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PRCTL(2) Linux Programmer's Manual PRCTL(2)

NAME
prctl - operations on a process

SYNOPSIS
#include <sys/prctl.h>

int prctl(int option, unsigned long arg2, unsigned long arg3,
unsigned long arg4, unsigned long arg5);

DESCRIPTION
prctl() is called with a first argument describing what to do (with values defined in
<linux/prctl.h>), and further arguments with a significance depending on the first one.
The first argument can be:

PR_CAPBSET_READ (since Linux 2.6.25)
Return (as the function result) 1 if the capability specified in arg2 is in the
calling thread's capability bounding set, or 0 if it is not. (The capability con‐
stants are defined in <linux/capability.h>.) The capability bounding set dictates
whether the process can receive the capability through a file's permitted capabil‐
ity set on a subsequent call to execve(2).

If the capability specified in arg2 is not valid, then the call fails with the
error EINVAL.

PR_CAPBSET_DROP (since Linux 2.6.25)
If the calling thread has the CAP_SETPCAP capability, then drop the capability
specified by arg2 from the calling thread's capability bounding set. Any children
of the calling thread will inherit the newly reduced bounding set.

The call fails with the error: EPERM if the calling thread does not have the
CAP_SETPCAP; EINVAL if arg2 does not represent a valid capability; or EINVAL if
file capabilities are not enabled in the kernel, in which case bounding sets are
not supported.

PR_SET_CHILD_SUBREAPER (since Linux 3.4)
If arg2 is nonzero, set the "child subreaper" attribute of the calling process; if
arg2 is zero, unset the attribute. When a process is marked as a child subreaper,
all of the children that it creates, and their descendants, will be marked as hav‐
ing a subreaper. In effect, a subreaper fulfills the role of init(1) for its
descendant processes. Upon termination of a process that is orphaned (i.e., its
immediate parent has already terminated) and marked as having a subreaper, the
nearest still living ancestor subreaper will receive a SIGCHLD signal and be able
to wait(2) on the process to discover its termination status.

PR_GET_CHILD_SUBREAPER (since Linux 3.4)
Return the "child subreaper" setting of the caller, in the location pointed to by
(int *) arg2.

PR_SET_DUMPABLE (since Linux 2.3.20)
Set the state of the flag determining whether core dumps are produced for the call‐
ing process upon delivery of a signal whose default behavior is to produce a core
dump. (Normally, this flag is set for a process by default, but it is cleared when
a set-user-ID or set-group-ID program is executed and also by various system calls
that manipulate process UIDs and GIDs). In kernels up to and including 2.6.12,
arg2 must be either 0 (process is not dumpable) or 1 (process is dumpable).
Between kernels 2.6.13 and 2.6.17, the value 2 was also permitted, which caused any
binary which normally would not be dumped to be dumped readable by root only; for
security reasons, this feature has been removed. (See also the description of
/proc/sys/fs/suid_dumpable in proc(5).) Processes that are not dumpable can not be
attached via ptrace(2) PTRACE_ATTACH.

PR_GET_DUMPABLE (since Linux 2.3.20)
Return (as the function result) the current state of the calling process's dumpable
flag.

PR_SET_ENDIAN (since Linux 2.6.18, PowerPC only)
Set the endian-ness of the calling process to the value given in arg2, which should
be one of the following: PR_ENDIAN_BIG, PR_ENDIAN_LITTLE, or PR_ENDIAN_PPC_LITTLE
(PowerPC pseudo little endian).

PR_GET_ENDIAN (since Linux 2.6.18, PowerPC only)
Return the endian-ness of the calling process, in the location pointed to by
(int *) arg2.

PR_SET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
Set floating-point emulation control bits to arg2. Pass PR_FPEMU_NOPRINT to
silently emulate fp operations accesses, or PR_FPEMU_SIGFPE to not emulate fp oper‐
ations and send SIGFPE instead.

PR_GET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
Return floating-point emulation control bits, in the location pointed to by (int *)
arg2.

