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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.

       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.

       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.

       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  this  bit  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)
              Get  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 if 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  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.

              Since Linux 3.10, this feature is available all the time.  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).

                     The  second  limitation  is that such transitions can be done only once in a
                     process life time.  Any further attempts will be rejected.  This should help
                     system  administrators  monitor  unusual  symbolic-link transitions over all
                     processes running on a system.

              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 bit to the value in arg2.  With no_new_privs
              set to 1, execve(2) promises not to grant privileges to do anything that could  not
              have  been  done without the execve(2) call (for example, rendering the set-user-ID
              and set-group-ID mode bits, and file capabilities non-functional).  Once set,  this
              bit  cannot  be unset.  The setting of this bit is inherited by children created by
              fork(2) and clone(2), and preserved across execve(2).

              Since Linux 4.10, the value of a thread's no_new_privs bit 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 bit 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.  This value is cleared for the child of a
              fork(2) and (since Linux 2.4.36 / 2.6.23) when  executing  a  set-user-ID  or  set-
              group-ID   binary,   or   a   binary   that   has   associated   capabilities  (see
              capabilities(7)).  This value is preserved across execve(2).

              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.

       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_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  (via 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)
              Retrieve the clear_child_tid address set by  set_tid_address(2)  and  the  clone(2)
              CLONE_CHILD_CLEARTID  flag,  in  the  location  pointed  to by (int **) arg2.  This
              feature is available only if the kernel is built with the CONFIG_CHECKPOINT_RESTORE
              option  enabled.   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.  If the nanosecond value supplied in arg2  is  greater  than  zero,
              then  the  "current"  value is set to this value.  If arg2 is less than or 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 values are 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-test4)
              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-test4)
              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_KEEPCAPS,  PR_GET_NO_NEW_PRIVS,  PR_GET_THP_DISABLE,
       PR_CAPBSET_READ,  PR_GET_TIMING,  PR_GET_TIMERSLACK,  PR_GET_SECUREBITS,  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.

       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.

       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_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.

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 4.16 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/.