prctl(2)



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 am-
                     bient 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/ca-
              pability.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 pro-
              cesses.   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 sta-
              tus.

              The setting of this bit is not inherited by children created  by
              fork(2)  and  clone(2).   The  setting  is  preserved across ex-
              ecve(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 (per-
              haps so that it can restart that process).  Some init(1)  frame-
              works  (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 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 bi-
              nary 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  ca-
                 pabilities  (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 be-
                     low) can operate only when this bit is set  (FR=1).   Ap-
                     plications that use the O32 FPXX ABI can operate with ei-
                     ther 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.
              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_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 dy-
              namic 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  pro-
              vides  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.   Be-
              fore 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 exe-
                     cutable, 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  exe-
                     cutable, 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  ex-
                     panded  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  op-
                     tion.

              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 ex-
                     isting 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  at-
                     tempts  will be rejected.  This should help system admin-
                     istrators 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  un-
                     signed 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  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 en-
              forcement.  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  lo-
              cation  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 direc-
              tory 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 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  at-
              tribute  that can be set via pthread_setname_np(3) and retrieved
              using pthread_getname_np(3).  The attribute is likewise accessi-
              ble 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/userspace-api/no_new_privs.rst        (or        Documenta-
              tion/prctl/no_new_privs.txt  before  Linux 4.13).  See also sec-
              comp(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 (ei-
              ther 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 ex-
              ecve(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 an-
              cestor.  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/admin-guide/LSM/Yama.rst       (or      Documentation/secu-
              rity/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 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  ex-
              ecve(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/userspace-api/seccomp_filter.rst       (or       Documenta-
              tion/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 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 (re-
              taining 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 "de-
              fault" 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 an-
              other; 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  sys-
              tem 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 un-
              used 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
              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 in-
       troduced 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 4.16 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/.

Linux                             2018-02-02                          PRCTL(2)

Man(1) output converted with man2html
list of all man pages