
Signal-delivery-stop is observed by the tracer as waitpid(2)
returning with WIFSTOPPED(status) true, with the signal returned
by WSTOPSIG(status). If the signal is SIGTRAP, this may be a
different kind of ptrace-stop; see the "Syscall-stops" and
"execve" sections below for details. If WSTOPSIG(status) returns
a stopping signal, this may be a group-stop; see below.
Signal injection and suppression
After signal-delivery-stop is observed by the tracer, the tracer
should restart the tracee with the call
ptrace(PTRACE_restart, pid, 0, sig)
where PTRACE_restart is one of the restarting ptrace operations.
If sig is 0, then a signal is not delivered. Otherwise, the
signal sig is delivered. This operation is called signal
injection in this manual page, to distinguish it from signal-
delivery-stop.
The sig value may be different from the WSTOPSIG(status) value:
the tracer can cause a different signal to be injected.
Note that a suppressed signal still causes system calls to return
prematurely. In this case, system calls will be restarted: the
tracer will observe the tracee to reexecute the interrupted system
call (or restart_syscall(2) system call for a few system calls
which use a different mechanism for restarting) if the tracer uses
PTRACE_SYSCALL. Even system calls (such as poll(2)) which are not
restartable after signal are restarted after signal is suppressed;
however, kernel bugs exist which cause some system calls to fail
with EINTR even though no observable signal is injected to the
tracee.
Restarting ptrace commands issued in ptrace-stops other than
signal-delivery-stop are not guaranteed to inject a signal, even
if sig is nonzero. No error is reported; a nonzero sig may simply
be ignored. Ptrace users should not try to "create a new signal"
this way: use tgkill(2) instead.
The fact that signal injection operations may be ignored when
restarting the tracee after ptrace stops that are not signal-
delivery-stops is a cause of confusion among ptrace users. One
typical scenario is that the tracer observes group-stop, mistakes
it for signal-delivery-stop, restarts the tracee with
ptrace(PTRACE_restart, pid, 0, stopsig)
with the intention of injecting stopsig, but stopsig gets ignored
and the tracee continues to run.
The SIGCONT signal has a side effect of waking up (all threads of)
a group-stopped process. This side effect happens before signal-
delivery-stop. The tracer can't suppress this side effect (it can
only suppress signal injection, which only causes the SIGCONT
handler to not be executed in the tracee, if such a handler is
installed). In fact, waking up from group-stop may be followed by
signal-delivery-stop for signal(s) other than SIGCONT, if they
were pending when SIGCONT was delivered. In other words, SIGCONT
may be not the first signal observed by the tracee after it was
sent.
Stopping signals cause (all threads of) a process to enter group-
stop. This side effect happens after signal injection, and
therefore can be suppressed by the tracer.
In Linux 2.4 and earlier, the SIGSTOP signal can't be injected.
PTRACE_GETSIGINFO can be used to retrieve a siginfo_t structure
which corresponds to the delivered signal. PTRACE_SETSIGINFO may
be used to modify it. If PTRACE_SETSIGINFO has been used to alter
siginfo_t, the si_signo field and the sig parameter in the
restarting command must match, otherwise the result is undefined.
Group-stop
When a (possibly multithreaded) process receives a stopping
signal, all threads stop. If some threads are traced, they enter
a group-stop. Note that the stopping signal will first cause
signal-delivery-stop (on one tracee only), and only after it is
injected by the tracer (or after it was dispatched to a thread
which isn't traced), will group-stop be initiated on all tracees
within the multithreaded process. As usual, every tracee reports
its group-stop separately to the corresponding tracer.
Group-stop is observed by the tracer as waitpid(2) returning with
WIFSTOPPED(status) true, with the stopping signal available via
WSTOPSIG(status). The same result is returned by some other
classes of ptrace-stops, therefore the recommended practice is to
perform the call
ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)
The call can be avoided if the signal is not SIGSTOP, SIGTSTP,
SIGTTIN, or SIGTTOU; only these four signals are stopping signals.
If the tracer sees something else, it can't be a group-stop.
Otherwise, the tracer needs to call PTRACE_GETSIGINFO. If
PTRACE_GETSIGINFO fails with EINVAL, then it is definitely a
group-stop. (Other failure codes are possible, such as ESRCH ("no
such process") if a SIGKILL killed the tracee.)
