PRCTL(2) Linux Programmer's Manual PRCTL(2)
NAME
prctl - operations on a process or thread
SYNOPSIS
#include <sys/prctl.h>
int prctl(int option, unsigned long arg2, unsigned long arg3,
unsigned long arg4, unsigned long arg5);
DESCRIPTION
prctl() manipulates various aspects of the behavior of the calling
thread or process.
Note that careless use of some prctl() operations can confuse the user-
space run-time environment, so these operations should be used with
care.
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_CAP_AMBIENT (since Linux 4.3)
Reads or changes the ambient capability set of the calling
thread, 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 am-
bient 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.
Higher-level interfaces layered on top of the above operations
are provided in the libcap(3) library in the form of cap_get_am-
bient(3), cap_set_ambient(3), and cap_reset_ambient(3).
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/ca-
pability.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.
A higher-level interface layered on top of this operation is
provided in the libcap(3) library in the form of
cap_get_bound(3).
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.
A higher-level interface layered on top of this operation is
provided in the libcap(3) library in the form of
cap_drop_bound(3).
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.
A subreaper fulfills the role of init(1) for its descendant pro-
cesses. When a process becomes orphaned (i.e., its immediate
parent terminates), then that process will be reparented to the
nearest still living ancestor subreaper. Subsequently, calls to
getppid(2) in the orphaned process will now return the PID of
the subreaper process, and when the orphan terminates, it is the
subreaper process that will receive a SIGCHLD signal and will be
able to wait(2) on the process to discover its termination sta-
tus.
The setting of the "child subreaper" attribute is not inherited
by children created by fork(2) and clone(2). The setting is
preserved across execve(2).
Establishing a subreaper process is useful in session management
frameworks where a hierarchical group of processes is managed by
a subreaper process that needs to be informed when one of the
processes--for example, a double-forked daemon--terminates (per-
haps so that it can restart that process). Some init(1) frame-
works (e.g., systemd(1)) employ a subreaper process for similar
reasons.
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 "dumpable" attribute, which determines
whether core dumps are produced for the calling process upon de-
livery 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 bi-
nary 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, the "dumpable" attribute 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 follow-
ing 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, resulting in a change of either the effective
user ID or the effective group ID.
* The process executes (execve(2)) a program that has file ca-
pabilities (see capabilities(7)), but only if the permitted
capabilities gained exceed those already permitted for the
process.
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 attribute.
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 restric-
tions 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 con-
taining 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 be-
low) can operate only when this bit is set (FR=1). Ap-
plications that use the O32 FPXX ABI can operate with ei-
ther 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 regis-
ters 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 signifi-
cant 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)
Return (as the function result) 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 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
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_IO_FLUSHER (since Linux 5.6)
If a user process is involved in the block layer or filesystem
I/O path, and can allocate memory while processing I/O requests
it must set arg2 to 1. This will put the process in the
IO_FLUSHER state, which allows it special treatment to make
progress when allocating memory. If arg2 is 0, the process will
clear the IO_FLUSHER state, and the default behavior will be
used.
The calling process must have the CAP_SYS_RESOURCE capability.
arg3, arg4, and arg5 must be zero.
The IO_FLUSHER state is inherited by a child process created via
fork(2) and is preserved across execve(2).
Examples of IO_FLUSHER applications are FUSE daemons, SCSI de-
vice emulation daemons, and daemons that perform error handling
like multipath path recovery applications.
PR_GET_IO_FLUSHER (Since Linux 5.6)
Return (as the function result) the IO_FLUSHER state of the
caller. A value of 1 indicates that the caller is in the
IO_FLUSHER state; 0 indicates that the caller is not in the
IO_FLUSHER state.
The calling process must have the CAP_SYS_RESOURCE capability.
arg2, arg3, arg4, and arg5 must be zero.
PR_SET_KEEPCAPS (since Linux 2.2.18)
Set the state of the calling thread's "keep capabilities" flag.
The effect of this flag is described in capabilities(7). arg2
must be either 0 (clear the flag) or 1 (set the 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. See capabilities(7) for a
description of this flag.
PR_MCE_KILL (since Linux 2.6.32)
Set the machine check memory corruption kill policy for the
calling 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 (as the function result) 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 dy-
namic 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.
