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NAME

       prctl - operations on a process

SYNOPSIS

       #include <sys/prctl.h>

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

DESCRIPTION

       prctl()  is  called  with  a  first argument describing what to do (with values defined in
       <linux/prctl.h>), and further arguments with a significance depending on  the  first  one.
       The first argument can be:

       PR_CAP_AMBIENT (since Linux 4.3)
              Reads or changes the ambient capability set of the calling thread, according to the
              value of arg2, which must be one of the following:

              PR_CAP_AMBIENT_RAISE
                     The capability specified in arg3 is added to the ambient set.  The specified
                     capability must already be present in both the permitted and the inheritable
                     sets  of  the  process.   This   operation   is   not   permitted   if   the
                     SECBIT_NO_CAP_AMBIENT_RAISE securebit is set.

              PR_CAP_AMBIENT_LOWER
                     The capability specified in arg3 is removed from the ambient set.

              PR_CAP_AMBIENT_IS_SET
                     The  prctl()  call returns 1 if the capability in arg3 is in the ambient set
                     and 0 if it is not.

              PR_CAP_AMBIENT_CLEAR_ALL
                     All capabilities will be removed  from  the  ambient  set.   This  operation
                     requires setting arg3 to zero.

              In all of the above operations, arg4 and arg5 must be specified as 0.

              Higher-level  interfaces layered on top of the above operations are provided in the
              libcap(3) library  in  the  form  of  cap_get_ambient(3),  cap_set_ambient(3),  and
              cap_reset_ambient(3).

       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.

              A higher-level interface layered on top  of  this  operation  is  provided  in  the
              libcap(3) library in the form of cap_get_bound(3).

       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.

              A  higher-level  interface  layered  on  top  of  this operation is provided in the
              libcap(3) library in the form of cap_drop_bound(3).

       PR_SET_CHILD_SUBREAPER (since Linux 3.4)
              If arg2 is nonzero, set the "child subreaper" attribute of the calling process;  if
              arg2 is zero, unset the attribute.

              A  subreaper  fulfills  the  role  of init(1) for its descendant processes.  When a
              process becomes orphaned (i.e., its immediate parent terminates) then that  process
              will  be  reparented to the nearest still living ancestor subreaper.  Subsequently,
              calls to getppid() in the orphaned process will now return the PID of the subreaper
              process,  and  when  the  orphan  terminates, it is the subreaper process that will
              receive a SIGCHLD signal and will be able to wait(2) on the process to discover its
              termination status.

              The setting of the "child subreaper" attribute is not inherited by children created
              by fork(2) and clone(2).  The setting is preserved across execve(2).

              Establishing a subreaper process is useful in session management frameworks where a
              hierarchical  group of processes is managed by a subreaper process that needs to be
              informed when one of the processes—for example, a  double-forked  daemon—terminates
              (perhaps  so  that  it  can  restart that process).  Some init(1) frameworks (e.g.,
              systemd(1)) employ a subreaper process for similar reasons.

       PR_GET_CHILD_SUBREAPER (since Linux 3.4)
              Return the "child subreaper" setting of the caller, in the location pointed  to  by
              (int *) arg2.

       PR_SET_DUMPABLE (since Linux 2.3.20)
              Set  the  state  of  the  "dumpable"  flag, which determines whether core dumps are
              produced for the calling process upon delivery of a signal whose  default  behavior
              is to produce a core dump.

              In  kernels  up  to and including 2.6.12, arg2 must be either 0 (SUID_DUMP_DISABLE,
              process is not dumpable) or  1  (SUID_DUMP_USER,  process  is  dumpable).   Between
              kernels  2.6.13 and 2.6.17, the value 2 was also permitted, which caused any binary
              which normally would not be dumped to be dumped readable by root only; for security
              reasons, this feature has been removed.  (See also the description of /proc/sys/fs/
              suid_dumpable in proc(5).)

              Normally, this flag is set to 1.   However,  it  is  reset  to  the  current  value
              contained  in  the  file /proc/sys/fs/suid_dumpable (which by default has the value
              0), in the following circumstances:

              *  The process's effective user or group ID is changed.

              *  The process's filesystem user or group ID is changed (see credentials(7)).

              *  The  process  executes  (execve(2))  a  set-user-ID  or  set-group-ID   program,
                 resulting in a change of either the effective user ID or the effective group ID.

              *  The  process  executes  (execve(2))  a  program  that has file capabilities (see
                 capabilities(7)), but only if the permitted  capabilities  gained  exceed  those
                 already permitted for the process.

              Processes  that  are  not dumpable can not be attached via ptrace(2) PTRACE_ATTACH;
              see ptrace(2) for further details.

              If a process is not dumpable, the ownership of files in the  process's  /proc/[pid]
              directory is affected as described in proc(5).

       PR_GET_DUMPABLE (since Linux 2.3.20)
              Return (as the function result) the current state of the calling process's dumpable
              flag.

