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PRCTL(2) Linux Programmer's Manual PRCTL(2)

NAME

prctl - operations on a process

SYNOPSIS

#include <sys/prctl.h>

int prctl(int option, unsigned long arg2, unsigned long arg3,
          unsigned long arg4, unsigned long arg5);

DESCRIPTION

prctl() is called with a first argument describing what to do (with values defined in <linux/prctl.h>), and further arguments with a significance depending on the first one. The first argument can be:

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.

If the calling thread has the CAP_SETPCAP capability, 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.

If arg2 is nonzero, set the "child subreaper" attribute of the calling process; if arg2 is zero, unset the attribute. When a process is marked as a child subreaper, all of the children that it creates, and their descendants, will be marked as having a subreaper. In effect, a subreaper fulfills the role of init(1) for its descendant processes. Upon termination of a process that is orphaned (i.e., its immediate parent has already terminated) and marked as having a subreaper, the nearest still living ancestor subreaper will receive a SIGCHLD signal and be able to wait(2) on the process to discover its termination status.
Return the "child subreaper" setting of the caller, in the location pointed to by (int *) arg2.
Set the state of the flag determining whether core dumps are produced for the calling process upon delivery of a signal whose default behavior is to produce a core dump. (Normally, this flag is set for a process by default, but it is cleared when a set-user-ID or set-group-ID program is executed and also by various system calls that manipulate process UIDs and GIDs). In kernels up to and including 2.6.12, arg2 must be either 0 (process is not dumpable) or 1 (process is dumpable). Between kernels 2.6.13 and 2.6.17, the value 2 was also permitted, which caused any binary which normally would not be dumped to be dumped readable by root only; for security reasons, this feature has been removed. (See also the description of /proc/sys/fs/suid_dumpable in proc(5).) Processes that are not dumpable can not be attached via ptrace(2) PTRACE_ATTACH.
Return (as the function result) the current state of the calling process's dumpable flag.
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.
Set floating-point emulation control bits to arg2. Pass PR_FPEMU_NOPRINT to silently emulate fp operations accesses, or PR_FPEMU_SIGFPE to not emulate fp operations and send SIGFPE instead.
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.
Set the state of the thread's "keep capabilities" flag, which determines whether the threads's permitted capability set is cleared when a change is made to the threads's user IDs such that the threads's real UID, effective UID, and saved set-user-ID all become nonzero when at least one of them previously had the value 0. By default, the permitted capability set is cleared when such a change is made; setting the "keep capabilities" flag prevents it from being cleared. arg2 must be either 0 (permitted capabilities are cleared) or 1 (permitted capabilities are kept). (A thread's effective capability set is always cleared when such a credential change is made, regardless of the setting of the "keep capabilities" flag.) The "keep capabilities" value will be reset to 0 on subsequent calls to execve(2).
Return (as the function result) the current state of the calling threads's "keep capabilities" flag.
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, and should be null-terminated if it contains fewer bytes. This is the same attribute that can be set via pthread_setname_np(3) and retrieved using pthread_getname_np(3). The attribute is likewise accessible via /proc/self/task/[tid]/comm, where tid is the name of the calling thread.
Return the name of the calling thread, in the buffer pointed to by (char *) arg2. The buffer should allow space for up to 16 bytes; the returned string will be null-terminated if it is shorter than that.
Set the calling process's no_new_privs bit to the value in arg2. With no_new_privs set to 1, execve(2) promises not to grant privileges to do anything that could not have been done without the execve(2) call (for example, rendering the set-user-ID and set-group-ID permission bits, and file capabilities non-functional). Once set, this bit cannot be unset. The setting of this bit is inherited by children created by fork(2) and clone(2), and preserved across execve(2).

For more information, see the kernel source file Documentation/prctl/no_new_privs.txt.

Return the value of the no_new_privs bit for the current process. A value of 0 indicates the regular execve(2) behavior. A value of 1 indicates execve(2) will operate in the privilege-restricting mode described above.
Set the parent process death signal of the calling process to arg2 (either a signal value in the range 1..maxsig, or 0 to clear). This is the signal that the calling process will get when its parent dies. This value is cleared for the child of a fork(2) and (since Linux 2.4.36 / 2.6.23) when executing a set-user-ID or set-group-ID binary.
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/security/Yama.txt.

Set the secure computing (seccomp) mode for the calling thread, to limit the available system calls. The seccomp mode is selected via arg2. (The seccomp constants are defined in <linux/seccomp.h>.)

With arg2 set to SECCOMP_MODE_STRICT the only system calls that the thread is permitted to make are read(2), write(2), _exit(2), and sigreturn(2). Other system calls result in the delivery of a SIGKILL signal. Strict secure computing mode is useful for number-crunching applications that may need to execute untrusted byte code, perhaps obtained by reading from a pipe or socket. This operation is available only if the kernel is configured with CONFIG_SECCOMP enabled.

With arg2 set to SECCOMP_MODE_FILTER (since Linux 3.5) the system calls allowed are defined by a pointer to a Berkeley Packet Filter passed in arg3. This argument is a pointer to struct sock_fprog; it can be designed to filter arbitrary system calls and system call arguments. This mode is available only if the kernel is configured with CONFIG_SECCOMP_FILTER enabled.

