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_CAP_AMBIENT (since Linux 4.3)
Reads or changes the ambient capability set, according to the
value of arg2, which must be one of the following:
PR_CAP_AMBIENT_RAISE
The capability specified in arg3 is added to the ambient
set. The specified capability must already be present in
both the permitted and the inheritable sets of the
process. This operation is not permitted if the
SECBIT_NO_CAP_AMBIENT_RAISE securebit is set.
PR_CAP_AMBIENT_LOWER
The capability specified in arg3 is removed from the
ambient set.
PR_CAP_AMBIENT_IS_SET
The prctl() call returns 1 if the capability in arg3 is
in the ambient set and 0 if it is not.
PR_CAP_AMBIENT_CLEAR_ALL
All capabilities will be removed from the ambient set.
This operation requires setting arg3 to zero.
In all of the above operations, arg4 and arg5 must be specified
as 0.
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 within its
user namespace, 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 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
terminated) and marked as having a subreaper, the nearest still
living ancestor subreaper will receive a SIGCHLD signal and will
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 "dumpable" flag, which determines whether
core dumps are produced for the calling process upon delivery of
a signal whose default behavior is to produce a core dump.
In kernels up to and including 2.6.12, arg2 must be either 0
(SUID_DUMP_DISABLE, process is not dumpable) or 1
(SUID_DUMP_USER, 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).)
Normally, this flag is set to 1. However, it is reset to the
current value contained in the file /proc/sys/fs/suid_dumpable
(which by default has the value 0), in the following
circumstances:
* The process's effective user or group ID is changed.
* The process's filesystem user or group ID is changed (see
credentials(7)).
* The process executes (execve(2)) a set-user-ID or set-group-
ID program, or a program that has capabilities (see
capabilities(7)).
Processes that are not dumpable can not be attached via
ptrace(2) PTRACE_ATTACH; see ptrace(2) for further details.
If a process is not dumpable, the ownership of files in the
process's /proc/[pid] directory is affected as described in
proc(5).
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_FP_MODE (since Linux 4.0, only on MIPS)
On the MIPS architecture, user-space code can be built using an
ABI which permits linking with code that has more restrictive
floating-point (FP) requirements. For example, user-space code
may be built to target the O32 FPXX ABI and linked with code
built for either one of the more restrictive FP32 or FP64 ABIs.
When more restrictive code is linked in, the overall requirement
for the process is to use the more restrictive floating-point
mode.
Because the kernel has no means of knowing in advance which mode
the process should be executed in, and because these
restrictions can change over the lifetime of the process, the
PR_SET_FP_MODE operation is provided to allow control of the
floating-point mode from user space.
The (unsigned int) arg2 argument is a bit mask describing the
floating-point mode used:
PR_FP_MODE_FR
When this bit is unset (so called FR=0 or FR0 mode), the
32 floating-point registers are 32 bits wide, and 64-bit
registers are represented as a pair of registers (even-
and odd- numbered, with the even-numbered register
containing the lower 32 bits, and the odd-numbered
register containing the higher 32 bits).
When this bit is set (on supported hardware), the 32
floating-point registers are 64 bits wide (so called FR=1
or FR1 mode). Note that modern MIPS implementations
(MIPS R6 and newer) support FR=1 mode only.
Applications that use the O32 FP32 ABI can operate only
when this bit is unset (FR=0; or they can be used with
FRE enabled, see below). Applications that use the O32
FP64 ABI (and the O32 FP64A ABI, which exists to provide
the ability to operate with existing FP32 code; see
below) can operate only when this bit is set (FR=1).
Applications that use the O32 FPXX ABI can operate with
either FR=0 or FR=1.