If tracee was attached using PTRACE_SEIZE, group-stop is indicated
by PTRACE_EVENT_STOP: status>>16 == PTRACE_EVENT_STOP. This
allows detection of group-stops without requiring an extra
PTRACE_GETSIGINFO call.
As of Linux 2.6.38, after the tracer sees the tracee ptrace-stop
and until it restarts or kills it, the tracee will not run, and
will not send notifications (except SIGKILL death) to the tracer,
even if the tracer enters into another waitpid(2) call.
The kernel behavior described in the previous paragraph causes a
problem with transparent handling of stopping signals. If the
tracer restarts the tracee after group-stop, the stopping signal
is effectively ignored—the tracee doesn't remain stopped, it runs.
If the tracer doesn't restart the tracee before entering into the
next waitpid(2), future SIGCONT signals will not be reported to
the tracer; this would cause the SIGCONT signals to have no effect
on the tracee.
Since Linux 3.4, there is a method to overcome this problem:
instead of PTRACE_CONT, a PTRACE_LISTEN command can be used to
restart a tracee in a way where it does not execute, but waits for
a new event which it can report via waitpid(2) (such as when it is
restarted by a SIGCONT).
PTRACE_EVENT stops
If the tracer sets PTRACE_O_TRACE_* options, the tracee will enter
ptrace-stops called PTRACE_EVENT stops.
PTRACE_EVENT stops are observed by the tracer as waitpid(2)
returning with WIFSTOPPED(status), and WSTOPSIG(status) returns
SIGTRAP (or for PTRACE_EVENT_STOP, returns the stopping signal if
tracee is in a group-stop). An additional bit is set in the
higher byte of the status word: the value status>>8 will be
((PTRACE_EVENT_foo<<8) | SIGTRAP).
The following events exist:
PTRACE_EVENT_VFORK
Stop before return from vfork(2) or clone(2) with the
CLONE_VFORK flag. When the tracee is continued after this
stop, it will wait for child to exit/exec before continuing
its execution (in other words, the usual behavior on
vfork(2)).
PTRACE_EVENT_FORK
Stop before return from fork(2) or clone(2) with the exit
signal set to SIGCHLD.
PTRACE_EVENT_CLONE
Stop before return from clone(2).
PTRACE_EVENT_VFORK_DONE
Stop before return from vfork(2) or clone(2) with the
CLONE_VFORK flag, but after the child unblocked this tracee
by exiting or execing.
For all four stops described above, the stop occurs in the parent
(i.e., the tracee), not in the newly created thread.
PTRACE_GETEVENTMSG can be used to retrieve the new thread's ID.
PTRACE_EVENT_EXEC
Stop before return from execve(2). Since Linux 3.0,
PTRACE_GETEVENTMSG returns the former thread ID.
PTRACE_EVENT_EXIT
Stop before exit (including death from exit_group(2)),
signal death, or exit caused by execve(2) in a
multithreaded process. PTRACE_GETEVENTMSG returns the exit
status. Registers can be examined (unlike when "real" exit
happens). The tracee is still alive; it needs to be
PTRACE_CONTed or PTRACE_DETACHed to finish exiting.
PTRACE_EVENT_STOP
Stop induced by PTRACE_INTERRUPT command, or group-stop, or
initial ptrace-stop when a new child is attached (only if
attached using PTRACE_SEIZE).
PTRACE_EVENT_SECCOMP
Stop triggered by a seccomp(2) rule on tracee syscall entry
when PTRACE_O_TRACESECCOMP has been set by the tracer. The
seccomp event message data (from the SECCOMP_RET_DATA
portion of the seccomp filter rule) can be retrieved with
PTRACE_GETEVENTMSG. The semantics of this stop are
described in detail in a separate section below.
PTRACE_GETSIGINFO on PTRACE_EVENT stops returns SIGTRAP in
si_signo, with si_code set to (event<<8) | SIGTRAP.
Syscall-stops
If the tracee was restarted by PTRACE_SYSCALL or PTRACE_SYSEMU,
the tracee enters syscall-enter-stop just prior to entering any
system call (which will not be executed if the restart was using
PTRACE_SYSEMU, regardless of any change made to registers at this
point or how the tracee is restarted after this stop). No matter
which method caused the syscall-entry-stop, if the tracer restarts
the tracee with PTRACE_SYSCALL, the tracee enters syscall-exit-
stop when the system call is finished, or if it is interrupted by
a signal. (That is, signal-delivery-stop never happens between
syscall-enter-stop and syscall-exit-stop; it happens after
syscall-exit-stop.). If the tracee is continued using any other
method (including PTRACE_SYSEMU), no syscall-exit-stop occurs.