PR_SET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
Set floating-point exception mode to arg2. Pass PR_FP_EXC_SW_ENABLE to use FPEXC
for FP exception enables, PR_FP_EXC_DIV for floating-point divide by zero,
PR_FP_EXC_OVF for floating-point overflow, PR_FP_EXC_UND for floating-point under‐
flow, PR_FP_EXC_RES for floating-point inexact result, PR_FP_EXC_INV for floating-
point invalid operation, PR_FP_EXC_DISABLED for FP exceptions disabled,
PR_FP_EXC_NONRECOV for async nonrecoverable exception mode, PR_FP_EXC_ASYNC for
async recoverable exception mode, PR_FP_EXC_PRECISE for precise exception mode.

PR_GET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
Return floating-point exception mode, in the location pointed to by (int *) arg2.

PR_SET_KEEPCAPS (since Linux 2.2.18)
Set the state of the thread's "keep capabilities" flag, which determines whether
the threads's permitted capability set is cleared when a change is made to the
threads's user IDs such that the threads's real UID, effective UID, and saved set-
user-ID all become nonzero when at least one of them previously had the value 0.
By default, the permitted capability set is cleared when such a change is made;
setting the "keep capabilities" flag prevents it from being cleared. arg2 must be
either 0 (permitted capabilities are cleared) or 1 (permitted capabilities are
kept). (A thread's effective capability set is always cleared when such a creden‐
tial change is made, regardless of the setting of the "keep capabilities" flag.)
The "keep capabilities" value will be reset to 0 on subsequent calls to execve(2).

PR_GET_KEEPCAPS (since Linux 2.2.18)
Return (as the function result) the current state of the calling threads's "keep
capabilities" flag.

PR_SET_NAME (since Linux 2.6.9)
Set the name of the calling thread, using the value in the location pointed to by
(char *) arg2. The name can be up to 16 bytes long, and should be null-terminated
if it contains fewer bytes. This is the same attribute that can be set via
pthread_setname_np(3) and retrieved using pthread_getname_np(3). The attribute is
likewise accessible via /proc/self/task/[tid]/comm, where tid is the name of the
calling thread.

PR_GET_NAME (since Linux 2.6.11)
Return the name of the calling thread, in the buffer pointed to by (char *) arg2.
The buffer should allow space for up to 16 bytes; the returned string will be null-
terminated if it is shorter than that.

PR_SET_NO_NEW_PRIVS (since Linux 3.5)
Set the calling process's no_new_privs bit to the value in arg2. With no_new_privs
set to 1, execve(2) promises not to grant privileges to do anything that could not
have been done without the execve(2) call (for example, rendering the set-user-ID
and set-group-ID permission bits, and file capabilities non-functional). Once set,
this bit cannot be unset. The setting of this bit is inherited by children created
by fork(2) and clone(2), and preserved across execve(2).

For more information, see the kernel source file Documenta‐
tion/prctl/no_new_privs.txt.

PR_GET_NO_NEW_PRIVS (since Linux 3.5)
Return (as the function result) the value of the no_new_privs bit for the current
process. A value of 0 indicates the regular execve(2) behavior. A value of 1
indicates execve(2) will operate in the privilege-restricting mode described above.

PR_SET_PDEATHSIG (since Linux 2.1.57)
Set the parent process death signal of the calling process to arg2 (either a signal
value in the range 1..maxsig, or 0 to clear). This is the signal that the calling
process will get when its parent dies. This value is cleared for the child of a
fork(2) and (since Linux 2.4.36 / 2.6.23) when executing a set-user-ID or set-
group-ID binary. This value is preserved across execve(2).

PR_GET_PDEATHSIG (since Linux 2.3.15)
Return the current value of the parent process death signal, in the location
pointed to by (int *) arg2.

PR_SET_PTRACER (since Linux 3.4)
This is meaningful only when the Yama LSM is enabled and in mode 1 ("restricted
ptrace", visible via /proc/sys/kernel/yama/ptrace_scope). When a "ptracer process
ID" is passed in arg2, the caller is declaring that the ptracer process can
ptrace(2) the calling process as if it were a direct process ancestor. Each
PR_SET_PTRACER operation replaces the previous "ptracer process ID". Employing
PR_SET_PTRACER with arg2 set to 0 clears the caller's "ptracer process ID". If
arg2 is PR_SET_PTRACER_ANY, the ptrace restrictions introduced by Yama are effec‐
tively disabled for the calling process.

