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



NAME
       prctl - operations on a process

SYNOPSIS
       #include <sys/prctl.h>

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

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

	      When a process is marked as a child subreaper, all of the	 chil-
	      dren  that  it creates, and their descendants, will be marked as
	      having a subreaper.  In effect, a subreaper fulfills the role of
	      init(1)  for  its	 descendant  processes.	 Upon termination of a
	      process that is orphaned (i.e., its immediate parent has already
	      terminated)  and marked as having a subreaper, the nearest still
	      living ancestor subreaper will receive a SIGCHLD signal and will
	      be  able	to  wait(2) on the process to discover its termination
	      status.

       PR_GET_CHILD_SUBREAPER (since Linux 3.4)
	      Return the "child subreaper" setting of the caller, in the loca-
	      tion 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  circum-
	      stances:

	      *	 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 restric-
	      tions  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  con-
		     taining  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 regis-
		     ters 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 signifi-
		     cant 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 oper-
	      ations 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_NONRE-
	      COV 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,
	      which determines whether the thread's permitted  capability  set
	      is  cleared  when a change is made to the thread's user IDs such
	      that the thread's real UID, effective UID, and saved set-user-ID
	      all  become nonzero when at least one of them previously had the
	      value 0.	By default, the permitted capability  set  is  cleared
	      when such a change is made; setting the "keep capabilities" flag
	      prevents it from being cleared.  arg2 must be either 0  (permit-
	      ted  capabilities	 are cleared) or 1 (permitted capabilities are
	      kept).  (A thread's effective capability set is  always  cleared
	      when such a credential change is made, regardless of the setting
	      of the "keep capabilities" flag.)	 The "keep capabilities" value
	      will be reset to 0 on subsequent calls to execve(2).

       PR_GET_KEEPCAPS (since Linux 2.2.18)
	      Return (as the function result) the current state of the calling
	      thread's "keep capabilities" 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_fail-
	      ure_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 sys-
	      tem-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  sig-
	      nal.  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 call-
	      ing process.  Usually these fields are set  by  the  kernel  and
	      dynamic loader (see ld.so(8) for more information) and a regular
	      application should not use this  feature.	  However,  there  are
	      cases,  such  as	self-modifying programs, where a program might
	      find it useful to change its own memory map.   This  feature  is
	      available	 only  if  the	kernel is built with the CONFIG_CHECK-
	      POINT_RESTORE option enabled.  The calling process must have the
	      CAP_SYS_RESOURCE	capability.   The  value in arg2 is one of the
	      options below, while arg3 provides a new value for the option.

	      PR_SET_MM_START_CODE
		     Set the address above which the  program  text  can  run.
		     The  corresponding	 memory area must be readable and exe-
		     cutable, but not writable or  sharable  (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  exe-
		     cutable, but not writable or sharable.

	      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
		     sharable.

	      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
		     sharable.

	      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  seg-
		     ment.   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  descrip-
		     tor 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 cre-
		     ates 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.

       PR_MPX_ENABLE_MANAGEMENT, PR_MPX_DISABLE_MANAGEMENT (since Linux 3.19)
	      Enable  or disable kernel management of Memory Protection eXten-
	      sions (MPX) bounds tables.  The arg2, arg3, arg4, and arg5 argu-
	      ments 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	 allo-
	      cates  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 allo-
	      cated, 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	direc-
	      tory in the bndcfgu register.

	      These calls will 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 loca-
	      tion 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 like-
	      wise 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 pre-
	      served 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  Documenta-
	      tion/prctl/no_new_privs.txt.  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 oper-
	      ate 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 capabil-
	      ities (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/ker-
	      nel/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 Documenta-
	      tion/security/Yama.txt.

       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 com-
	      puting 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 sys-
	      tem 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 Documenta-
	      tion/prctl/seccomp_filter.txt.

       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 com-
	      puting  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, with-
	      out 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 modi-
	      fied, and using a malloc hook with madvise(2) is not  an	option
	      (i.e., statically allocated data).  The setting of the "THP dis-
	      able" 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	 (with
	      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, includ-
	      ing    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 pro-
	      cesses, 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]/timer-
	      slack_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_TIM-
	      ING_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	 addi-
	      tional  flag with the value of 4 and no corresponding named con-
	      stant, 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  architec-
	      tures)  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_AMBI-
       ENT_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  sys-
	      tem 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 correspond-
		 ing 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	speci-
		 fies  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 num-
	      ber.

       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
	      SECURE_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	 maxi-
       mum stack size, and so on.

SEE ALSO
       signal(2), core(5)

COLOPHON
       This  page  is  part of release 4.10 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				  2016-12-12			      PRCTL(2)