Note that all mentions PTRACE_SYSEMU apply equally to
PTRACE_SYSEMU_SINGLESTEP.
However, even if the tracee was continued using PTRACE_SYSCALL, it
is not guaranteed that the next stop will be a syscall-exit-stop.
Other possibilities are that the tracee may stop in a PTRACE_EVENT
stop (including seccomp stops), exit (if it entered _exit(2) or
exit_group(2)), be killed by SIGKILL, or die silently (if it is a
thread group leader, the execve(2) happened in another thread, and
that thread is not traced by the same tracer; this situation is
discussed later).
Syscall-enter-stop and syscall-exit-stop are observed by the
tracer as waitpid(2) returning with WIFSTOPPED(status) true, and
WSTOPSIG(status) giving SIGTRAP. If the PTRACE_O_TRACESYSGOOD
option was set by the tracer, then WSTOPSIG(status) will give the
value (SIGTRAP | 0x80).
Syscall-stops can be distinguished from signal-delivery-stop with
SIGTRAP by querying PTRACE_GETSIGINFO for the following cases:
si_code <= 0
SIGTRAP was delivered as a result of a user-space action,
for example, a system call (tgkill(2), kill(2),
sigqueue(3), etc.), expiration of a POSIX timer, change of
state on a POSIX message queue, or completion of an
asynchronous I/O operation.
si_code == SI_KERNEL (0x80)
SIGTRAP was sent by the kernel.
si_code == SIGTRAP or si_code == (SIGTRAP|0x80)
This is a syscall-stop.
However, syscall-stops happen very often (twice per system call),
and performing PTRACE_GETSIGINFO for every syscall-stop may be
somewhat expensive.
Some architectures allow the cases to be distinguished by
examining registers. For example, on x86, rax == -ENOSYS in
syscall-enter-stop. Since SIGTRAP (like any other signal) always
happens after syscall-exit-stop, and at this point rax almost
never contains -ENOSYS, the SIGTRAP looks like "syscall-stop which
is not syscall-enter-stop"; in other words, it looks like a "stray
syscall-exit-stop" and can be detected this way. But such
detection is fragile and is best avoided.
Using the PTRACE_O_TRACESYSGOOD option is the recommended method
to distinguish syscall-stops from other kinds of ptrace-stops,
since it is reliable and does not incur a performance penalty.
Syscall-enter-stop and syscall-exit-stop are indistinguishable
from each other by the tracer. The tracer needs to keep track of
the sequence of ptrace-stops in order to not misinterpret syscall-
enter-stop as syscall-exit-stop or vice versa. In general, a
syscall-enter-stop is always followed by syscall-exit-stop,
PTRACE_EVENT stop, or the tracee's death; no other kinds of
ptrace-stop can occur in between. However, note that seccomp
stops (see below) can cause syscall-exit-stops, without preceding
syscall-entry-stops. If seccomp is in use, care needs to be taken
not to misinterpret such stops as syscall-entry-stops.
If after syscall-enter-stop, the tracer uses a restarting command
other than PTRACE_SYSCALL, syscall-exit-stop is not generated.
PTRACE_GETSIGINFO on syscall-stops returns SIGTRAP in si_signo,
with si_code set to SIGTRAP or (SIGTRAP|0x80).
PTRACE_EVENT_SECCOMP stops (Linux 3.5 to Linux 4.7)
The behavior of PTRACE_EVENT_SECCOMP stops and their interaction
with other kinds of ptrace stops has changed between kernel
versions. This documents the behavior from their introduction
until Linux 4.7 (inclusive). The behavior in later kernel
versions is documented in the next section.
A PTRACE_EVENT_SECCOMP stop occurs whenever a SECCOMP_RET_TRACE
rule is triggered. This is independent of which methods was used
to restart the system call. Notably, seccomp still runs even if
the tracee was restarted using PTRACE_SYSEMU and this system call
is unconditionally skipped.
Restarts from this stop will behave as if the stop had occurred
right before the system call in question. In particular, both
PTRACE_SYSCALL and PTRACE_SYSEMU will normally cause a subsequent
syscall-entry-stop. However, if after the PTRACE_EVENT_SECCOMP
the system call number is negative, both the syscall-entry-stop
and the system call itself will be skipped. This means that if
the system call number is negative after a PTRACE_EVENT_SECCOMP
and the tracee is restarted using PTRACE_SYSCALL, the next
observed stop will be a syscall-exit-stop, rather than the
syscall-entry-stop that might have been expected.