If SECCOMP_MODE_FILTER filters permit fork(2), then the seccomp mode is inherited by children created by fork(2); if execve(2) is permitted, then the seccomp mode is preserved across execve(2). If the filters permit prctl() calls, then additional filters can be added; they are run in order until the first non-allow result is seen.

For further information, see the kernel source file Documentation/prctl/seccomp_filter.txt.

Return the secure computing mode of the calling thread. If the caller is not in secure computing mode, this operation returns 0; if the caller is in strict secure computing mode, then the prctl() call will cause a SIGKILL signal to be sent to the process. If the caller is in filter mode, and this system call is allowed by the seccomp filters, it returns 2. This operation is available only if the kernel is configured with CONFIG_SECCOMP enabled.
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).
Retrieve the clear_child_tid address set by set_tid_address(2) and the clone(2) CLONE_CHILD_CLEARTID flag, in the location pointed to by (int **) arg2. This feature is available only if the kernel is built with the CONFIG_CHECKPOINT_RESTORE option enabled.
Set the current timer slack for the calling thread to the nanosecond value supplied in arg2. If arg2 is less than or equal to zero, reset the current timer slack to the thread's default timer slack value. The timer slack is used by the 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 realtime scheduling policy (see sched_setscheduler(2)).

Each thread has two associated timer slack values: a "default" value, and a "current" value. The current value is the one that governs grouping of timer expirations. When a new thread is created, the two timer slack values are made the same as the current value of the creating thread. Thereafter, a thread can adjust its current timer slack value via PR_SET_TIMERSLACK (the default value can't be changed). The timer slack values of init (PID 1), the ancestor of all processes, are 50,000 nanoseconds (50 microseconds). The timer slack values are preserved across execve(2).

Return 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.
Disable all performance counters attached to the calling process, regardless of whether the counters were created by this process or another process. Performance counters created by the calling process for other processes are unaffected. For more information on performance counters, see the Linux kernel source file tools/perf/design.txt.
Originally called PR_TASK_PERF_COUNTERS_DISABLE; renamed (with same numerical value) in Linux 2.6.32.
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 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) 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.
(see PR_SET_UNALIGN for information on versions and architectures) Return unaligned access control bits, in the location pointed to by (int *) arg2.
Set the machine check memory corruption kill policy for the current thread. If arg2 is PR_MCE_KILL_CLEAR, clear the thread memory corruption kill policy and use the system-wide default. (The system-wide default is defined by /proc/sys/vm/memory_failure_early_kill; see proc(5).) If arg2 is PR_MCE_KILL_SET, use a thread-specific memory corruption kill policy. In this case, arg3 defines whether the policy is early kill (PR_MCE_KILL_EARLY), late kill (PR_MCE_KILL_LATE), or the system-wide default (PR_MCE_KILL_DEFAULT). Early kill means that the thread receives a SIGBUS signal as soon as hardware memory corruption is detected inside its address space. In late kill mode, the process is killed only when it accesses a corrupted page. See sigaction(2) for more information on the SIGBUS signal. The policy is inherited by children. The remaining unused prctl() arguments must be zero for future compatibility.
Return 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. This feature is available only if the kernel is built with the CONFIG_CHECKPOINT_RESTORE option enabled. The calling process must have the CAP_SYS_RESOURCE capability. The value in arg2 is one of the options below, while arg3 provides a new value for the option.
Set the address above which the program text can run. The corresponding memory area must be readable and executable, but not writable or sharable (see mprotect(2) and mmap(2) for more information).
Set the address below which the program text can run. The corresponding memory area must be readable and executable, but not writable or sharable.
Set the address above which initialized and uninitialized (bss) data are placed. The corresponding memory area must be readable and writable, but not executable or sharable.
Set the address below which initialized and uninitialized (bss) data are placed. The corresponding memory area must be readable and writable, but not executable or sharable.
Set the start address of the stack. The corresponding memory area must be readable and writable.
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)).
Set the current brk(2) value. The requirements for the address are the same as for the PR_SET_MM_START_BRK option.

RETURN VALUE

On success, PR_GET_DUMPABLE, PR_GET_KEEPCAPS, PR_GET_NO_NEW_PRIVS, PR_CAPBSET_READ, PR_GET_TIMING, PR_GET_SECUREBITS, PR_MCE_KILL_GET, and (if it returns) PR_GET_SECCOMP return the nonnegative values described above. All other option values return 0 on success. On error, -1 is returned, and errno is set appropriately.

ERRORS

arg2 is an invalid address.
The value of option is not recognized.
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_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.
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_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_KEEPCAPS, and the callers's SECURE_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.

VERSIONS

The prctl() system call was introduced in Linux 2.1.57.

CONFORMING TO

This call is Linux-specific. IRIX has a prctl() system call (also introduced in Linux 2.1.44 as irix_prctl on the MIPS architecture), with prototype

ptrdiff_t prctl(int option, int arg2, int arg3);

and options to get the maximum number of processes per user, get the maximum number of processors the calling process can use, find out whether a specified process is currently blocked, get or set the maximum stack size, and so on.

SEE ALSO

signal(2), core(5)

COLOPHON

This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

2013-05-21 Linux