Reads or changes the ambient capability set of the calling thread, according to the value of arg2, which must be one of the following:

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).

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).

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).

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(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 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.

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

Set the state of the "dumpable" attribute, 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, 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 following circumstances:

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).

Return (as the function result) the current state of the calling process's dumpable attribute.

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).

Return the endian-ness of the calling process, in the location pointed to by (int *) arg2.

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:

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.

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.

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.

Return floating-point emulation control bits, in the location pointed to by (int *) arg2.

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.

Return floating-point exception mode, in the location pointed to by (int *) arg2.

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 device emulation daemons, and daemons that perform error handling like multipath path recovery applications.

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.

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).

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.

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.

Return (as the function result) the current per-process machine check kill policy. All unused prctl() arguments must be zero.

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.

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:

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.

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 (see proc(5)), where [tid] is the thread ID of the calling thread, as returned by gettid(2).

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.

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, 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).

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.

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:

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 EINVAL.

	Warning
	Because the compiler or run-time environment may be using some or all of the keys, a successful PR_PAC_RESET_KEYS 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 doing.

For more information, see the kernel source file Documentation/arm64/pointer−authentication.rst (or Documentation/arm64/pointer−authentication.txt before Linux 5.3).

Set the parent-death signal of the calling process to arg2 (either 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_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). The parent-death signal setting is also cleared upon changes to any of the following thread credentials: effective user ID, effective group ID, filesystem user ID, or filesystem group ID.

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

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).

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).

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).

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

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

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:

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.

Sets the state of the speculation misfeature specified in arg2. The speculation-misfeature settings are per-thread attributes.

Currently, arg2 must be one of:

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:

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 kernel source file Documentation/admin−guide/kernel−parameters.txt.

Configure the thread's SVE vector length, as specified by (int) arg2. Arguments arg3, arg4, and arg5 are ignored.

The bits of arg2 corresponding to PR_SVE_VL_LEN_MASK must be set to the desired vector length in bytes. This is interpreted as an upper bound: the kernel will select the greatest available vector length that does not exceed the value specified. In particular, specifying SVE_VL_MAX (defined in <asm/sigcontext.h>) for the PR_SVE_VL_LEN_MASK bits requests the maximum supported vector length.

In addition, the other bits of arg2 must be set to one of the following combinations of flags:

In all cases, any previously pending deferred change is canceled.

The call fails with error EINVAL if SVE is not supported on the platform, if arg2 is unrecognized or invalid, or the value in the bits of arg2 corresponding to PR_SVE_VL_LEN_MASK is outside the range SVE_VL_MIN..SVE_VL_MAX or is not a multiple of 16.

On success, a nonnegative value is returned that describes the selected configuration. If PR_SVE_SET_VL_ONEXEC was included in arg2, then the configuration described by the return value will take effect at the next execve(). Otherwise, the configuration is already in effect when the PR_SVE_SET_VL call returns. In either case, the value is encoded in the same way as the return value of PR_SVE_GET_VL. Note that there is no explicit flag in the return value corresponding to PR_SVE_SET_VL_ONEXEC.

The configuration (including any pending deferred change) is inherited across fork(2) and clone(2).

For more information, see the kernel source file Documentation/arm64/sve.rst (or Documentation/arm64/sve.txt before Linux 5.3).

	Warning
	Because the compiler or run-time environment may be using SVE, using this call without the PR_SVE_SET_VL_ONEXEC flag may crash the calling process. The conditions for using it safely are complex and system-dependent. Don't use it unless you really know what you are doing.

Get the thread's current SVE vector length configuration.

Arguments arg2, arg3, arg4, and arg5 are ignored.

Provided that the kernel and platform support SVE, this operation always succeeds, returning a nonnegative value that describes the current configuration. The bits corresponding to PR_SVE_VL_LEN_MASK contain the currently configured vector length in bytes. The bit corresponding to PR_SVE_VL_INHERIT indicates whether the vector length will be inherited across execve(2).

Note that there is no way to determine whether there is a pending vector length change that has not yet taken effect.

For more information, see the kernel source file Documentation/arm64/sve.rst (or Documentation/arm64/sve.txt before Linux 5.3).

Configure the Syscall User Dispatch mechanism for the calling thread. This mechanism allows an application to selectively intercept system calls so that they can be handled within the application itself. Interception takes the form of a thread-directed SIGSYS signal that is delivered to the thread when it makes a system call. If intercepted, the system call is not executed by the kernel.

To enable this mechanism, arg2 should be set to PR_SYS_DISPATCH_ON. Once enabled, further system calls will be selectively intercepted, depending on a control variable provided by user space. In this case, arg3 and arg4 respectively identify the offset and length of a single contiguous memory region in the process address space from where system calls are always allowed to be executed, regardless of the control variable. (Typically, this area would include the area of memory containing the C library.)

arg5 points to a char-sized variable that is a fast switch to allow/block system call execution without the overhead of doing another system call to re-configure Syscall User Dispatch. This control variable can either be set to SYSCALL_DISPATCH_FILTER_BLOCK to block system calls from executing or to SYSCALL_DISPATCH_FILTER_ALLOW to temporarily allow them to be executed. This value is checked by the kernel on every system call entry, and any unexpected value will raise an uncatchable SIGSYS at that time, killing the application.