For further information, see the kernel source file Documentation/secu‐
rity/Yama.txt.

PR_SET_SECCOMP (since Linux 2.6.23)
Set the secure computing (seccomp) mode for the calling thread, to limit the avail‐
able system calls. The seccomp mode is selected via arg2. (The seccomp constants
are defined in <linux/seccomp.h>.)

With arg2 set to SECCOMP_MODE_STRICT the only system calls that the thread is per‐
mitted to make are read(2), write(2), _exit(2), and sigreturn(2). Other system
calls result in the delivery of a SIGKILL signal. Strict secure computing mode is
useful for number-crunching applications that may need to execute untrusted byte
code, perhaps obtained by reading from a pipe or socket. This operation is avail‐
able only if the kernel is configured with CONFIG_SECCOMP enabled.

With arg2 set to SECCOMP_MODE_FILTER (since Linux 3.5) the system calls allowed are
defined by a pointer to a Berkeley Packet Filter passed in arg3. This argument is
a pointer to struct sock_fprog; it can be designed to filter arbitrary system calls
and system call arguments. This mode is available only if the kernel is configured
with CONFIG_SECCOMP_FILTER enabled.

If SECCOMP_MODE_FILTER filters permit fork(2), then the seccomp mode is inherited
by children created by fork(2); if execve(2) is permitted, then the seccomp mode is
preserved across execve(2). If the filters permit prctl() calls, then additional
filters can be added; they are run in order until the first non-allow result is
seen.

For further information, see the kernel source file Documentation/prctl/sec‐
comp_filter.txt.

PR_GET_SECCOMP (since Linux 2.6.23)
Return (as the function result) the secure computing mode of the calling thread.
If the caller is not in secure computing mode, this operation returns 0; if the
caller is in strict secure computing mode, then the prctl() call will cause a
SIGKILL signal to be sent to the process. If the caller is in filter mode, and
this system call is allowed by the seccomp filters, it returns 2. This operation
is available only if the kernel is configured with CONFIG_SECCOMP enabled.

PR_SET_SECUREBITS (since Linux 2.6.26)
Set the "securebits" flags of the calling thread to the value supplied in arg2.
See capabilities(7).

PR_GET_SECUREBITS (since Linux 2.6.26)
Return (as the function result) the "securebits" flags of the calling thread. See
capabilities(7).

PR_SET_THP_DISABLE (since Linux 3.15)
Set the state of the "THP disable" flag for the calling thread. If arg2 has a
nonzero value, the flag is set, otherwise it is cleared. Setting this flag pro‐
vides a method for disabling transparent huge pages for jobs where the code cannot
be modified, and using a malloc hook with madvise(2) is not an option (i.e., stati‐
cally allocated data). The setting of the "THP disable" flag is inherited by a
child created via fork(2) and is preserved across execve(2).

PR_GET_THP_DISABLE (since Linux 3.15)
Return (via the function result) the current setting of the "THP disable" flag for
the calling thread: either 1, if the flag is set, or 0, if it is not.

PR_GET_TID_ADDRESS (since Linux 3.5)
Retrieve the clear_child_tid address set by set_tid_address(2) and the clone(2)
CLONE_CHILD_CLEARTID flag, in the location pointed to by (int **) arg2. This fea‐
ture is available only if the kernel is built with the CONFIG_CHECKPOINT_RESTORE
option enabled.

PR_SET_TIMERSLACK (since Linux 2.6.28)
Set the current timer slack for the calling thread to the nanosecond value supplied
in arg2. If arg2 is less than or equal to zero, reset the current timer slack to
the thread's default timer slack value. The timer slack is used by the kernel to
group timer expirations for the calling thread that are close to one another; as a
consequence, timer expirations for the thread may be up to the specified number of
nanoseconds late (but will never expire early). Grouping timer expirations can
help reduce system power consumption by minimizing CPU wake-ups.

