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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_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, 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 children 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 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
              location pointed to by (int *) arg2.

       PR_SET_DUMPABLE (since Linux 2.3.20)
              Set the state of the flag determining  whether  core  dumps  are
              produced for the calling process upon delivery of a signal whose
              default behavior is to produce a  core  dump.   (Normally,  this
              flag  is  set for a process by default, but it is cleared when a
              set-user-ID or set-group-ID program  is  executed  and  also  by
              various system calls that manipulate process UIDs and GIDs).  In
              kernels up to and  including  2.6.12,  arg2  must  be  either  0
              (process  is  not dumpable) or 1 (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).)  Processes that are  not
              dumpable can not be attached via ptrace(2) PTRACE_ATTACH.

       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_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 fp operations accesses, or
              PR_FPEMU_SIGFPE to not emulate fp  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 thread's "keep  capabilities"  flag,  which
              determines  whether  the  threads's  permitted capability set is
              cleared when a change is made to the  threads's  user  IDs  such
              that  the threads'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 (permitted 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
              threads's "keep capabilities" flag.

       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, and should be null-terminated if it  contains  fewer
              bytes.   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 if it is
              shorter than that.

       PR_SET_NO_NEW_PRIVS (since Linux 3.5)
              Set the calling process'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  permission  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).

              For    more    information,   see   the   kernel   source   file
              Documentation/prctl/no_new_privs.txt.

       PR_GET_NO_NEW_PRIVS (since Linux 3.5)
              Return the  value  of  the  no_new_privs  bit  for  the  current
              process.  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  process 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.

       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/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  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),
              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/prctl/seccomp_filter.txt.

       PR_GET_SECCOMP (since Linux 2.6.23)
              Return 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.  This operation
              is  available  only   if   the   kernel   is   configured   with
              CONFIG_SECCOMP enabled.

       PR_SET_SECUREBITS (since Linux 2.6.26)
              Set  the  "securebits"  flags of the calling thread to the value
              supplied in arg2.  See capabilities(7).

       PR_GET_SECUREBITS (since Linux 2.6.26)
              Return (as the function result) the "securebits"  flags  of  the
              calling thread.  See capabilities(7).

       PR_GET_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.

       PR_SET_TIMERSLACK (since Linux 2.6.28)
              Set  the  current  timer  slack  for  the  calling thread to the
              nanosecond value supplied in arg2.  If  arg2  is  less  than  or
              equal  to  zero,  reset  the current timer slack to the thread's
              default timer slack value.  The  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
              realtime scheduling policy (see sched_setscheduler(2)).

              Each  thread  has two associated timer slack values: a "default"
              value, and a "current" value.  The current value is the one that
              governs  grouping  of  timer  expirations.  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).

       PR_GET_TIMERSLACK (since Linux 2.6.28)
              Return 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_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_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)  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.

       PR_GET_UNALIGN
              (see    PR_SET_UNALIGN   for   information   on   versions   and
              architectures) Return unaligned  access  control  bits,  in  the
              location pointed to by (int *) arg2.

       PR_MCE_KILL (since Linux 2.6.32)
              Set  the  machine  check  memory  corruption kill policy for the
              current 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.   This  feature  is
              available    only    if   the   kernel   is   built   with   the
              CONFIG_CHECKPOINT_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
                     executable, 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
                     executable, 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
                     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.

RETURN VALUE

       On  success,  PR_GET_DUMPABLE,  PR_GET_KEEPCAPS,   PR_GET_NO_NEW_PRIVS,
       PR_CAPBSET_READ, PR_GET_TIMING, PR_GET_SECUREBITS, PR_MCE_KILL_GET, 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

       EFAULT arg2 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_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.

       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     callers'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.

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 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.