When a system call is intercepted, the kernel sends a thread-directed SIGSYS signal to the triggering thread. Various fields will be set in the siginfo_t structure (see sigaction(2)) associated with the signal:

The program counter will be as though the system call happened (i.e., the program counter will not point to the system call instruction).

When the signal handler returns to the kernel, the system call completes immediately and returns to the calling thread, without actually being executed. If necessary (i.e., when emulating the system call on user space.), the signal handler should set the system call return value to a sane value, by modifying the register context stored in the ucontext argument of the signal handler. See sigaction(2), sigreturn(2), and getcontext(3) for more information.

If arg2 is set to PR_SYS_DISPATCH_OFF, Syscall User Dispatch is disabled for that thread. the remaining arguments must be set to 0.

The setting is not preserved across fork(2), clone(2), or execve(2).

For more information, see the kernel source file Documentation/admin-guide/syscall-user-dispatch.rst

Controls support for passing tagged user-space addresses to the kernel (i.e., addresses where bits 56—63 are not all zero).

The level of support is selected by arg2, which can be one of the following:

The remaining arguments arg3, arg4, and arg5 must all be zero.

On success, the mode specified in arg2 is set for the calling thread and the return value is 0. If the arguments are invalid, the mode specified in arg2 is unrecognized, or if this feature is unsupported by the kernel or disabled via /proc/sys/abi/tagged_addr_disabled, the call fails with the error EINVAL.

In particular, if prctl(PR_SET_TAGGED_ADDR_CTRL, 0, 0, 0, 0) fails with EINVAL, then all addresses passed to the kernel must be untagged.

Irrespective of which mode is set, addresses passed to certain interfaces must always be untagged:

This list of exclusions may shrink when moving from one kernel version to a later kernel version. While the kernel may make some guarantees for backwards compatibility reasons, for the purposes of new software the effect of passing tagged addresses to these interfaces is unspecified.

The mode set by this call is inherited across fork(2) and clone(2). The mode is reset by execve(2) to 0 (i.e., tagged addresses not permitted in the user/kernel ABI).

For more information, see the kernel source file Documentation/arm64/tagged−address−abi.rst.

	Warning
	This call is primarily intended for use by the run-time environment. A successful PR_SET_TAGGED_ADDR_CTRL call elsewhere 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 doing.

Returns the current tagged address mode for the calling thread.

Arguments arg2, arg3, arg4, and arg5 must all be zero.

If the arguments are invalid or this feature is disabled or unsupported by the kernel, the call fails with EINVAL. In particular, if prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0) fails with EINVAL, then this feature is definitely either unsupported, or disabled via /proc/sys/abi/tagged_addr_disabled. In this case, all addresses passed to the kernel must be untagged.

Otherwise, the call returns a nonnegative value describing the current tagged address mode, encoded in the same way as the arg2 argument of PR_SET_TAGGED_ADDR_CTRL.

For more information, see the kernel source file Documentation/arm64/tagged−address−abi.rst.

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.

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.

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).

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.

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.

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).

Return (as the function result) the "current" timer slack value of the calling thread.

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).

Return (as the function result) which process timing method is currently in use.

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.

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

(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).

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

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).

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

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

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.

arg2 is an invalid address.

option is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER, the system was built with CONFIG_SECCOMP_FILTER, and arg3 is an invalid address.

option is PR_SET_SYSCALL_USER_DISPATCH and arg5 has an invalid address.

The value of option is not recognized, or not supported on this system.

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

arg2 is not valid value for this option.

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

option is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER, and the kernel was not configured with CONFIG_SECCOMP_FILTER.

option is PR_SET_MM, and one of the following is true

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

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

option is PR_SET_DUMPABLE and arg2 is neither SUID_DUMP_DISABLE nor SUID_DUMP_USER.

option is PR_SET_TIMING and arg2 is not PR_TIMING_STATISTICAL.

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

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

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

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

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.

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.

option is PR_SVE_SET_VL and the arguments are invalid or unsupported, or SVE is not available on this platform. See the description of PR_SVE_SET_VL above for details.

option is PR_SVE_GET_VL and SVE is not available on this platform.

option is PR_SET_SYSCALL_USER_DISPATCH and one of the following is true:

option is PR_SET_TAGGED_ADDR_CTRL and the arguments are invalid or unsupported. See the description of PR_SET_TAGGED_ADDR_CTRL above for details.

option is PR_GET_TAGGED_ADDR_CTRL and the arguments are invalid or unsupported. See the description of PR_GET_TAGGED_ADDR_CTRL above for details.

option was PR_SET_SPECULATION_CTRL the kernel or CPU does not support the requested speculation misfeature.

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.

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.

option is PR_SET_FP_MODE and arg2 has an invalid or unsupported value.

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)).

option is PR_SET_SPECULATION_CTRL wherein the speculation was disabled with PR_SPEC_FORCE_DISABLE and caller tried to enable it again.

option is PR_SET_KEEPCAPS, and the caller's SECBIT_KEEP_CAPS_LOCKED flag is set (see capabilities(7)).

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

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

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.

option was PR_SET_SPECULATION_CTRL and arg3 is not PR_SPEC_ENABLE, PR_SPEC_DISABLE, PR_SPEC_FORCE_DISABLE, nor PR_SPEC_DISABLE_NOEXEC.