       PR_SET_ENDIAN (since Linux 2.6.18, PowerPC only)
              Set the endian-ness of the calling process to the value given in arg2, which should
              be  one  of the following: PR_ENDIAN_BIG, PR_ENDIAN_LITTLE, or PR_ENDIAN_PPC_LITTLE
              (PowerPC pseudo little endian).

       PR_GET_ENDIAN (since Linux 2.6.18, PowerPC only)
              Return the endian-ness of the calling  process,  in  the  location  pointed  to  by
              (int *) arg2.

       PR_SET_FP_MODE (since Linux 4.0, only on MIPS)
              On  the  MIPS architecture, user-space code can be built using an ABI which permits
              linking with code that has more restrictive floating-point (FP) requirements.   For
              example,  user-space  code  may be built to target the O32 FPXX ABI and linked with
              code built for either one of the more restrictive FP32 or  FP64  ABIs.   When  more
              restrictive  code  is  linked in, the overall requirement for the process is to use
              the more restrictive floating-point mode.

              Because the kernel has no means of knowing in advance which mode the process should
              be  executed in, and because these restrictions can change over the lifetime of the
              process, the PR_SET_FP_MODE operation is provided to allow control of the floating-
              point mode from user space.

              The  (unsigned  int) arg2 argument is a bit mask describing the floating-point mode
              used:

              PR_FP_MODE_FR
                     When this bit is unset (so called FR=0 or FR0 mode), the  32  floating-point
                     registers  are  32 bits wide, and 64-bit registers are represented as a pair
                     of registers (even- and  odd-  numbered,  with  the  even-numbered  register
                     containing  the  lower 32 bits, and the odd-numbered register containing the
                     higher 32 bits).

                     When this  bit  is  set  (on  supported  hardware),  the  32  floating-point
                     registers  are  64 bits wide (so called FR=1 or FR1 mode).  Note that modern
                     MIPS implementations (MIPS R6 and newer) support FR=1 mode only.

                     Applications that use the O32 FP32 ABI can operate only  when  this  bit  is
                     unset (FR=0; or they can be used with FRE enabled, see below).  Applications
                     that use the O32 FP64 ABI (and the O32 FP64A ABI, which  exists  to  provide
                     the  ability to operate with existing FP32 code; see below) can operate only
                     when this bit is set (FR=1).  Applications that use the  O32  FPXX  ABI  can
                     operate with either FR=0 or FR=1.

              PR_FP_MODE_FRE
                     Enable  emulation of 32-bit floating-point mode.  When this mode is enabled,
                     it  emulates  32-bit  floating-point  operations  by  raising  a   reserved-
                     instruction  exception on every instruction that uses 32-bit formats and the
                     kernel then handles the instruction in software.  (The problem lies  in  the
                     discrepancy of handling odd-numbered registers which are the high 32 bits of
                     64-bit registers with even numbers in FR=0 mode and the lower  32-bit  parts
                     of  odd-numbered  64-bit  registers  in  FR=1  mode.)   Enabling this bit is
                     necessary when code with the O32 FP32 ABI  should  operate  with  code  with
                     compatible  the  O32 FPXX or O32 FP64A ABIs (which require FR=1 FPU mode) or
                     when it is executed on newer hardware (MIPS R6  onwards)  which  lacks  FR=0
                     mode support when a binary with the FP32 ABI is used.

                     Note that this mode makes sense only when the FPU is in 64-bit mode (FR=1).

                     Note  that the use of emulation inherently has a significant performance hit
                     and should be avoided if possible.

              In the N32/N64 ABI, 64-bit floating-point mode is always used, so FPU emulation  is
              not required and the FPU always operates in FR=1 mode.

              This option is mainly intended for use by the dynamic linker (ld.so(8)).

              The arguments arg3, arg4, and arg5 are ignored.

       PR_GET_FP_MODE (since Linux 4.0, only on MIPS)
              Get  the  current  floating-point  mode  (see the description of PR_SET_FP_MODE for
              details).

              On success, the call returns a bit mask which represents the current floating-point
              mode.

              The arguments arg2, arg3, arg4, and arg5 are ignored.

       PR_SET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
              Set  floating-point  emulation  control  bits  to  arg2.   Pass PR_FPEMU_NOPRINT to
              silently emulate floating-point  operation  accesses,  or  PR_FPEMU_SIGFPE  to  not
              emulate floating-point operations and send SIGFPE instead.

       PR_GET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
              Return floating-point emulation control bits, in the location pointed to by (int *)
              arg2.

       PR_SET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
              Set floating-point exception mode to arg2.  Pass PR_FP_EXC_SW_ENABLE to  use  FPEXC
              for  FP  exception  enables,  PR_FP_EXC_DIV  for  floating-point  divide  by  zero,
              PR_FP_EXC_OVF  for  floating-point  overflow,  PR_FP_EXC_UND   for   floating-point
              underflow,  PR_FP_EXC_RES  for  floating-point  inexact  result,  PR_FP_EXC_INV for
              floating-point invalid operation, PR_FP_EXC_DISABLED for  FP  exceptions  disabled,
              PR_FP_EXC_NONRECOV  for  async  nonrecoverable  exception mode, PR_FP_EXC_ASYNC for
              async recoverable exception mode, PR_FP_EXC_PRECISE for precise exception mode.

