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 sig-
nificance 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 constants are defined in
<linux/capability.h>.) The capability bounding set dictates
whether the process can receive the capability through a file's
permitted capability 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 capa-
bility 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 having 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 termi-
nated) and marked as having a subreaper, the nearest still liv-
ing 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 loca-
tion 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 calling 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 var-
ious 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 operations 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
underflow, 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_NONRE-
COV 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 capabili-
ties" flag prevents it from being cleared. arg2 must be either
0 (permitted capabilities are cleared) or 1 (permitted capabili-
ties are kept). (A thread's effective capability set is always
cleared when such a credential change is made, regardless of the
setting of the "keep capabilities" flag.) The "keep capabili-
ties" 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 loca-
tion 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-func-
tional). 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 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.
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/ker-
nel/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 effectively disabled for the calling process.
For further information, see the kernel source file Documenta-
tion/security/Yama.txt.
PR_SET_SECCOMP (since Linux 2.6.23)
Set the secure computing (seccomp) mode for the calling thread,
to limit the available 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 permitted 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 available only if the kernel is configured
with CONFIG_SECCOMP enabled.
With arg2 set to SECCOMP_MODE_FILTER (since Linux 3.5) the sys-
tem 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 Documenta-
tion/prctl/seccomp_filter.txt.
PR_GET_SECCOMP (since Linux 2.6.23)
Return 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_SEC-
COMP 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_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 feature 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 ker-
nel 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), pselect(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, includ-
ing pthread_cond_timedwait(3), pthread_mutex_timedlock(3),
pthread_rwlock_timedrdlock(3), pthread_rwlock_timedwrlock(3),
and sem_timedwait(3)).
Timer slack is not applied to threads that are scheduled under a
realtime scheduling policy (see sched_setscheduler(2)).
Each thread has two associated timer slack values: a "default"
value, and a "current" value. The current value is the one that
governs grouping of timer expirations. 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 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_TIMESTAMP to arg2. PR_TIM-
ING_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 architec-
tures) 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 cur-
rent 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/memory_fail-
ure_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 sig-
nal. 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() arguments must be zero.
PR_SET_MM (since Linux 3.3)
Modify certain kernel memory map descriptor fields of the call-
ing process. Usually these fields are set by the kernel and
dynamic loader (see ld.so(8) for more information) 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_CHECK-
POINT_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 exe-
cutable, 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 exe-
cutable, 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 seg-
ment. 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.
RETURN VALUE
On success, PR_GET_DUMPABLE, PR_GET_KEEPCAPS, PR_GET_NO_NEW_PRIVS,
PR_CAPBSET_READ, PR_GET_TIMING, 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 appropriately.
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 correspond-
ing 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 speci-
fies 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.
EPERM option is PR_SET_SECUREBITS, and the caller does not have the
CAP_SETPCAP capability, 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.
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 maxi-
mum stack size, and so on.
SEE ALSO
signal(2), core(5)
COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
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