The calling process must have the CAP_SYS_RESOURCE capability.
The value in arg2 is one of the options below, while arg3 pro-
vides a new value for the option. The arg4 and arg5 arguments
must be zero if unused.
Before Linux 3.10, this feature is available only if the kernel
is built with the CONFIG_CHECKPOINT_RESTORE option enabled.
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 shareable (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 shareable.
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
shareable.
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
shareable.
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 ex-
panded 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 op-
tion.
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 descrip-
tor should be obtained with a regular open(2) call.
To change the symbolic link, one needs to unmap all ex-
isting executable memory areas, including those created
by the kernel itself (for example the kernel usually cre-
ates at least one executable memory area for the ELF
.text section).
In Linux 4.9 and earlier, the PR_SET_MM_EXE_FILE opera-
tion can be performed only once in a process's lifetime;
attempting to perform the operation a second time results
in the error EPERM. This restriction was enforced for
security reasons that were subsequently deemed specious,
and the restriction was removed in Linux 4.10 because
some user-space applications needed to perform this oper-
ation more than once.
The following options are available since Linux 3.18.
PR_SET_MM_MAP
Provides one-shot access to all the addresses by passing
in a struct prctl_mm_map (as defined in <linux/prctl.h>).
The arg4 argument should provide the size of the struct.
This feature is available only if the kernel is built
with the CONFIG_CHECKPOINT_RESTORE option enabled.
PR_SET_MM_MAP_SIZE
Returns the size of the struct prctl_mm_map the kernel
expects. This allows user space to find a compatible
struct. The arg4 argument should be a pointer to an un-
signed int.
This feature is available only if the kernel is built
with the CONFIG_CHECKPOINT_RESTORE option enabled.
PR_MPX_ENABLE_MANAGEMENT, PR_MPX_DISABLE_MANAGEMENT (since Linux 3.19,
removed in Linux 5.4; only on x86)
Enable or disable kernel management of Memory Protection eXten-
sions (MPX) bounds tables. The arg2, arg3, arg4, and arg5 argu-
ments 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 en-
forcement. 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 allo-
cates the table and populates the bounds directory with the lo-
cation 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 direc-
tory in the bndcfgu register.
These calls 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.
Due to a lack of toolchain support, PR_MPX_ENABLE_MANAGEMENT and
PR_MPX_DISABLE_MANAGEMENT are not supported in Linux 5.4 and
later.
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, 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 at-
tribute that can be set via pthread_setname_np(3) and retrieved
using pthread_getname_np(3). The attribute is likewise accessi-
ble via /proc/self/task/[tid]/comm (see proc(5)), where [tid] is
the thread ID of the calling thread, as returned by gettid(2).
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 thread's no_new_privs attribute 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-func-
tional). Once set, the no_new_privs attribute cannot be unset.
The setting of this attribute is inherited by children created
by fork(2) and clone(2), and preserved across execve(2).
Since Linux 4.10, the value of a thread's no_new_privs attribute
can be viewed via the NoNewPrivs field in the /proc/[pid]/status
file.
For more information, see the kernel source file Documenta-
tion/userspace-api/no_new_privs.rst (or Documenta-
tion/prctl/no_new_privs.txt before Linux 4.13). See also sec-
comp(2).
PR_GET_NO_NEW_PRIVS (since Linux 3.5)
Return (as the function result) the value of the no_new_privs
attribute for the calling thread. 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_PAC_RESET_KEYS (since Linux 5.0, only on arm64)
Securely reset the thread's pointer authentication keys to fresh
random values generated by the kernel.
The set of keys to be reset is specified by arg2, which must be
a logical OR of zero or more of the following:
PR_PAC_APIAKEY
instruction authentication key A
PR_PAC_APIBKEY
instruction authentication key B
PR_PAC_APDAKEY
data authentication key A
PR_PAC_APDBKEY
data authentication key B
PR_PAC_APGAKEY
generic authentication "A" key.
(Yes folks, there really is no generic B key.)
As a special case, if arg2 is zero, then all the keys are reset.