PTRACE_EVENT_SECCOMP stops (since Linux 4.8)
Starting with Linux 4.8, the PTRACE_EVENT_SECCOMP stop was
reordered to occur between syscall-entry-stop and syscall-exit-
stop. Note that seccomp no longer runs (and no
PTRACE_EVENT_SECCOMP will be reported) if the system call is
skipped due to PTRACE_SYSEMU.
Functionally, a PTRACE_EVENT_SECCOMP stop functions comparably to
a syscall-entry-stop (i.e., continuations using PTRACE_SYSCALL
will cause syscall-exit-stops, the system call number may be
changed and any other modified registers are visible to the to-be-
executed system call as well). Note that there may be, but need
not have been a preceding syscall-entry-stop.
After a PTRACE_EVENT_SECCOMP stop, seccomp will be rerun, with a
SECCOMP_RET_TRACE rule now functioning the same as a
SECCOMP_RET_ALLOW. Specifically, this means that if registers are
not modified during the PTRACE_EVENT_SECCOMP stop, the system call
will then be allowed.
PTRACE_SINGLESTEP stops
[Details of these kinds of stops are yet to be documented.]
Informational and restarting ptrace commands
Most ptrace commands (all except PTRACE_ATTACH, PTRACE_SEIZE,
PTRACE_TRACEME, PTRACE_INTERRUPT, and PTRACE_KILL) require the
tracee to be in a ptrace-stop, otherwise they fail with ESRCH.
When the tracee is in ptrace-stop, the tracer can read and write
data to the tracee using informational commands. These commands
leave the tracee in ptrace-stopped state:
ptrace(PTRACE_PEEKTEXT/PEEKDATA/PEEKUSER, pid, addr, 0);
ptrace(PTRACE_POKETEXT/POKEDATA/POKEUSER, pid, addr, long_val);
ptrace(PTRACE_GETREGS/GETFPREGS, pid, 0, &struct);
ptrace(PTRACE_SETREGS/SETFPREGS, pid, 0, &struct);
ptrace(PTRACE_GETREGSET, pid, NT_foo, &iov);
ptrace(PTRACE_SETREGSET, pid, NT_foo, &iov);
ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo);
ptrace(PTRACE_SETSIGINFO, pid, 0, &siginfo);
ptrace(PTRACE_GETEVENTMSG, pid, 0, &long_var);
ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);
Note that some errors are not reported. For example, setting
signal information (siginfo) may have no effect in some ptrace-
stops, yet the call may succeed (return 0 and not set errno);
querying PTRACE_GETEVENTMSG may succeed and return some random
value if current ptrace-stop is not documented as returning a
meaningful event message.
The call
ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);
affects one tracee. The tracee's current flags are replaced.
Flags are inherited by new tracees created and "auto-attached" via
active PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, or
PTRACE_O_TRACECLONE options.
Another group of commands makes the ptrace-stopped tracee run.
They have the form:
ptrace(cmd, pid, 0, sig);
where cmd is PTRACE_CONT, PTRACE_LISTEN, PTRACE_DETACH,
PTRACE_SYSCALL, PTRACE_SINGLESTEP, PTRACE_SYSEMU, or
PTRACE_SYSEMU_SINGLESTEP. If the tracee is in signal-delivery-
stop, sig is the signal to be injected (if it is nonzero).
Otherwise, sig may be ignored. (When restarting a tracee from a
ptrace-stop other than signal-delivery-stop, recommended practice
is to always pass 0 in sig.)
Attaching and detaching
A thread can be attached to the tracer using the call
ptrace(PTRACE_ATTACH, pid, 0, 0);
or
ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);
PTRACE_ATTACH sends SIGSTOP to this thread. If the tracer wants
this SIGSTOP to have no effect, it needs to suppress it. Note
that if other signals are concurrently sent to this thread during
attach, the tracer may see the tracee enter signal-delivery-stop
with other signal(s) first! The usual practice is to reinject
these signals until SIGSTOP is seen, then suppress SIGSTOP
injection. The design bug here is that a ptrace attach and a
concurrently delivered SIGSTOP may race and the concurrent SIGSTOP
may be lost.