PR_FP_MODE_FRE
Enable emulation of 32-bit floating-point mode. When
this mode is enabled, it emulates 32-bit floating-point
operations by raising a reserved-instruction exception on
every instruction that uses 32-bit formats and the kernel
then handles the instruction in software. (The problem
lies in the discrepancy of handling odd-numbered
registers which are the high 32 bits of 64-bit registers
with even numbers in FR=0 mode and the lower 32-bit parts
of odd-numbered 64-bit registers in FR=1 mode.) Enabling
this bit is necessary when code with the O32 FP32 ABI
should operate with code with compatible the O32 FPXX or
O32 FP64A ABIs (which require FR=1 FPU mode) or when it
is executed on newer hardware (MIPS R6 onwards) which
lacks FR=0 mode support when a binary with the FP32 ABI
is used.
Note that this mode makes sense only when the FPU is in
64-bit mode (FR=1).
Note that the use of emulation inherently has a
significant performance hit and should be avoided if
possible.
In the N32/N64 ABI, 64-bit floating-point mode is always used,
so FPU emulation is not required and the FPU always operates in
FR=1 mode.
This option is mainly intended for use by the dynamic linker
(ld.so(8)).
The arguments arg3, arg4, and arg5 are ignored.
PR_GET_FP_MODE (since Linux 4.0, only on MIPS)
Get the current floating-point mode (see the description of
PR_SET_FP_MODE for details).
On success, the call returns a bit mask which represents the
current floating-point mode.
The arguments arg2, arg3, arg4, and arg5 are ignored.
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 floating-point operation
accesses, or PR_FPEMU_SIGFPE to not emulate floating-point
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_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 thread's permitted capability set is
cleared when a change is made to the thread's user IDs such that
the thread'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 credential 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
thread's "keep capabilities" flag.
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/memory_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
system-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() arguments 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 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_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 memory 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.
PR_MPX_ENABLE_MANAGEMENT, PR_MPX_DISABLE_MANAGEMENT (since Linux 3.19)
Enable or disable kernel management of Memory Protection
eXtensions (MPX) bounds tables. The arg2, arg3, arg4, and arg5
arguments must be zero.
MPX is a hardware-assisted mechanism for performing bounds
checking on pointers. It consists of a set of registers storing
bounds information and a set of special instruction prefixes
that tell the CPU on which instructions it should do bounds
enforcement. There is a limited number of these registers and
when there are more pointers than registers, their contents must
be "spilled" into a set of tables. These tables are called
"bounds tables" and the MPX prctl() operations control whether
the kernel manages their allocation and freeing.
When management is enabled, the kernel will take over allocation
and freeing of the bounds tables. It does this by trapping the
#BR exceptions that result at first use of missing bounds tables
and instead of delivering the exception to user space, it
allocates the table and populates the bounds directory with the
location of the new table. For freeing, the kernel checks to
see if bounds tables are present for memory which is not
allocated, and frees them if so.
Before enabling MPX management using PR_MPX_ENABLE_MANAGEMENT,
the application must first have allocated a user-space buffer
for the bounds directory and placed the location of that
directory in the bndcfgu register.
These calls will fail if the CPU or kernel does not support MPX.
Kernel support for MPX is enabled via the CONFIG_X86_INTEL_MPX
configuration option. You can check whether the CPU supports
MPX by looking for the 'mpx' CPUID bit, like with the following
command:
cat /proc/cpuinfo | grep ' mpx '
A thread may not switch in or out of long (64-bit) mode while
MPX is enabled.
All threads in a process are affected by these calls.
The child of a fork(2) inherits the state of MPX management.
During execve(2), MPX management is reset to a state as if
PR_MPX_DISABLE_MANAGEMENT had been called.
For further information on Intel MPX, see the kernel source file
Documentation/x86/intel_mpx.txt.
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, including the terminating null byte. (If the length
of the string, including the terminating null byte, exceeds 16
bytes, the string is silently truncated.) 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.
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 mode 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
Documentation/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 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, or a binary that has associated
capabilities (see capabilities(7)). This value is preserved
across execve(2).
Warning: the "parent" in this case is considered to be the
thread that created this process. In other words, the signal
will be sent when that thread terminates (via, for example,
pthread_exit(3)), rather than after all of the threads in the
parent process terminate.
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 effectively disabled for the calling process.
For further information, see the kernel source file
Documentation/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 more recent seccomp(2)
system call provides a superset of the functionality of
PR_SET_SECCOMP.
