ptrace(2) System Calls Manual ptrace(2)
NAME
ptrace - process trace
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/ptrace.h>
long ptrace(enum __ptrace_request op, pid_t pid,
void *addr, void *data);
DESCRIPTION
The ptrace() system call provides a means by which one process (the
"tracer") may observe and control the execution of another process (the
"tracee"), and examine and change the tracee's memory and registers. It
is primarily used to implement breakpoint debugging and system call
tracing.
A tracee first needs to be attached to the tracer. Attachment and sub-
sequent 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 result-
ing 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 deliv-
ered, 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 ig-
noring the delivered signal (or even delivering a different signal in-
stead).
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 pro-
gram 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 re-
maining 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 ig-
nored; 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 modifica-
tions 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_SE-
TREGS 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 ad-
dress 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 indi-
cates that there is no signal corresponding to the specified or-
dinal 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 con-
tains 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 inter-
preted as a bit mask of options, which are specified by the fol-
lowing 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 en-
sure 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 re-
trieved 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 reg-
isters are still available, allowing the tracer to see
where the exit occurred, whereas the normal exit notifica-
tion 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 re-
trieved 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_SUS-
PEND_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 SEC-
COMP_RET_DATA associated with the triggered rule. (addr is ig-
nored.)
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 de-
livered 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 sys-
tem call, or after execution of a single instruction, respec-
tively. (The tracee will also, as usual, be stopped upon receipt
of a signal.) From the tracer's perspective, the tracee will ap-
pear 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 cur-
rently supported only on arm (and arm64, though only for back-
wards 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_LIS-
TEN 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-deliv-
ery-stop, otherwise it may not work (i.e., may complete success-
fully 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 inter-
rupted and syscall-exit-stop is reported. (The interrupted sys-
tem 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(sta-
tus) returns the stop signal. If any other ptrace-stop is gener-
ated 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 nec-
essarily have stopped by the completion of this call; use wait-
pid(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 ac-
cess 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 at-
tached 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_LIS-
TEN commands. The "seized" behavior just described is inherited
by children that are automatically attached using PTRACE_O_TRACE-
FORK, PTRACE_O_TRACEVFORK, and PTRACE_O_TRACECLONE. addr must be
zero. data contains a bit mask of ptrace options to activate im-
mediately.
Permission to perform a PTRACE_SEIZE is governed by a ptrace ac-
cess 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 opera-
tion 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 EMEDIUM-
TYPE.
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 re-
gardless 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, as-
signing 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 ar-
gument, 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 re-
lating to a system call entry stop.
PTRACE_SYSCALL_INFO_EXIT
The exit component of the union contains information re-
lating 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 multi-
threaded 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 er-
ror on any ptrace operation. Unfortunately, the same error is returned
if the tracee exists but is not ptrace-stopped (for commands which re-
quire a stopped tracee), or if it is not traced by the process which is-
sued 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 en-
ter ptrace-stop. Note that there is no guarantee that waitpid(WNOHANG)
will reliably report the tracee's death status if a ptrace operation re-
turned ESRCH. waitpid(WNOHANG) may return 0 instead. In other words,
the tracee may be "not yet fully dead", but already refusing ptrace op-
erations.
The tracer can't assume that the tracee always ends its life by report-
ing 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) noti-
fications), 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 man-
ual 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 im-
plies 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 re-
sults available yet") even if the tracer knows there should be a notifi-
cation. 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 (sta-
tus>>8) & 0xff.)
Signal-delivery-stop
When a (possibly multithreaded) process receives any signal except
SIGKILL, the kernel selects an arbitrary thread which handles the sig-
nal. (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(sta-
tus). 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 de-
livered. 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 prema-
turely. 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 dif-
ferent mechanism for restarting) if the tracer uses PTRACE_SYSCALL.
Even system calls (such as poll(2)) which are not restartable after sig-
nal are restarted after signal is suppressed; however, kernel bugs exist
which cause some system calls to fail with EINTR even though no observ-
able signal is injected to the tracee.
Restarting ptrace commands issued in ptrace-stops other than signal-de-
livery-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 exe-
cuted 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 WIF-
STOPPED(status) true, with the stopping signal available via WSTOP-
SIG(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 EIN-
VAL, then it is definitely a group-stop. (Other failure codes are pos-
sible, 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 detec-
tion 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 un-
til 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 SIG-
CONT 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 ex-
ecing.
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 ex-
amined (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 ini-
tial 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 sec-
comp filter rule) can be retrieved with PTRACE_GETEVENTMSG. The
semantics of this stop are described in detail in a separate sec-
tion 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, re-
gardless 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-de-
livery-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 (in-
cluding 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 SIG-
TRAP by querying PTRACE_GETSIGINFO for the following cases:
si_code <= 0
SIGTRAP was delivered as a result of a user-space action, for ex-
ample, a system call (tgkill(2), kill(2), sigqueue(3), etc.), ex-
piration 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 ex-
pensive.
Some architectures allow the cases to be distinguished by examining reg-
isters. 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 dis-
tinguish syscall-stops from other kinds of ptrace-stops, since it is re-
liable 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 preced-
ing 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 (inclu-
sive). 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 sec-
comp 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 SEC-
COMP_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 in-
formation (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 con-
current 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-de-
livery-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 de-
livered to the children, causing them to enter signal-delivery-stop af-
ter 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 de-
sign bug is that this can race with concurrent SIGSTOPs. Another com-
plication is that the tracee may enter other ptrace-stops and needs to
be restarted and waited for again, until SIGSTOP is seen. Yet another
complication is to be sure that the tracee is not already ptrace-
stopped, because no signal delivery happens while it is—not even
SIGSTOP.