Since new keys could be added in future, this is the recommended
way to completely wipe the existing keys when establishing a
clean execution context. Note that there is no need to use
PR_PAC_RESET_KEYS in preparation for calling execve(2), since
execve(2) resets all the pointer authentication keys.
The remaining arguments arg3, arg4, and arg5 must all be zero.
If the arguments are invalid, and in particular if arg2 contains
set bits that are unrecognized or that correspond to a key not
available on this platform, then the call fails with error EIN-
VAL.
Warning: Because the compiler or run-time environment may be us-
ing some or all of the keys, a successful may crash the calling
process. The conditions for using it safely are complex and
system-dependent. Don't use it unless you know what you are do-
ing.
For more information, see the kernel source file Documenta-
tion/arm64/pointer-authentication.rst (or Documenta-
tion/arm64/pointer-authentication.txt before Linux 5.3).
PR_PAC_RESET_KEYS
PR_SET_PDEATHSIG (since Linux 2.1.57)
Set the parent-death signal of the calling process to arg2 (ei-
ther a signal value in the range 1..NSIG-1, or 0 to clear).
This is the signal that the calling process will get when its
parent dies.
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.
The parent-death signal is sent upon subsequent termination of
the parent thread and also upon termination of each subreaper
process (see the description of PR_SET_CHILD_SUBREAPER above) to
which the caller is subsequently reparented. If the parent
thread and all ancestor subreapers have already terminated by
the time of the PR_SET_PDEATHSIG operation, then no parent-death
signal is sent to the caller.
The parent-death signal is process-directed (see signal(7)) and,
if the child installs a handler using the sigaction(2) SA_SIG-
INFO flag, the si_pid field of the siginfo_t argument of the
handler contains the PID of the terminating parent process.
The parent-death signal setting is cleared for the child of a
fork(2). It is also (since Linux 2.4.36 / 2.6.23) cleared when
executing a set-user-ID or set-group-ID binary, or a binary that
has associated capabilities (see capabilities(7)); otherwise,
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/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 an-
cestor. 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/admin-guide/LSM/Yama.rst (or Documentation/secu-
rity/Yama.txt before Linux 4.13).
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 com-
puting 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 ex-
ecve(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/userspace-api/seccomp_filter.rst (or Documenta-
tion/prctl/seccomp_filter.txt before Linux 4.13).
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 com-
puting 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, with-
out 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_GET_SPECULATION_CTRL (since Linux 4.17)
Return (as the function result) the state of the speculation
misfeature specified in arg2. Currently, the only permitted
value for this argument is PR_SPEC_STORE_BYPASS (otherwise the
call fails with the error ENODEV).
The return value uses bits 0-3 with the following meaning:
PR_SPEC_PRCTL
Mitigation can be controlled per thread by PR_SET_SPECU-
LATION_CTRL.
PR_SPEC_ENABLE
The speculation feature is enabled, mitigation is dis-
abled.
PR_SPEC_DISABLE
The speculation feature is disabled, mitigation is en-
abled.
PR_SPEC_FORCE_DISABLE
Same as PR_SPEC_DISABLE but cannot be undone.
PR_SPEC_DISABLE_NOEXEC (since Linux 5.1)
Same as PR_SPEC_DISABLE, but the state will be cleared on
execve(2).
If all bits are 0, then the CPU is not affected by the specula-
tion misfeature.
If PR_SPEC_PRCTL is set, then per-thread control of the mitiga-
tion is available. If not set, prctl() for the speculation mis-
feature will fail.
The arg3, arg4, and arg5 arguments must be specified as 0; oth-
erwise the call fails with the error EINVAL.
PR_SET_SPECULATION_CTRL (since Linux 4.17)
Sets the state of the speculation misfeature specified in arg2.
The speculation-misfeature settings are per-thread attributes.
Currently, arg2 must be one of:
PR_SPEC_STORE_BYPASS
Set the state of the speculative store bypass misfeature.
PR_SPEC_INDIRECT_BRANCH (since Linux 4.20)
Set the state of the indirect branch speculation misfea-
ture.
If arg2 does not have one of the above values, then the call
fails with the error ENODEV.
The arg3 argument is used to hand in the control value, which is
one of the following:
PR_SPEC_ENABLE
The speculation feature is enabled, mitigation is dis-
abled.