The timer expirations affected by timer slack are those set by select(2), pse‐
lect(2), poll(2), ppoll(2), epoll_wait(2), epoll_pwait(2), clock_nanosleep(2),
nanosleep(2), and futex(2) (and thus the library functions implemented via futexes,
including pthread_cond_timedwait(3), pthread_mutex_timedlock(3),
pthread_rwlock_timedrdlock(3), pthread_rwlock_timedwrlock(3), and sem_timed‐
wait(3)).

Timer slack is not applied to threads that are scheduled under a real-time schedul‐
ing policy (see sched_setscheduler(2)).

Each thread has two associated timer slack values: a "default" value, and a "cur‐
rent" value. The current value is the one that governs grouping of timer expira‐
tions. When a new thread is created, the two timer slack values are made the same
as the current value of the creating thread. Thereafter, a thread can adjust its
current timer slack value via PR_SET_TIMERSLACK (the default value can't be
changed). The timer slack values of init (PID 1), the ancestor of all processes,
are 50,000 nanoseconds (50 microseconds). The timer slack values are preserved
across execve(2).

PR_GET_TIMERSLACK (since Linux 2.6.28)
Return (as the function result) the current timer slack value of the calling
thread.

PR_SET_TIMING (since Linux 2.6.0-test4)
Set whether to use (normal, traditional) statistical process timing or accurate
timestamp-based process timing, by passing PR_TIMING_STATISTICAL or PR_TIMING_TIME‐
STAMP to arg2. PR_TIMING_TIMESTAMP is not currently implemented (attempting to set
this mode will yield the error EINVAL).

PR_GET_TIMING (since Linux 2.6.0-test4)
Return (as the function result) which process timing method is currently in use.

PR_TASK_PERF_EVENTS_DISABLE (since Linux 2.6.31)
Disable all performance counters attached to the calling process, regardless of
whether the counters were created by this process or another process. Performance
counters created by the calling process for other processes are unaffected. For
more information on performance counters, see the Linux kernel source file
tools/perf/design.txt.

Originally called PR_TASK_PERF_COUNTERS_DISABLE; renamed (with same numerical
value) in Linux 2.6.32.

PR_TASK_PERF_EVENTS_ENABLE (since Linux 2.6.31)
The converse of PR_TASK_PERF_EVENTS_DISABLE; enable performance counters attached
to the calling process.

Originally called PR_TASK_PERF_COUNTERS_ENABLE; renamed in Linux 2.6.32.

PR_SET_TSC (since Linux 2.6.26, x86 only)
Set the state of the flag determining whether the timestamp counter can be read by
the process. Pass PR_TSC_ENABLE to arg2 to allow it to be read, or PR_TSC_SIGSEGV
to generate a SIGSEGV when the process tries to read the timestamp counter.

PR_GET_TSC (since Linux 2.6.26, x86 only)
Return the state of the flag determining whether the timestamp counter can be read,
in the location pointed to by (int *) arg2.

PR_SET_UNALIGN
(Only on: ia64, since Linux 2.3.48; parisc, since Linux 2.6.15; PowerPC, since
Linux 2.6.18; Alpha, since Linux 2.6.22) Set unaligned access control bits to arg2.
Pass PR_UNALIGN_NOPRINT to silently fix up unaligned user accesses, or
PR_UNALIGN_SIGBUS to generate SIGBUS on unaligned user access.

PR_GET_UNALIGN
(see PR_SET_UNALIGN for information on versions and architectures) Return unaligned
access control bits, in the location pointed to by (int *) arg2.