       PR_GET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
              Return floating-point exception mode, in the location pointed to by (int *) arg2.

       PR_SET_KEEPCAPS (since Linux 2.2.18)
              Set the state of the calling thread's "keep capabilities" flag.  The effect of this
              flag  is described in capabilities(7).  arg2 must be either 0 (clear the flag) or 1
              (set the flag).  The "keep capabilities" value will be reset  to  0  on  subsequent
              calls to execve(2).

       PR_GET_KEEPCAPS (since Linux 2.2.18)
              Return  (as  the  function  result) the current state of the calling thread's "keep
              capabilities" flag.  See capabilities(7) for a description of this flag.

       PR_MCE_KILL (since Linux 2.6.32)
              Set the machine check memory corruption kill policy for  the  calling  thread.   If
              arg2  is  PR_MCE_KILL_CLEAR, clear the thread memory corruption kill policy and use
              the   system-wide   default.    (The   system-wide   default    is    defined    by
              /proc/sys/vm/memory_failure_early_kill;  see proc(5).)  If arg2 is PR_MCE_KILL_SET,
              use a thread-specific memory corruption kill policy.  In this  case,  arg3  defines
              whether the policy is early kill (PR_MCE_KILL_EARLY), late kill (PR_MCE_KILL_LATE),
              or the system-wide default (PR_MCE_KILL_DEFAULT).  Early kill means that the thread
              receives  a  SIGBUS signal as soon as hardware memory corruption is detected inside
              its address space.  In late kill mode, the process is killed only when it  accesses
              a corrupted page.  See sigaction(2) for more information on the SIGBUS signal.  The
              policy is inherited by children.  The remaining unused prctl()  arguments  must  be
              zero for future compatibility.

       PR_MCE_KILL_GET (since Linux 2.6.32)
              Return  the  current  per-process  machine  check  kill policy.  All unused prctl()
              arguments must be zero.

       PR_SET_MM (since Linux 3.3)
              Modify certain kernel memory map descriptor fields of the calling process.  Usually
              these  fields  are  set  by  the  kernel  and dynamic loader (see ld.so(8) for more
              information) and a regular application should not use this feature.  However, there
              are cases, such as self-modifying programs, where a program might find it useful to
              change its own memory map.

              The calling process must have the CAP_SYS_RESOURCE capability.  The value  in  arg2
              is  one  of the options below, while arg3 provides a new value for the option.  The
              arg4 and arg5 arguments must be zero if unused.

              Before Linux 3.10, this feature is available only if the kernel is built  with  the
              CONFIG_CHECKPOINT_RESTORE option enabled.

              PR_SET_MM_START_CODE
                     Set  the  address  above  which the program text can run.  The corresponding
                     memory area must be readable and executable, but not writable  or  shareable
                     (see mprotect(2) and mmap(2) for more information).

              PR_SET_MM_END_CODE
                     Set  the  address  below  which the program text can run.  The corresponding
                     memory area must be readable and executable, but not writable or shareable.

              PR_SET_MM_START_DATA
                     Set the address above which initialized and  uninitialized  (bss)  data  are
                     placed.   The  corresponding  memory area must be readable and writable, but
                     not executable or shareable.

              PR_SET_MM_END_DATA
                     Set the address below which initialized and  uninitialized  (bss)  data  are
                     placed.   The  corresponding  memory area must be readable and writable, but
                     not executable or shareable.

              PR_SET_MM_START_STACK
                     Set the start address of the stack.  The corresponding memory area  must  be
                     readable and writable.

              PR_SET_MM_START_BRK
                     Set  the  address  above  which the program heap can be expanded with brk(2)
                     call.  The address must be greater than the ending address  of  the  current
                     program  data segment.  In addition, the combined size of the resulting heap
                     and the size of the data segment can't exceed the RLIMIT_DATA resource limit
                     (see setrlimit(2)).

              PR_SET_MM_BRK
                     Set the current brk(2) value.  The requirements for the address are the same
                     as for the PR_SET_MM_START_BRK option.

              The following options are available since Linux 3.5.

              PR_SET_MM_ARG_START
                     Set the address above which the program command line is placed.

              PR_SET_MM_ARG_END
                     Set the address below which the program command line is placed.

              PR_SET_MM_ENV_START
                     Set the address above which the program environment is placed.

              PR_SET_MM_ENV_END
                     Set the address below which the program environment is placed.

                     The   address   passed    with    PR_SET_MM_ARG_START,    PR_SET_MM_ARG_END,
                     PR_SET_MM_ENV_START,  and PR_SET_MM_ENV_END should belong to a process stack
                     area.  Thus, the corresponding memory area must be readable,  writable,  and
                     (depending on the kernel configuration) have the MAP_GROWSDOWN attribute set
                     (see mmap(2)).

