prctl(2) — Linux manual page

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

NAME         top

       prctl - operations on a process or thread

SYNOPSIS         top

       #include <sys/prctl.h>

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

DESCRIPTION         top

       prctl() manipulates various aspects of the behavior of the
       calling thread or process.

       Note that careless use of some prctl() operations can confuse the
       user-space run-time environment, so these operations should be
       used with care.

       prctl() is called with a first argument describing what to do
       (with values defined in <linux/prctl.h>), and further arguments
       with a 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(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.

       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" 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:

              *  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 attribute.

       PR_SET_ENDIAN (since Linux 2.6.18, PowerPC only)
              Set the endian-ness of the calling process to the value
              given in arg2, which should be one of the following:
              PR_ENDIAN_BIG, PR_ENDIAN_LITTLE, or PR_ENDIAN_PPC_LITTLE
              (PowerPC pseudo little endian).

       PR_GET_ENDIAN (since Linux 2.6.18, PowerPC only)
              Return the endian-ness of the calling process, in the
              location pointed to by (int *) arg2.

       PR_SET_FP_MODE (since Linux 4.0, only on MIPS)
              On the MIPS architecture, user-space code can be built
              using an ABI which permits linking with code that has more
              restrictive floating-point (FP) requirements.  For
              example, user-space code may be built to target the O32
              FPXX ABI and linked with code built for either one of the
              more restrictive FP32 or FP64 ABIs.  When more restrictive
              code is linked in, the overall requirement for the process
              is to use the more restrictive floating-point mode.

              Because the kernel has no means of knowing in advance
              which mode the process should be executed in, and because
              these 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_IO_FLUSHER (since Linux 5.6)
              If a user process is involved in the block layer or
              filesystem I/O path, and can allocate memory while
              processing I/O requests it must set arg2 to 1.  This will
              put the process in the IO_FLUSHER state, which allows it
              special treatment to make progress when allocating memory.
              If arg2 is 0, the process will clear the IO_FLUSHER state,
              and the default behavior will be used.

              The calling process must have the CAP_SYS_RESOURCE
              capability.

              arg3, arg4, and arg5 must be zero.

              The IO_FLUSHER state is inherited by a child process
              created via fork(2) and is preserved across execve(2).

              Examples of IO_FLUSHER applications are FUSE daemons, SCSI
              device emulation daemons, and daemons that perform error
              handling like multipath path recovery applications.

       PR_GET_IO_FLUSHER (Since Linux 5.6)
              Return (as the function result) the IO_FLUSHER state of
              the caller.  A value of 1 indicates that the caller is in
              the IO_FLUSHER state; 0 indicates that the caller is not
              in the IO_FLUSHER state.

              The calling process must have the CAP_SYS_RESOURCE
              capability.

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

       PR_SET_KEEPCAPS (since Linux 2.2.18)
              Set the state of the calling thread's "keep capabilities"
              flag.  The effect of this flag is described in
              capabilities(7).  arg2 must be either 0 (clear the flag)
              or 1 (set the flag).  The "keep capabilities" value will
              be reset to 0 on subsequent calls to execve(2).

       PR_GET_KEEPCAPS (since Linux 2.2.18)
              Return (as the function result) the current state of the
              calling thread's "keep capabilities" flag.  See
              capabilities(7) for a description of this flag.

       PR_MCE_KILL (since Linux 2.6.32)
              Set the machine check memory corruption kill policy for
              the calling thread.  If arg2 is PR_MCE_KILL_CLEAR, clear
              the thread memory corruption kill policy and use the
              system-wide default.  (The system-wide default is defined
              by /proc/sys/vm/memory_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, removed in Linux 5.4; only on x86)
              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.

              Due to a lack of toolchain support,
              PR_MPX_ENABLE_MANAGEMENT and PR_MPX_DISABLE_MANAGEMENT are
              not supported in Linux 5.4 and later.

       PR_SET_NAME (since Linux 2.6.9)
              Set the name of the calling thread, using the value in the
              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).