PR_MCE_KILL (since Linux 2.6.32)
Set the machine check memory corruption kill policy for the current thread. If
arg2 is PR_MCE_KILL_CLEAR, clear the thread memory corruption kill policy and use
the system-wide default. (The system-wide default is defined by /proc/sys/vm/mem‐
ory_failure_early_kill; see proc(5).) If arg2 is PR_MCE_KILL_SET, use a thread-
specific memory corruption kill policy. In this case, arg3 defines whether the
policy is early kill (PR_MCE_KILL_EARLY), late kill (PR_MCE_KILL_LATE), or the sys‐
tem-wide default (PR_MCE_KILL_DEFAULT). Early kill means that the thread receives
a SIGBUS signal as soon as hardware memory corruption is detected inside its
address space. In late kill mode, the process is killed only when it accesses a
corrupted page. See sigaction(2) for more information on the SIGBUS signal. The
policy is inherited by children. The remaining unused prctl() arguments must be
zero for future compatibility.

PR_MCE_KILL_GET (since Linux 2.6.32)
Return the current per-process machine check kill policy. All unused prctl() argu‐
ments must be zero.

PR_SET_MM (since Linux 3.3)
Modify certain kernel memory map descriptor fields of the calling process. Usually
these fields are set by the kernel and dynamic loader (see ld.so(8) for more infor‐
mation) and a regular application should not use this feature. However, there are
cases, such as self-modifying programs, where a program might find it useful to
change its own memory map. This feature is available only if the kernel is built
with the CONFIG_CHECKPOINT_RESTORE option enabled. The calling process must have
the CAP_SYS_RESOURCE capability. The value in arg2 is one of the options below,
while arg3 provides a new value for the option.

PR_SET_MM_START_CODE
Set the address above which the program text can run. The corresponding
memory area must be readable and executable, but not writable or sharable
(see mprotect(2) and mmap(2) for more information).

PR_SET_MM_END_CODE
Set the address below which the program text can run. The corresponding
memory area must be readable and executable, but not writable or sharable.

PR_SET_MM_START_DATA
Set the address above which initialized and uninitialized (bss) data are
placed. The corresponding memory area must be readable and writable, but
not executable or sharable.

PR_SET_MM_END_DATA
Set the address below which initialized and uninitialized (bss) data are
placed. The corresponding memory area must be readable and writable, but
not executable or sharable.

PR_SET_MM_START_STACK
Set the start address of the stack. The corresponding memory area must be
readable and writable.

PR_SET_MM_START_BRK
Set the address above which the program heap can be expanded with brk(2)
call. The address must be greater than the ending address of the current
program data segment. In addition, the combined size of the resulting heap
and the size of the data segment can't exceed the RLIMIT_DATA resource limit
(see setrlimit(2)).

PR_SET_MM_BRK
Set the current brk(2) value. The requirements for the address are the same
as for the PR_SET_MM_START_BRK option.

The following options are available since Linux 3.5.

PR_SET_MM_ARG_START
Set the address above which the program command line is placed.

PR_SET_MM_ARG_END
Set the address below which the program command line is placed.

PR_SET_MM_ENV_START
Set the address above which the program environment is placed.

PR_SET_MM_ENV_END
Set the address below which the program environment is placed.

The address passed with PR_SET_MM_ARG_START, PR_SET_MM_ARG_END,
PR_SET_MM_ENV_START, and PR_SET_MM_ENV_END should belong to a process stack
area. Thus, the corresponding memory area must be readable, writable, and
(depending on the kernel configuration) have the MAP_GROWSDOWN attribute set
(see mmap(2)).

PR_SET_MM_AUXV
Set a new auxiliary vector. The arg3 argument should provide the address of
the vector. The arg4 is the size of the vector.

PR_SET_MM_EXE_FILE
Supersede the /proc/pid/exe symbolic link with a new one pointing to a new
executable file identified by the file descriptor provided in arg3 argument.
The file descriptor should be obtained with a regular open(2) call.

To change the symbolic link, one needs to unmap all existing executable mem‐
ory areas, including those created by the kernel itself (for example the
kernel usually creates at least one executable memory area for the ELF .text
section).

The second limitation is that such transitions can be done only once in a
process life time. Any further attempts will be rejected. This should help
system administrators monitor unusual symbolic-link transitions over all
processes running on a system.