              PR_SET_MM_AUXV
                     Set a new auxiliary vector.  The arg3 argument should provide the address of
                     the vector.  The arg4 is the size of the vector.

              PR_SET_MM_EXE_FILE
                     Supersede  the  /proc/pid/exe symbolic link with a new one pointing to a new
                     executable file identified by the file descriptor provided in arg3 argument.
                     The file descriptor should be obtained with a regular open(2) call.

                     To  change  the  symbolic  link,  one needs to unmap all existing executable
                     memory areas, including those created by the kernel itself (for example  the
                     kernel usually creates at least one executable memory area for the ELF .text
                     section).

                     In Linux 4.9 and earlier, the PR_SET_MM_EXE_FILE operation can be  performed
                     only  once  in  a  process's lifetime; attempting to perform the operation a
                     second time results in the error EPERM.  This restriction was  enforced  for
                     security reasons that were subsequently deemed specious, and the restriction
                     was removed in Linux 4.10 because some  user-space  applications  needed  to
                     perform this operation more than once.

              The following options are available since Linux 3.18.

              PR_SET_MM_MAP
                     Provides  one-shot  access  to  all  the  addresses  by  passing in a struct
                     prctl_mm_map (as defined in  <linux/prctl.h>).   The  arg4  argument  should
                     provide the size of the struct.

                     This   feature   is   available  only  if  the  kernel  is  built  with  the
                     CONFIG_CHECKPOINT_RESTORE option enabled.

              PR_SET_MM_MAP_SIZE
                     Returns the size of the struct prctl_mm_map the kernel expects.  This allows
                     user  space  to  find  a  compatible  struct.  The arg4 argument should be a
                     pointer to an unsigned int.

                     This  feature  is  available  only  if  the  kernel  is   built   with   the
                     CONFIG_CHECKPOINT_RESTORE option enabled.

       PR_MPX_ENABLE_MANAGEMENT, PR_MPX_DISABLE_MANAGEMENT (since Linux 3.19)
              Enable  or  disable  kernel management of Memory Protection eXtensions (MPX) bounds
              tables.  The arg2, arg3, arg4, and arg5 arguments must be zero.

              MPX is a hardware-assisted mechanism for performing bounds  checking  on  pointers.
              It  consists  of a set of registers storing bounds information and a set of special
              instruction prefixes that tell the CPU on which instructions it  should  do  bounds
              enforcement.   There is a limited number of these registers and when there are more
              pointers than registers, their contents must be "spilled" into  a  set  of  tables.
              These  tables  are  called  "bounds  tables" and the MPX prctl() operations control
              whether the kernel manages their allocation and freeing.

              When management is enabled, the kernel will take over allocation and freeing of the
              bounds  tables.   It  does this by trapping the #BR exceptions that result at first
              use of missing bounds tables and instead of delivering the exception to user space,
              it  allocates the table and populates the bounds directory with the location of the
              new table.  For freeing, the kernel checks to see if bounds tables are present  for
              memory which is not allocated, and frees them if so.

              Before enabling MPX management using PR_MPX_ENABLE_MANAGEMENT, the application must
              first have allocated a user-space buffer for the bounds directory  and  placed  the
              location of that directory in the bndcfgu register.

              These calls fail if the CPU or kernel does not support MPX.  Kernel support for MPX
              is enabled via  the  CONFIG_X86_INTEL_MPX  configuration  option.   You  can  check
              whether  the  CPU  supports  MPX  by looking for the 'mpx' CPUID bit, like with the
              following command:

                  cat /proc/cpuinfo | grep ' mpx '

              A thread may not switch in or out of long (64-bit) mode while MPX is enabled.

              All threads in a process are affected by these calls.

              The child of a fork(2) inherits the state of MPX management.  During execve(2), MPX
              management is reset to a state as if PR_MPX_DISABLE_MANAGEMENT had been called.

              For   further   information   on   Intel   MPX,   see   the   kernel   source  file
              Documentation/x86/intel_mpx.txt.

       PR_SET_NAME (since Linux 2.6.9)
              Set the name of the calling thread, using the value in the location pointed  to  by
              (char *) arg2.  The name can be up to 16 bytes long, including the terminating null
              byte.  (If the length of the string, including the terminating null  byte,  exceeds
              16  bytes,  the string is silently truncated.)  This is the same attribute that can
              be set via pthread_setname_np(3) and retrieved  using  pthread_getname_np(3).   The
              attribute  is  likewise accessible via /proc/self/task/[tid]/comm, where tid is the
              name of the calling thread.

       PR_GET_NAME (since Linux 2.6.11)
              Return the name of the calling thread, in the buffer pointed to by  (char *)  arg2.
              The buffer should allow space for up to 16 bytes; the returned string will be null-
              terminated.