If the tracer dies, all tracees are automatically detached and
restarted, unless they were in group-stop. Handling of restart from
group-stop is currently buggy, but the "as planned" behavior is to leave
tracee stopped and waiting for SIGCONT. If the tracee is restarted from
signal-delivery-stop, the pending signal is injected.
execve(2) under ptrace
When one thread in a multithreaded process calls execve(2), the kernel
destroys all other threads in the process, and resets the thread ID of
the execing thread to the thread group ID (process ID). (Or, to put
things another way, when a multithreaded process does an execve(2), at
completion of the call, it appears as though the execve(2) occurred in
the thread group leader, regardless of which thread did the execve(2).)
This resetting of the thread ID looks very 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 ex-
cept 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 ex-
ecve(2). (Remember, under ptrace, the "pid" returned from wait-
pid(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 op-
tion 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 "re-
turned 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 differ-
ent 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 sin-
gle 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) re-
turns. This is an ordinary signal (similar to one which can be gener-
ated 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 de-
livery to the tracee (if SIGTRAP is set to SIG_DFL, it is a killing sig-
nal). However, determining which SIGTRAP to suppress is not easy. Set-
ting the PTRACE_O_TRACEEXEC option or using PTRACE_SEIZE and thus sup-
pressing this extra SIGTRAP is the recommended approach.
Real parent
The ptrace API (ab)uses the standard UNIX parent/child signaling over
waitpid(2). This used to cause the real parent of the process to stop
receiving several kinds of waitpid(2) notifications when the child
process is traced by some other process.
Many of these bugs have been fixed, but as of Linux 2.6.38 several still
exist; see BUGS below.
As of Linux 2.6.38, the following is believed to work correctly:
• exit/death by signal is reported first to the tracer, then, when the
tracer consumes the waitpid(2) result, to the real parent (to the
real parent only when the whole multithreaded process exits). If the
tracer and the real parent are the same process, the report is sent
only once.
RETURN VALUE
On success, the PTRACE_PEEK* operations return the requested data (but
see NOTES), the PTRACE_SECCOMP_GET_FILTER operation returns the number
of instructions in the BPF program, the PTRACE_GET_SYSCALL_INFO opera-
tion 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 er-
ror. Since the value returned by a successful PTRACE_PEEK* operation
may be -1, the caller must clear errno before the call, and then check
it afterward to determine whether or not an error occurred.
ERRORS
EBUSY (i386 only) There was an error with allocating or freeing a debug
register.
EFAULT There was an attempt to read from or write to an invalid area in
the tracer's or the tracee's memory, probably because the area
wasn't mapped or accessible. Unfortunately, under Linux, differ-
ent 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 re-
quire a stopped tracee).
STANDARDS
None.
HISTORY
SVr4, 4.3BSD.
Before Linux 2.6.26, init(1), the process with PID 1, may not be traced.
NOTES
Although arguments to ptrace() are interpreted according to the proto-
type 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, set-
ting 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 operat-
ing-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 ar-
chitecture.
Ptrace access mode checking
Various parts of the kernel-user-space API (not just ptrace() opera-
tions), require so-called "ptrace access mode" checks, whose outcome de-
termines 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 re-
sults of checks performed by any enabled Linux Security Module (LSM)—for
example, SELinux, Yama, or Smack—and by the commoncap LSM (which is al-
ways 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 danger-
ous, such as: get_robust_list(2); kcmp(2); reading
/proc/pid/auxv, /proc/pid/environ, or /proc/pid/stat; or read-
link(2) of a /proc/pid/ns/* file.
PTRACE_MODE_ATTACH
For "write" operations, or other operations that are more danger-
ous, such as: ptrace attaching (PTRACE_ATTACH) to another process
or calling process_vm_writev(2). (PTRACE_MODE_ATTACH was effec-
tively the default before Linux 2.6.27.)
Since Linux 4.5, the above access mode checks are combined (ORed) with
one of the following modifiers:
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 afore-
mentioned 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 vi-
olation 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 subsec-
tion 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 ac-
cesses 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 ac-
tion 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 al-
ways 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 name-
space 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 capa-
bility 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_RE-
ALCREDS, 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 tar-
get 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/ker-
nel/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 val-
ues:
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 de-
fault, 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 Documenta-
tion/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 name-
space (and further-removed descendants of that namespace). Conse-
quently, 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 re-
turn value.
BUGS
On hosts with Linux 2.6 kernel headers, PTRACE_SETOPTIONS is declared
with a different value than the one for Linux 2.4. This leads to appli-
cations 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 subse-
quent WIFEXITED notification will not be delivered until all other
threads exit. As explained above, if one of other threads calls ex-
ecve(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_DE-
TACH 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 opera-
tion: 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 er-
rno to zero.
SEE ALSO
gdb(1), ltrace(1), strace(1), clone(2), execve(2), fork(2), gettid(2),
prctl(2), seccomp(2), sigaction(2), tgkill(2), vfork(2), waitpid(2),
exec(3), capabilities(7), signal(7)
Linux man-pages 6.9.1 2024-05-02 ptrace(2)
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