RETURN VALUE
On success, PR_GET_DUMPABLE, PR_GET_KEEPCAPS, PR_GET_NO_NEW_PRIVS, PR_GET_THP_DISABLE,
PR_CAPBSET_READ, PR_GET_TIMING, PR_GET_TIMERSLACK, PR_GET_SECUREBITS, PR_MCE_KILL_GET, and
(if it returns) PR_GET_SECCOMP return the nonnegative values described above. All other
option values return 0 on success. On error, -1 is returned, and errno is set appropri‐
ately.

ERRORS
EFAULT arg2 is an invalid address.

EINVAL The value of option is not recognized.

EINVAL option is PR_MCE_KILL or PR_MCE_KILL_GET or PR_SET_MM, and unused prctl() arguments
were not specified as zero.

EINVAL arg2 is not valid value for this option.

EINVAL option is PR_SET_SECCOMP or PR_GET_SECCOMP, and the kernel was not configured with
CONFIG_SECCOMP.

EINVAL option is PR_SET_MM, and one of the following is true

* arg4 or arg5 is nonzero;

* arg3 is greater than TASK_SIZE (the limit on the size of the user address space
for this architecture);

* arg2 is PR_SET_MM_START_CODE, PR_SET_MM_END_CODE, PR_SET_MM_START_DATA,
PR_SET_MM_END_DATA, or PR_SET_MM_START_STACK, and the permissions of the corre‐
sponding memory area are not as required;

* arg2 is PR_SET_MM_START_BRK or PR_SET_MM_BRK, and arg3 is less than or equal to
the end of the data segment or specifies a value that would cause the
RLIMIT_DATA resource limit to be exceeded.

EINVAL option is PR_SET_PTRACER and arg2 is not 0, PR_SET_PTRACER_ANY, or the PID of an
existing process.

EINVAL option is PR_SET_PDEATHSIG and arg2 is not a valid signal number.

EINVAL option is PR_SET_DUMPABLE and arg2 is neither SUID_DUMP_DISABLE nor SUID_DUMP_USER.

EINVAL option is PR_SET_TIMING and arg2 is not PR_TIMING_STATISTICAL.

EINVAL option is PR_SET_NO_NEW_PRIVS and arg2 is not equal to 1 or arg3, arg4, or arg5 is
nonzero.

EINVAL option is PR_GET_NO_NEW_PRIVS and arg2, arg3, arg4, or arg5 is nonzero.

EINVAL option is PR_SET_THP_DISABLE and arg3, arg4, or arg5 is nonzero.

EINVAL option is PR_GET_THP_DISABLE and arg2, arg3, arg4, or arg5 is nonzero.

EPERM option is PR_SET_SECUREBITS, and the caller does not have the CAP_SETPCAP capabil‐
ity, or tried to unset a "locked" flag, or tried to set a flag whose corresponding
locked flag was set (see capabilities(7)).

EPERM option is PR_SET_KEEPCAPS, and the callers's SECURE_KEEP_CAPS_LOCKED flag is set
(see capabilities(7)).

EPERM option is PR_CAPBSET_DROP, and the caller does not have the CAP_SETPCAP capability.

EPERM option is PR_SET_MM, and the caller does not have the CAP_SYS_RESOURCE capability.

EACCES option is PR_SET_MM, and arg3 is PR_SET_MM_EXE_FILE, the file is not executable.

EBUSY option is PR_SET_MM, arg3 is PR_SET_MM_EXE_FILE, and this the second attempt to
change the /proc/pid/exe symbolic link, which is prohibited.

EBADF option is PR_SET_MM, arg3 is PR_SET_MM_EXE_FILE, and the file descriptor passed in
arg4 is not valid.

VERSIONS
The prctl() system call was introduced in Linux 2.1.57.

CONFORMING TO
This call is Linux-specific. IRIX has a prctl() system call (also introduced in Linux
2.1.44 as irix_prctl on the MIPS architecture), with prototype

ptrdiff_t prctl(int option, int arg2, int arg3);

and options to get the maximum number of processes per user, get the maximum number of
processors the calling process can use, find out whether a specified process is currently
blocked, get or set the maximum stack size, and so on.