       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,
              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_PAC_RESET_KEYS (since Linux 5.0, only on arm64)
              Securely reset the thread's pointer authentication keys to
              fresh random values generated by the kernel.

              The set of keys to be reset is specified by arg2, which
              must be a logical OR of zero or more of the following:

              PR_PAC_APIAKEY
                     instruction authentication key A

              PR_PAC_APIBKEY
                     instruction authentication key B

              PR_PAC_APDAKEY
                     data authentication key A

              PR_PAC_APDBKEY
                     data authentication key B

              PR_PAC_APGAKEY
                     generic authentication “A” key.

                     (Yes folks, there really is no generic B key.)

              As a special case, if arg2 is zero, then all the keys are
              reset.  Since new keys could be added in future, this is
              the recommended way to completely wipe the existing keys
              when establishing a clean execution context.  Note that
              there is no need to use PR_PAC_RESET_KEYS in preparation
              for calling execve(2), since execve(2) resets all the
              pointer authentication keys.

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

              If the arguments are invalid, and in particular if arg2
              contains set bits that are unrecognized or that correspond
              to a key not available on this platform, then the call
              fails with error 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).

       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..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.

       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.

              PR_SPEC_DISABLE_NOEXEC (since Linux 5.1)
                     Same as PR_SPEC_DISABLE, but the state will be
                     cleared on execve(2).

              If all bits are 0, then the CPU is not affected by the
              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.  The speculation-misfeature settings are per-thread
              attributes.

              Currently, arg2 must be one of:

              PR_SPEC_STORE_BYPASS
                     Set the state of the speculative store bypass
                     misfeature.

              PR_SPEC_INDIRECT_BRANCH (since Linux 4.20)
                     Set the state of the indirect branch speculation
                     misfeature.

              If arg2 does not have one of the above values, then the
              call fails with the error ENODEV.

              The arg3 argument is used to hand in the control value,
              which is one of the following:

              PR_SPEC_ENABLE
                     The speculation feature is enabled, mitigation is
                     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(arg2, PR_SPEC_ENABLE) with the
                     same value for arg2 will fail with the error EPERM.

              PR_SPEC_DISABLE_NOEXEC (since Linux 5.1)
                     Same as PR_SPEC_DISABLE, but the state will be
                     cleared on execve(2).  Currently only supported for
                     arg2 equal to PR_SPEC_STORE_BYPASS.

              Any unsupported value in arg3 will result in the call
              failing with the error ERANGE.

              The arg4 and arg5 arguments must be specified as 0;
              otherwise the call fails with the error EINVAL.

              The speculation feature can also be controlled by the
              spec_store_bypass_disable boot parameter.  This parameter
              may enforce a read-only policy which will result in the
              prctl() call failing with the error ENXIO.  For further
              details, see the kernel source file
              Documentation/admin-guide/kernel-parameters.txt.

       PR_SVE_SET_VL (since Linux 4.15, only on arm64)
              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:

              0      Perform the change immediately.  At the next
                     execve(2) in the thread, the vector length will be
                     reset to the value configured in
                     /proc/sys/abi/sve_default_vector_length.

              PR_SVE_VL_INHERIT
                     Perform the change immediately.  Subsequent
                     execve(2) calls will preserve the new vector
                     length.

              PR_SVE_SET_VL_ONEXEC
                     Defer the change, so that it is performed at the
                     next execve(2) in the thread.  Further execve(2)
                     calls will reset the vector length to the value
                     configured in
                     /proc/sys/abi/sve_default_vector_length.

              PR_SVE_SET_VL_ONEXEC | PR_SVE_VL_INHERIT
                     Defer the change, so that it is performed at the
                     next execve(2) in the thread.  Further execve(2)
                     calls will preserve the new vector length.

              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.

       PR_SVE_GET_VL (since Linux 4.15, only on arm64)
              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).

       PR_SET_SYSCALL_USER_DISPATCH (since Linux 5.11, x86 only)
              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:

              *  si_signo will contain SIGSYS.

