prctl


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_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 capa-
              bility  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 termi-
              nated) and marked as having a subreaper, the nearest still  liv-
              ing 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 loca-
              tion 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

       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_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 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 capabili-
              ties" flag prevents it from being cleared.  arg2 must be  either
              0 (permitted capabilities are cleared) or 1 (permitted capabili-
              ties 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  capabili-
              ties" value will be reset to 0 on subsequent calls to execve(2).
              /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-func-
              tional).   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 Documenta-
              tion/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/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.
              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 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_SEC-
              COMP 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  ker-
              nel  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
              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_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_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_MCE_KILL (since Linux 2.6.32)
              Set the machine check memory corruption kill policy for the cur-
              rent  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
                     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 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.

       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
       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 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/.



Linux                             2013-05-21                          PRCTL(2)
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