       PR_SET_NO_NEW_PRIVS (since Linux 3.5)
              Set the calling thread's  no_new_privs  attribute  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 the no_new_privs attribute cannot be unset.   The  setting  of  this
              attribute  is  inherited by children created by fork(2) and clone(2), and preserved
              across execve(2).

              Since Linux 4.10, the value of a thread's no_new_privs attribute can be viewed  via
              the NoNewPrivs field in the /proc/[pid]/status file.

              For      more      information,      see      the      kernel      source      file
              Documentation/userspace-api/no_new_privs.rst                                    (or
              Documentation/prctl/no_new_privs.txt before Linux 4.13).  See also seccomp(2).

       PR_GET_NO_NEW_PRIVS (since Linux 3.5)
              Return  (as  the  function  result) the value of the no_new_privs attribute for the
              calling thread.  A value of 0 indicates the regular execve(2) behavior.  A value of
              1  indicates  execve(2)  will  operate  in the privilege-restricting mode described
              above.

       PR_SET_PDEATHSIG (since Linux 2.1.57)
              Set the parent-death signal of the calling process to arg2 (either a  signal  value
              in  the  range  1..maxsig,  or  0  to  clear).  This is the signal that the calling
              process will get when its parent dies.

              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.

              The  parent-death  signal  is sent upon subsequent termination of the parent thread
              and also upon termination  of  each  subreaper  process  (see  the  description  of
              PR_SET_CHILD_SUBREAPER  above)  to which the caller is subsequently reparented.  If
              the parent thread and all ancestor subreapers have already terminated by  the  time
              of  the  PR_SET_PDEATHSIG  operation,  then  no  parent-death signal is sent to the
              caller.

              The parent-death signal is process-directed  (see  signal(7))  and,  if  the  child
              installs  a handler using the sigaction(2) SA_SIGINFO flag, the si_pid field of the
              siginfo_t argument of the handler  contains  the  PID  of  the  terminating  parent
              process.

              The  parent-death signal setting is cleared for the child of a fork(2).  It is also
              (since Linux 2.4.36 / 2.6.23) cleared when executing a set-user-ID or  set-group-ID
              binary,  or  a  binary  that  has  associated  capabilities  (see capabilities(7));
              otherwise, this value is preserved across execve(2).

       PR_GET_PDEATHSIG (since Linux 2.3.15)
              Return the current value of the  parent  process  death  signal,  in  the  location
              pointed to by (int *) arg2.

       PR_SET_PTRACER (since Linux 3.4)
              This  is  meaningful  only  when the Yama LSM is enabled and in mode 1 ("restricted
              ptrace", visible via /proc/sys/kernel/yama/ptrace_scope).  When a "ptracer  process
              ID"  is  passed  in  arg2,  the  caller  is  declaring that the ptracer process can
              ptrace(2) the calling process as if  it  were  a  direct  process  ancestor.   Each
              PR_SET_PTRACER  operation  replaces  the  previous "ptracer process ID".  Employing
              PR_SET_PTRACER with arg2 set to 0 clears the caller's  "ptracer  process  ID".   If
              arg2  is  PR_SET_PTRACER_ANY,  the  ptrace  restrictions  introduced  by  Yama  are
              effectively disabled for the calling process.

              For     further     information,     see      the      kernel      source      file
              Documentation/admin-guide/LSM/Yama.rst  (or  Documentation/security/Yama.txt before
              Linux 4.13).

       PR_SET_SECCOMP (since Linux 2.6.23)
              Set the secure computing (seccomp) mode  for  the  calling  thread,  to  limit  the
              available system calls.  The more recent seccomp(2) system call provides a superset
              of the functionality of PR_SET_SECCOMP.

              The seccomp mode is selected via arg2.   (The  seccomp  constants  are  defined  in
              <linux/seccomp.h>.)

              With  arg2  set  to  SECCOMP_MODE_STRICT,  the only system calls that the thread is
              permitted to make are read(2), write(2),  _exit(2)  (but  not  exit_group(2)),  and
              sigreturn(2).   Other  system  calls  result  in  the delivery of a SIGKILL signal.
              Strict secure computing mode is useful for number-crunching applications  that  may
              need  to  execute  untrusted  byte code, perhaps obtained by reading from a pipe or
              socket.  This operation  is  available  only  if  the  kernel  is  configured  with
              CONFIG_SECCOMP enabled.

              With  arg2  set  to SECCOMP_MODE_FILTER (since Linux 3.5), the system calls allowed
              are defined by a pointer to a Berkeley Packet Filter passed in arg3.  This argument
              is  a  pointer  to struct sock_fprog; it can be designed to filter arbitrary system
              calls and system call arguments.  This mode is available  only  if  the  kernel  is
              configured with CONFIG_SECCOMP_FILTER enabled.

              If  SECCOMP_MODE_FILTER  filters permit fork(2), then the seccomp mode is inherited
              by children created by fork(2); if execve(2) is permitted, then the seccomp mode is
              preserved  across  execve(2).  If the filters permit prctl() calls, then additional
              filters can be added; they are run in order until the  first  non-allow  result  is
              seen.

