Ubuntu Manpage: prctl - operations on a process

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 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 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.

Higher-level interfaces layered on top of the above operations are provided in the
libcap(3) library in the form of cap_get_ambient(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/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.

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 processes. 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() 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 status.

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
(perhaps so that it can restart that process). Some init(1) frameworks (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 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,
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 capabilities (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
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)
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 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 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_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 (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 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.

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. 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 executable, 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 executable, 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 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).

In Linux 4.9 and earlier, the PR_SET_MM_EXE_FILE operation 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 operation 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 unsigned 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)
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 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 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-functional).
Once set, this 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
Documentation/userspace-api/no_new_privs.rst (or
Documentation/prctl/no_new_privs.txt before Linux 4.13). See also seccomp(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_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.

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_SIGINFO 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/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/admin-guide/LSM/Yama.rst (or Documentation/security/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 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/userspace-api/seccomp_filter.rst (or
Documentation/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 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_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_SPECULATION_CTRL

PR_SPEC_ENABLE
The speculation feature is enabled, mitigation is disabled.

PR_SPEC_DISABLE
The speculation feature is disabled, mitigation is enabled

PR_SPEC_FORCE_DISABLE
Same as PR_SPEC_DISABLE but cannot be undone.

If all bits are 0, then the CPU is not affected by the speculation misfeature.

If PR_SPEC_PRCTL is set, then per-thread control of the mitigation is available.
If not set, prctl() for the speculation misfeature will fail.

The arg3, arg4, and arg5 arguments must be specified as 0; otherwise 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. Currently, the
only permitted value for this argument is PR_SPEC_STORE_BYPASS (otherwise the call
fails with the error ENODEV). This setting is a per-thread attribute. 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 disabled.

PR_SPEC_DISABLE
The speculation feature is disabled, mitigation is enabled

PR_SPEC_FORCE_DISABLE
Same as PR_SPEC_DISABLE but cannot be undone. A subsequent prctl(...,
PR_SPEC_ENABLE) will fail with the error EPERM.

Any other 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 kernel
source file Documentation/admin-guide/kernel-parameters.txt.

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 (retaining 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_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 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. 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 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 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]/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)
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)
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_FP_MODE,   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_GET_SPECULATION_CTRL, 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.

EINVAL option was PR_GET_SPECULATION_CTRL or PR_SET_SPECULATION_CTRL and unused arguments
to prctl() are not 0.

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 neither PR_SPEC_ENABLE,
PR_SPEC_DISABLE, nor PR_SPEC_FORCE_DISABLE.

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.

COLOPHON

       This page is part of release 5.05 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/.