              *  si_call_addr will show the address of the system call
                 instruction.

              *  si_syscall and si_arch will indicate which system call
                 was attempted.

              *  si_code will contain SYS_USER_DISPATCH.

              *  si_errno will be set to 0.

              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

       PR_SET_TAGGED_ADDR_CTRL (since Linux 5.4, only on arm64)
              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:

              0      Addresses that are passed for the purpose of being
                     dereferenced by the kernel must be untagged.

              PR_TAGGED_ADDR_ENABLE
                     Addresses that are passed for the purpose of being
                     dereferenced by the kernel may be tagged, with the
                     exceptions summarized below.

              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:

              • brk(2), mmap(2), shmat(2), shmdt(2), and the new_address
                argument of mremap(2).

                (Prior to Linux 5.6 these accepted tagged addresses, but
                the behaviour may not be what you expect.  Don't rely on
                it.)

              • ‘polymorphic’ interfaces that accept pointers to
                arbitrary types cast to a void * or other generic type,
                specifically prctl(), ioctl(2), and in general
                setsockopt(2) (only certain specific setsockopt(2)
                options allow tagged addresses).

              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.

       PR_GET_TAGGED_ADDR_CTRL (since Linux 5.4, only on arm64)
              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.

       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_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_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         top

       On success, PR_CAP_AMBIENT+PR_CAP_AMBIENT_IS_SET,
       PR_CAPBSET_READ, PR_GET_DUMPABLE, PR_GET_FP_MODE,
       PR_GET_IO_FLUSHER, PR_GET_KEEPCAPS, PR_MCE_KILL_GET,
       PR_GET_NO_NEW_PRIVS, PR_GET_SECUREBITS, PR_GET_SPECULATION_CTRL,
       PR_SVE_GET_VL, PR_SVE_SET_VL, PR_GET_TAGGED_ADDR_CTRL,
       PR_GET_THP_DISABLE, PR_GET_TIMING, PR_GET_TIMERSLACK, and (if it
       returns) PR_GET_SECCOMP return the nonnegative values described
       above.  All other option values return 0 on success.  On error,
       -1 is returned, and errno is set to indicate the error.

ERRORS         top

       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.

       EFAULT option is PR_SET_SYSCALL_USER_DISPATCH and arg5 has an
              invalid address.

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

       EINVAL option is PR_MCE_KILL or PR_MCE_KILL_GET or PR_SET_MM, and
              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.  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.

       EINVAL 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.

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

       EINVAL option is PR_SET_SYSCALL_USER_DISPATCH and one of the
              following is true:

              *  arg2 is PR_SYS_DISPATCH_OFF and the remaining arguments
                 are not 0;

              *  arg2 is PR_SYS_DISPATCH_ON and the memory range
                 specified is outside the address space of the process.

              *  arg2 is invalid.

       EINVAL 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.

       EINVAL 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.

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

       ENXIO  option was PR_MPX_ENABLE_MANAGEMENT or
              PR_MPX_DISABLE_MANAGEMENT and the kernel or the CPU does
              not support MPX management.  Check that the kernel and
              processor have MPX support.

       ENXIO  option was PR_SET_SPECULATION_CTRL implies that the
              control of the selected speculation misfeature is not
              possible.  See PR_GET_SPECULATION_CTRL for the bit fields
              to determine which option is available.

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

       EPERM  option is PR_SET_SECUREBITS, and the caller does not have
              the CAP_SETPCAP capability, or tried to unset a "locked"
              flag, or tried to set a flag whose corresponding locked
              flag was set (see capabilities(7)).

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

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

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

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

       EPERM  option is PR_CAP_AMBIENT and arg2 is PR_CAP_AMBIENT_RAISE,
              but either the capability specified in arg3 is not present
              in the process's permitted and inheritable capability
              sets, or the PR_CAP_AMBIENT_LOWER securebit has been set.

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

VERSIONS         top

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

CONFORMING TO         top

       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         top

       signal(2), core(5)

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                       PRCTL(2)

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