PR_SPEC_DISABLE
The speculation feature is disabled, mitigation is en-
abled.
PR_SPEC_FORCE_DISABLE
Same as PR_SPEC_DISABLE, but cannot be undone. A subse-
quent prctl(arg2, PR_SPEC_ENABLE) with the same value for
arg2 will fail with the error EPERM.
PR_SPEC_DISABLE_NOEXEC (since Linux 5.1)
Same as PR_SPEC_DISABLE, but the state will be cleared on
execve(2). Currently only supported for arg2 equal to
PR_SPEC_STORE_BYPASS.
Any unsupported value in arg3 will result in the call failing
with the error ERANGE.
The arg4 and arg5 arguments must be specified as 0; otherwise
the call fails with the error EINVAL.
The speculation feature can also be controlled by the
spec_store_bypass_disable boot parameter. This parameter may
enforce a read-only policy which will result in the prctl() call
failing with the error ENXIO. For further details, see the ker-
nel source file Documentation/admin-guide/kernel-parameters.txt.
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 (re-
taining the 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_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 modi-
fied, and using a malloc hook with madvise(2) is not an option
(i.e., statically allocated data). The setting of the "THP dis-
able" 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 (as 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)
Return 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 ker-
nel 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 ker-
nel 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. arg2 is an unsigned
long value, then maximum "current" value is ULONG_MAX and the
minimum "current" value is 1. If the nanosecond value supplied
in arg2 is greater than zero, then the "current" value is set to
this value. If arg2 is 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 an-
other; 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 sys-
tem 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
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 pro-
cesses, are 50,000 nanoseconds (50 microseconds). The timer
slack value is inherited by a child created via fork(2), and is
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]/timer-
slack_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)
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)
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 addi-
tional flag with the value of 4 and no corresponding named con-
stant, 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 architec-
tures.) Return unaligned access control bits, in the location
pointed to by (unsigned int *) arg2.
RETURN VALUE
On success, PR_CAP_AMBIENT+PR_CAP_AMBIENT_IS_SET, PR_CAPBSET_READ,
PR_GET_DUMPABLE, PR_GET_FP_MODE, PR_GET_IO_FLUSHER, PR_GET_KEEPCAPS,
PR_MCE_KILL_GET, PR_GET_NO_NEW_PRIVS, PR_GET_SECUREBITS, PR_GET_SPECU-
LATION_CTRL, PR_GET_THP_DISABLE, PR_GET_TIMING, PR_GET_TIMERSLACK, 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 sys-
tem was built with CONFIG_SECCOMP_FILTER, and arg3 is an invalid
address.
EINVAL The value of option is not recognized, or not supported on this
system.
EINVAL option is PR_MCE_KILL or PR_MCE_KILL_GET or PR_SET_MM, and un-
used 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 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.
EINVAL option is PR_SET_PDEATHSIG and arg2 is not a valid signal num-
ber.
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.
EINVAL option was PR_GET_SPECULATION_CTRL or PR_SET_SPECULATION_CTRL
and unused arguments to prctl() are not 0. EINVAL option is
PR_PAC_RESET_KEYS and the arguments are invalid or unsupported.
See the description of PR_PAC_RESET_KEYS above for details.
ENODEV option was PR_SET_SPECULATION_CTRL the kernel or CPU does not
support the requested speculation misfeature.
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.
ENXIO option was PR_SET_SPECULATION_CTRL implies that the control of
the selected speculation misfeature is not possible. See
PR_GET_SPECULATION_CTRL for the bit fields to determine which
option is available.
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_SPECULATION_CTRL wherein the speculation was
disabled with PR_SPEC_FORCE_DISABLE and caller tried to enable
it again.
EPERM option is PR_SET_KEEPCAPS, and the caller's
SECBIT_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.
ERANGE option was PR_SET_SPECULATION_CTRL and arg3 is not PR_SPEC_EN-
ABLE, PR_SPEC_DISABLE, PR_SPEC_FORCE_DISABLE, nor PR_SPEC_DIS-
ABLE_NOEXEC.
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 in-
troduced 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 5.07 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/.
Linux 2020-04-11 PRCTL(2)