Since attaching sends SIGSTOP and the tracer usually suppresses
it, this may cause a stray EINTR return from the currently
executing system call in the tracee, as described in the "Signal
injection and suppression" section.
Since Linux 3.4, PTRACE_SEIZE can be used instead of
PTRACE_ATTACH. PTRACE_SEIZE does not stop the attached process.
If you need to stop it after attach (or at any other time) without
sending it any signals, use PTRACE_INTERRUPT command.
The operation
ptrace(PTRACE_TRACEME, 0, 0, 0);
turns the calling thread into a tracee. The thread continues to
run (doesn't enter ptrace-stop). A common practice is to follow
the PTRACE_TRACEME with
raise(SIGSTOP);
and allow the parent (which is our tracer now) to observe our
signal-delivery-stop.
If the PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, or
PTRACE_O_TRACECLONE options are in effect, then children created
by, respectively, vfork(2) or clone(2) with the CLONE_VFORK flag,
fork(2) or clone(2) with the exit signal set to SIGCHLD, and other
kinds of clone(2), are automatically attached to the same tracer
which traced their parent. SIGSTOP is delivered to the children,
causing them to enter signal-delivery-stop after they exit the
system call which created them.
Detaching of the tracee is performed by:
ptrace(PTRACE_DETACH, pid, 0, sig);
PTRACE_DETACH is a restarting operation; therefore it requires the
tracee to be in ptrace-stop. If the tracee is in signal-delivery-
stop, a signal can be injected. Otherwise, the sig parameter may
be silently ignored.
If the tracee is running when the tracer wants to detach it, the
usual solution is to send SIGSTOP (using tgkill(2), to make sure
it goes to the correct thread), wait for the tracee to stop in
signal-delivery-stop for SIGSTOP and then detach it (suppressing
SIGSTOP injection). A design bug is that this can race with
concurrent SIGSTOPs. Another complication is that the tracee may
enter other ptrace-stops and needs to be restarted and waited for
again, until SIGSTOP is seen. Yet another complication is to be
sure that the tracee is not already ptrace-stopped, because no
signal delivery happens while it is—not even SIGSTOP.
If the tracer dies, all tracees are automatically detached and
restarted, unless they were in group-stop. Handling of restart
from group-stop is currently buggy, but the "as planned" behavior
is to leave tracee stopped and waiting for SIGCONT. If the tracee
is restarted from signal-delivery-stop, the pending signal is
injected.
execve(2) under ptrace
When one thread in a multithreaded process calls execve(2), the
kernel destroys all other threads in the process, and resets the
thread ID of the execing thread to the thread group ID (process
ID). (Or, to put things another way, when a multithreaded process
does an execve(2), at completion of the call, it appears as though
the execve(2) occurred in the thread group leader, regardless of
which thread did the execve(2).) This resetting of the thread ID
looks very confusing to tracers:
• All other threads stop in PTRACE_EVENT_EXIT stop, if the
PTRACE_O_TRACEEXIT option was turned on. Then all other
threads except the thread group leader report death as if they
exited via _exit(2) with exit code 0.
• The execing tracee changes its thread ID while it is in the
execve(2). (Remember, under ptrace, the "pid" returned from
waitpid(2), or fed into ptrace calls, is the tracee's thread
ID.) That is, the tracee's thread ID is reset to be the same
as its process ID, which is the same as the thread group
leader's thread ID.
• Then a PTRACE_EVENT_EXEC stop happens, if the
PTRACE_O_TRACEEXEC option was turned on.
• If the thread group leader has reported its PTRACE_EVENT_EXIT
stop by this time, it appears to the tracer that the dead
thread leader "reappears from nowhere". (Note: the thread
group leader does not report death via WIFEXITED(status) until
there is at least one other live thread. This eliminates the
possibility that the tracer will see it dying and then
reappearing.) If the thread group leader was still alive, for
the tracer this may look as if thread group leader returns from
a different system call than it entered, or even "returned from
a system call even though it was not in any system call". If
the thread group leader was not traced (or was traced by a
different tracer), then during execve(2) it will appear as if
it has become a tracee of the tracer of the execing tracee.
All of the above effects are the artifacts of the thread ID change
in the tracee.
The PTRACE_O_TRACEEXEC option is the recommended tool for dealing
with this situation. First, it enables PTRACE_EVENT_EXEC stop,
which occurs before execve(2) returns. In this stop, the tracer
can use PTRACE_GETEVENTMSG to retrieve the tracee's former thread
ID. (This feature was introduced in Linux 3.0.) Second, the
PTRACE_O_TRACEEXEC option disables legacy SIGTRAP generation on
execve(2).