              For      further      information,      see      the     kernel     source     file
              Documentation/userspace-api/seccomp_filter.rst                                  (or
              Documentation/prctl/seccomp_filter.txt before Linux 4.13).

       PR_GET_SECCOMP (since Linux 2.6.23)
              Return  (as  the  function result) the secure computing mode of the calling thread.
              If the caller is not in secure computing mode, this operation  returns  0;  if  the
              caller  is  in  strict  secure  computing  mode, then the prctl() call will cause a
              SIGKILL signal to be sent to the process.  If the caller is  in  filter  mode,  and
              this  system  call  is allowed by the seccomp filters, it returns 2; otherwise, the
              process is killed with a SIGKILL signal.  This operation is available only  if  the
              kernel is configured with CONFIG_SECCOMP enabled.

              Since Linux 3.8, the Seccomp field of the /proc/[pid]/status file provides a method
              of obtaining the same information, without the risk that the process is killed; see
              proc(5).

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

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

       PR_GET_SPECULATION_CTRL (since Linux 4.17)
              Returns  the state of the speculation misfeature specified in arg2.  Currently, the
              only permitted value for this argument is PR_SPEC_STORE_BYPASS (otherwise the  call
              fails with the error ENODEV).

              The return value uses bits 0-3 with the following meaning:

              PR_SPEC_PRCTL
                     Mitigation can be controlled per thread by PR_SET_SPECULATION_CTRL

              PR_SPEC_ENABLE
                     The speculation feature is enabled, mitigation is disabled.

              PR_SPEC_DISABLE
                     The speculation feature is disabled, mitigation is enabled

              PR_SPEC_FORCE_DISABLE
                     Same as PR_SPEC_DISABLE but cannot be undone.

              If all bits are 0, then the CPU is not affected by the speculation misfeature.

              If  PR_SPEC_PRCTL  is  set, then per-thread control of the mitigation is available.
              If not set, prctl() for the speculation misfeature will fail.

              The arg3, arg4, and arg5 arguments must be specified as 0; otherwise the call fails
              with the error EINVAL.

       PR_SET_SPECULATION_CTRL (since Linux 4.17)
              Sets  the  state  of  the speculation misfeature specified in arg2.  Currently, the
              only permitted value for this argument is PR_SPEC_STORE_BYPASS (otherwise the  call
              fails  with  the  error ENODEV).  This setting is a per-thread attribute.  The arg3
              argument is used to hand in the control value, which is one of the following:

              PR_SPEC_ENABLE
                     The speculation feature is enabled, mitigation is disabled.

              PR_SPEC_DISABLE
                     The speculation feature is disabled, mitigation is enabled

              PR_SPEC_FORCE_DISABLE
                     Same as PR_SPEC_DISABLE but  cannot  be  undone.   A  subsequent  prctl(...,
                     PR_SPEC_ENABLE) will fail with the error EPERM.

              Any other value in arg3 will result in the call failing with the error ERANGE.

              The  arg4  and arg5 arguments must be specified as 0; otherwise the call fails with
              the error EINVAL.

              The speculation feature can also be  controlled  by  the  spec_store_bypass_disable
              boot parameter.  This parameter may enforce a read-only policy which will result in
              the prctl(2) call failing with the error  ENXIO.   For  further  details,  see  the
              kernel source file Documentation/admin-guide/kernel-parameters.txt.

       PR_SET_THP_DISABLE (since Linux 3.15)
              Set  the  state  of  the  "THP disable" flag for the calling thread.  If arg2 has a
              nonzero value, the flag is  set,  otherwise  it  is  cleared.   Setting  this  flag
              provides  a  method  for  disabling  transparent huge pages for jobs where the code
              cannot be modified, and using a malloc hook with madvise(2) is not an option (i.e.,
              statically  allocated data).  The setting of the "THP disable" flag is inherited by
              a child created via fork(2) and is preserved across execve(2).

       PR_TASK_PERF_EVENTS_DISABLE (since Linux 2.6.31)
              Disable all performance counters attached to the  calling  process,  regardless  of
              whether  the counters were created by this process or another process.  Performance
              counters created by the calling process for other processes  are  unaffected.   For
              more  information  on  performance  counters,  see  the  Linux  kernel  source file
              tools/perf/design.txt.

              Originally  called  PR_TASK_PERF_COUNTERS_DISABLE;  renamed  (retaining  the   same
              numerical value) in Linux 2.6.32.

       PR_TASK_PERF_EVENTS_ENABLE (since Linux 2.6.31)
              The  converse  of PR_TASK_PERF_EVENTS_DISABLE; enable performance counters attached
              to the calling process.

              Originally called PR_TASK_PERF_COUNTERS_ENABLE; renamed in Linux 2.6.32.

       PR_GET_THP_DISABLE (since Linux 3.15)
              Return (via the function result) the current setting of the "THP disable" flag  for
              the calling thread: either 1, if the flag is set, or 0, if it is not.