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)
(but not exit_group(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
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/seccomp_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; otherwise, the process is killed with a SIGKILL
signal. This operation is available only if the kernel is
configured with CONFIG_SECCOMP enabled.
Since Linux 3.8, the Seccomp field of the /proc/[pid]/status
file provides a method of obtaining the same information,
without the risk that the process is killed; see proc(5).
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 provides 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., statically 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_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_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 feature is available only if
the kernel is built with the CONFIG_CHECKPOINT_RESTORE option
enabled. Note that since the prctl() system call does not have
a compat implementation for the AMD64 x32 and MIPS n32 ABIs, and
the kernel writes out a pointer using the kernel's pointer size,
this operation expects a user-space buffer of 8 (not 4) bytes on
these ABIs.
PR_SET_TIMERSLACK (since Linux 2.6.28)
Each thread has two associated timer slack values: a "default"
value, and a "current" value. This operation sets the "current"
timer slack value for the calling thread. If the nanosecond
value supplied in arg2 is greater than zero, then the "current"
value is set to this value. If arg2 is less than or equal to
zero, the "current" timer slack is reset to the thread's
"default" timer slack value.
The "current" 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), 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,
including 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
real-time scheduling policy (see sched_setscheduler(2)).
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).
Since Linux 4.6, the "current" timer slack value of any process
can be examined and changed via the file
/proc/[pid]/timerslack_ns. See proc(5).
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_TIMESTAMP 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_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; sh,
since Linux 2.6.34; tile, since Linux 3.12) 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. Alpha also supports an
additional flag with the value of 4 and no corresponding named
constant, which instructs kernel to not fix up unaligned
accesses (it is analogous to providing the UAC_NOFIX flag in
SSI_NVPAIRS operation of the setsysinfo() system call on Tru64).
PR_GET_UNALIGN
(see PR_SET_UNALIGN for information on versions and
architectures) Return unaligned access control bits, in the
location pointed to by (unsigned int *) arg2.
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, PR_CAP_AMBIENT+PR_CAP_AMBIENT_IS_SET, 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
EACCES option is PR_SET_SECCOMP and arg2 is SECCOMP_MODE_FILTER, but
the process does not have the CAP_SYS_ADMIN capability or has
not set the no_new_privs attribute (see the discussion of
PR_SET_NO_NEW_PRIVS above).
EACCES option is PR_SET_MM, and arg3 is PR_SET_MM_EXE_FILE, the file is
not executable.
EBADF option is PR_SET_MM, arg3 is PR_SET_MM_EXE_FILE, and the file
descriptor passed in arg4 is not valid.
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.
EFAULT arg2 is an invalid address.
EFAULT option is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER, the
system was built with CONFIG_SECCOMP_FILTER, and arg3 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_SECCOMP, arg2 is SECCOMP_MODE_FILTER, and the
kernel was not configured with CONFIG_SECCOMP_FILTER.
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
corresponding 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.
EINVAL option is PR_CAP_AMBIENT and an unused argument (arg4, arg5, or,
in the case of PR_CAP_AMBIENT_CLEAR_ALL, arg3) is nonzero; or
arg2 has an invalid value; or arg2 is PR_CAP_AMBIENT_LOWER,
PR_CAP_AMBIENT_RAISE, or PR_CAP_AMBIENT_IS_SET and arg3 does not
specify a valid capability.
ENXIO option was PR_MPX_ENABLE_MANAGEMENT or PR_MPX_DISABLE_MANAGEMENT
and the kernel or the CPU does not support MPX management.
Check that the kernel and processor have MPX support.
EOPNOTSUPP
option is PR_SET_FP_MODE and arg2 has an invalid or unsupported
value.
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 caller'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.
EPERM option is PR_CAP_AMBIENT and arg2 is PR_CAP_AMBIENT_RAISE, but
either the capability specified in arg3 is not present in the
process's permitted and inheritable capability sets, or the
PR_CAP_AMBIENT_LOWER securebit has been set.
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.
SEE ALSO
signal(2), core(5)
COLOPHON
This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