When the tracer receives PTRACE_EVENT_EXEC stop notification, it
is guaranteed that except this tracee and the thread group leader,
no other threads from the process are alive.
On receiving the PTRACE_EVENT_EXEC stop notification, the tracer
should clean up all its internal data structures describing the
threads of this process, and retain only one data structure—one
which describes the single still running tracee, with
thread ID == thread group ID == process ID.
Example: two threads call execve(2) at the same time:
*** we get syscall-enter-stop in thread 1: **
PID1 execve("/bin/foo", "foo" <unfinished ...>
*** we issue PTRACE_SYSCALL for thread 1 **
*** we get syscall-enter-stop in thread 2: **
PID2 execve("/bin/bar", "bar" <unfinished ...>
*** we issue PTRACE_SYSCALL for thread 2 **
*** we get PTRACE_EVENT_EXEC for PID0, we issue PTRACE_SYSCALL **
*** we get syscall-exit-stop for PID0: **
PID0 <... execve resumed> ) = 0
If the PTRACE_O_TRACEEXEC option is not in effect for the execing
tracee, and if the tracee was PTRACE_ATTACHed rather that
PTRACE_SEIZEd, the kernel delivers an extra SIGTRAP to the tracee
after execve(2) returns. This is an ordinary signal (similar to
one which can be generated by kill -TRAP), not a special kind of
ptrace-stop. Employing PTRACE_GETSIGINFO for this signal returns
si_code set to 0 (SI_USER). This signal may be blocked by signal
mask, and thus may be delivered (much) later.
Usually, the tracer (for example, strace(1)) would not want to
show this extra post-execve SIGTRAP signal to the user, and would
suppress its delivery to the tracee (if SIGTRAP is set to SIG_DFL,
it is a killing signal). However, determining which SIGTRAP to
suppress is not easy. Setting the PTRACE_O_TRACEEXEC option or
using PTRACE_SEIZE and thus suppressing this extra SIGTRAP is the
recommended approach.
Real parent
The ptrace API (ab)uses the standard UNIX parent/child signaling
over waitpid(2). This used to cause the real parent of the
process to stop receiving several kinds of waitpid(2)
notifications when the child process is traced by some other
process.
Many of these bugs have been fixed, but as of Linux 2.6.38 several
still exist; see BUGS below.
As of Linux 2.6.38, the following is believed to work correctly:
• exit/death by signal is reported first to the tracer, then,
when the tracer consumes the waitpid(2) result, to the real
parent (to the real parent only when the whole multithreaded
process exits). If the tracer and the real parent are the same
process, the report is sent only once.
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.
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).
None.
SVr4, 4.3BSD.
Before Linux 2.6.26, init(1), the process with PID 1, may not be
traced.
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.
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.
gdb(1), ltrace(1), strace(1), clone(2), execve(2), fork(2),
gettid(2), prctl(2), seccomp(2), sigaction(2), tgkill(2),
vfork(2), waitpid(2), exec(3), capabilities(7), signal(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.15 2025-05-17 ptrace(2)
Pages that refer to this page: coresched(1), ltrace(1), setpriv(1), strace(1), clone(2), execve(2), get_robust_list(2), kcmp(2), memfd_secret(2), move_pages(2), perf_event_open(2), pidfd_getfd(2), process_madvise(2), process_vm_readv(2), PR_SET_DUMPABLE(2const), PR_SET_PTRACER(2const), seccomp(2), set_thread_area(2), sigaction(2), syscalls(2), wait(2), exec(3), seccomp_init(3), seccomp_rule_add(3), proc_pid_auxv(5), proc_pid_cwd(5), proc_pid_environ(5), proc_pid_exe(5), proc_pid_fd(5), proc_pid_io(5), proc_pid_map_files(5), proc_pid_maps(5), proc_pid_mem(5), proc_pid_pagemap(5), proc_pid_personality(5), proc_pid_root(5), proc_pid_stack(5), proc_pid_stat(5), proc_pid_syscall(5), proc_pid_timerslack_ns(5), proc_pid_wchan(5), proc_sys_fs(5), proc_sys_kernel(5), systemd.exec(5), capabilities(7), credentials(7), landlock(7), namespaces(7), user_namespaces(7), stapdyn(8)
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