       PR_GET_TID_ADDRESS (since Linux 3.5)
              Retrieve  the  clear_child_tid  address  set by set_tid_address(2) and the clone(2)
              CLONE_CHILD_CLEARTID flag, in the  location  pointed  to  by  (int **) arg2.   This
              feature is available only if the kernel is built with the CONFIG_CHECKPOINT_RESTORE
              option enabled.  Note that since the prctl() system call does  not  have  a  compat
              implementation  for  the  AMD64  x32 and MIPS n32 ABIs, and the kernel writes out a
              pointer using the kernel's pointer size, this operation expects a user-space buffer
              of 8 (not 4) bytes on these ABIs.

       PR_SET_TIMERSLACK (since Linux 2.6.28)
              Each  thread  has  two  associated  timer  slack  values:  a "default" value, and a
              "current" value.  This operation sets the  "current"  timer  slack  value  for  the
              calling  thread.   arg2  is an unsigned long value, then maximum "current" value is
              ULONG_MAX and the minimum "current" value is 1.  If the nanosecond  value  supplied
              in  arg2  is  greater than zero, then the "current" value is set to this value.  If
              arg2 is equal to zero, the "current" timer slack is reset to the thread's "default"
              timer slack value.

              The  "current" timer slack is used by the kernel to group timer expirations for the
              calling thread that are close to one another; as a consequence,  timer  expirations
              for  the  thread  may  be  up to the specified number of nanoseconds late (but will
              never expire early).  Grouping timer  expirations  can  help  reduce  system  power
              consumption by minimizing CPU wake-ups.

              The  timer  expirations  affected  by  timer  slack  are  those  set  by select(2),
              pselect(2), poll(2), ppoll(2), epoll_wait(2),  epoll_pwait(2),  clock_nanosleep(2),
              nanosleep(2), and futex(2) (and thus the library functions implemented via futexes,
              including          pthread_cond_timedwait(3),           pthread_mutex_timedlock(3),
              pthread_rwlock_timedrdlock(3),          pthread_rwlock_timedwrlock(3),          and
              sem_timedwait(3)).

              Timer slack is not  applied  to  threads  that  are  scheduled  under  a  real-time
              scheduling policy (see sched_setscheduler(2)).

              When  a  new thread is created, the two timer slack values are made the same as the
              "current" value of the creating  thread.   Thereafter,  a  thread  can  adjust  its
              "current"  timer  slack  value via PR_SET_TIMERSLACK.  The "default" value can't be
              changed.  The timer slack values of init (PID 1), the ancestor  of  all  processes,
              are  50,000 nanoseconds (50 microseconds).  The timer slack value is inherited by a
              child created via fork(2), and is preserved across execve(2).

              Since Linux 4.6, the "current" timer slack value of any process can be examined and
              changed via the file /proc/[pid]/timerslack_ns.  See proc(5).

       PR_GET_TIMERSLACK (since Linux 2.6.28)
              Return  (as  the  function  result)  the "current" timer slack value of the calling
              thread.

       PR_SET_TIMING (since Linux 2.6.0)
              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)
              Return (as the function result) which process timing method is currently in use.

       PR_SET_TSC (since Linux 2.6.26, x86 only)
              Set  the state of the flag determining whether the timestamp counter can be read by
              the process.  Pass PR_TSC_ENABLE to arg2 to allow it to be read, or  PR_TSC_SIGSEGV
              to generate a SIGSEGV when the process tries to read the timestamp counter.

       PR_GET_TSC (since Linux 2.6.26, x86 only)
              Return the state of the flag determining whether the timestamp counter can be read,
              in the location pointed to by (int *) arg2.

       PR_SET_UNALIGN
              (Only on: ia64, since Linux 2.3.48; parisc,  since  Linux  2.6.15;  PowerPC,  since
              Linux  2.6.18; Alpha, since Linux 2.6.22; sh, since Linux 2.6.34; tile, since Linux
              3.12) Set unaligned access  control  bits  to  arg2.   Pass  PR_UNALIGN_NOPRINT  to
              silently fix up unaligned user accesses, or PR_UNALIGN_SIGBUS to generate SIGBUS on
              unaligned user access.  Alpha also supports an additional flag with the value of  4
              and no corresponding named constant, which instructs kernel to not fix up unaligned
              accesses (it is analogous to providing the UAC_NOFIX flag in SSI_NVPAIRS  operation
              of the setsysinfo() system call on Tru64).

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

RETURN VALUE

       On success,  PR_GET_DUMPABLE,  PR_GET_KEEPCAPS,  PR_GET_NO_NEW_PRIVS,  PR_GET_THP_DISABLE,
       PR_CAPBSET_READ,  PR_GET_TIMING,  PR_GET_TIMERSLACK,  PR_GET_SECUREBITS,  PR_MCE_KILL_GET,
       PR_CAP_AMBIENT+PR_CAP_AMBIENT_IS_SET,  and  (if  it  returns)  PR_GET_SECCOMP  return  the
       nonnegative  values  described  above.   All  other option values return 0 on success.  On
       error, -1 is returned, and errno is set appropriately.

