prctl

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:

       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.

SEE ALSO
       signal(2), core(5)

COLOPHON
       This page is part of release 5.03 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-08-02                          PRCTL(2)