ERRORS

       EACCES option is PR_SET_SECCOMP and arg2 is SECCOMP_MODE_FILTER, but the process does  not
              have  the  CAP_SYS_ADMIN  capability or has not set the no_new_privs attribute (see
              the discussion of PR_SET_NO_NEW_PRIVS above).

       EACCES option is PR_SET_MM, and arg3 is PR_SET_MM_EXE_FILE, the file is not executable.

       EBADF  option is PR_SET_MM, arg3 is PR_SET_MM_EXE_FILE, and the file descriptor passed  in
              arg4 is not valid.

       EBUSY  option  is  PR_SET_MM,  arg3  is PR_SET_MM_EXE_FILE, and this the second attempt to
              change the /proc/pid/exe symbolic link, which is prohibited.

       EFAULT arg2 is an invalid address.

       EFAULT option is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER, the system  was  built  with
              CONFIG_SECCOMP_FILTER, and arg3 is an invalid address.

       EINVAL The value of option is not recognized.

       EINVAL option is PR_MCE_KILL or PR_MCE_KILL_GET or PR_SET_MM, and unused prctl() arguments
              were not specified as zero.

       EINVAL arg2 is not valid value for this option.

       EINVAL option is PR_SET_SECCOMP or PR_GET_SECCOMP, and the kernel was not configured  with
              CONFIG_SECCOMP.

       EINVAL option  is  PR_SET_SECCOMP,  arg2  is  SECCOMP_MODE_FILTER,  and the kernel was not
              configured with CONFIG_SECCOMP_FILTER.

       EINVAL option is PR_SET_MM, and one of the following is true

              *  arg4 or arg5 is nonzero;

              *  arg3 is greater than TASK_SIZE (the limit on the size of the user address  space
                 for this architecture);

              *  arg2    is   PR_SET_MM_START_CODE,   PR_SET_MM_END_CODE,   PR_SET_MM_START_DATA,
                 PR_SET_MM_END_DATA,  or  PR_SET_MM_START_STACK,  and  the  permissions  of   the
                 corresponding memory area are not as required;

              *  arg2  is PR_SET_MM_START_BRK or PR_SET_MM_BRK, and arg3 is less than or equal to
                 the end of  the  data  segment  or  specifies  a  value  that  would  cause  the
                 RLIMIT_DATA resource limit to be exceeded.

       EINVAL option  is  PR_SET_PTRACER  and arg2 is not 0, PR_SET_PTRACER_ANY, or the PID of an
              existing process.

       EINVAL option is PR_SET_PDEATHSIG and arg2 is not a valid signal number.

       EINVAL option is PR_SET_DUMPABLE and arg2 is neither SUID_DUMP_DISABLE nor SUID_DUMP_USER.

       EINVAL option is PR_SET_TIMING and arg2 is not PR_TIMING_STATISTICAL.

       EINVAL option is PR_SET_NO_NEW_PRIVS and arg2 is not equal to 1 or arg3, arg4, or arg5  is
              nonzero.

       EINVAL option is PR_GET_NO_NEW_PRIVS and arg2, arg3, arg4, or arg5 is nonzero.

       EINVAL option is PR_SET_THP_DISABLE and arg3, arg4, or arg5 is nonzero.

       EINVAL option is PR_GET_THP_DISABLE and arg2, arg3, arg4, or arg5 is nonzero.

       EINVAL option  is  PR_CAP_AMBIENT  and  an unused argument (arg4, arg5, or, in the case of
              PR_CAP_AMBIENT_CLEAR_ALL, arg3) is nonzero; or arg2 has an invalid value;  or  arg2
              is  PR_CAP_AMBIENT_LOWER,  PR_CAP_AMBIENT_RAISE,  or PR_CAP_AMBIENT_IS_SET and arg3
              does not specify a valid capability.

       ENODEV option was PR_SET_SPECULATION_CTRL the kernel or CPU does not support the requested
              speculation misfeature.

       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.

       ENXIO  option  was  PR_SET_SPECULATION_CTRL  implies  that  the  control  of  the selected
              speculation misfeature is not possible.  See PR_GET_SPECULATION_CTRL  for  the  bit
              fields to determine which option is available.

       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_SPECULATION_CTRL  wherein  the  speculation  was  disabled  with
              PR_SPEC_FORCE_DISABLE and caller tried to enable it again.

       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.

       ERANGE option   was   PR_SET_SPECULATION_CTRL   and   arg3   is   neither  PR_SPEC_ENABLE,
              PR_SPEC_DISABLE, nor PR_SPEC_FORCE_DISABLE.

       EINVAL option was PR_GET_SPECULATION_CTRL or PR_SET_SPECULATION_CTRL and unused  arguments
              to prctl() are not 0.

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 5.02 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/.