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man:prctl

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

NAME

     prctl - operations on a process

SYNOPSIS

     #include <sys/prctl.h>
     int prctl(int option, unsigned long arg2, unsigned long arg3,
               unsigned long arg4, unsigned long arg5);

DESCRIPTION

     prctl()  is  called  with  a first argument describing what to do (with
     values defined in <linux/prctl.h>), and further arguments with  a  sig-
     nificance depending on the first one.  The first argument can be:
     PR_CAP_AMBIENT (since Linux 4.3)
            Reads  or  changes  the  ambient  capability  set of the calling
            thread, according to the value of arg2, which must be one of the
            following:
            PR_CAP_AMBIENT_RAISE
                   The  capability specified in arg3 is added to the ambient
                   set.  The specified capability must already be present in
                   both  the  permitted  and  the  inheritable  sets  of the
                   process.   This  operation  is  not  permitted   if   the
                   SECBIT_NO_CAP_AMBIENT_RAISE securebit is set.
            PR_CAP_AMBIENT_LOWER
                   The  capability  specified  in  arg3  is removed from the
                   ambient set.
            PR_CAP_AMBIENT_IS_SET
                   The prctl() call returns 1 if the capability in  arg3  is
                   in the ambient set and 0 if it is not.
            PR_CAP_AMBIENT_CLEAR_ALL
                   All  capabilities  will  be removed from the ambient set.
                   This operation requires setting arg3 to zero.
            In all of the above operations, arg4 and arg5 must be  specified
            as 0.
     PR_CAPBSET_READ (since Linux 2.6.25)
            Return (as the function result) 1 if the capability specified in
            arg2 is in the calling thread's capability bounding set, or 0 if
            it   is   not.    (The   capability  constants  are  defined  in
            <linux/capability.h>.)  The  capability  bounding  set  dictates
            whether  the process can receive the capability through a file's
            permitted capability set on a subsequent call to execve(2).
            If the capability specified in arg2 is not valid, then the  call
            fails with the error EINVAL.
     PR_CAPBSET_DROP (since Linux 2.6.25)
            If  the calling thread has the CAP_SETPCAP capability within its
            user namespace, then drop the capability specified by arg2  from
            the  calling  thread's capability bounding set.  Any children of
            the calling thread will inherit the newly reduced bounding  set.
            The  call fails with the error: EPERM if the calling thread does
            not have the CAP_SETPCAP; EINVAL if arg2 does  not  represent  a
            valid capability; or EINVAL if file capabilities are not enabled
            in the kernel, in which case bounding sets are not supported.
     PR_SET_CHILD_SUBREAPER (since Linux 3.4)
            If arg2 is nonzero, set the "child subreaper" attribute  of  the
            calling process; if arg2 is zero, unset the attribute.
            A subreaper fulfills the role of init(1) for its descendant pro-
            cesses.  When a process becomes orphaned  (i.e.,  its  immediate
            parent  terminates)  then that process will be reparented to the
            nearest still living ancestor subreaper.  Subsequently, calls to
            getppid() in the orphaned process will now return the PID of the
            subreaper process, and when the orphan  terminates,  it  is  the
            subreaper process that will receive a SIGCHLD signal and will be
            able to wait(2) on the process to discover its termination  sta-
            tus.
            The  setting of this bit is not inherited by children created by
            fork(2)  and  clone(2).   The  setting   is   preserved   across
            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 (per-
            haps  so that it can restart that process).  Some init(1) frame-
            works (e.g., systemd(1)) employ a subreaper process for  similar
            reasons.
     PR_GET_CHILD_SUBREAPER (since Linux 3.4)
            Return the "child subreaper" setting of the caller, in the loca-
            tion pointed to by (int *) arg2.
     PR_SET_DUMPABLE (since Linux 2.3.20)
            Set the state of the "dumpable" flag, which  determines  whether
            core dumps are produced for the calling process upon delivery of
            a signal whose default behavior is to produce a core dump.
            In kernels up to and including 2.6.12, arg2  must  be  either  0
            (SUID_DUMP_DISABLE,    process    is    not   dumpable)   or   1
            (SUID_DUMP_USER, process is dumpable).  Between  kernels  2.6.13
            and  2.6.17,  the  value  2 was also permitted, which caused any
            binary which normally would not be dumped to be dumped  readable
            by  root  only;  for  security  reasons,  this  feature has been
            removed.    (See   also   the   description   of   /proc/sys/fs/
            suid_dumpable in proc(5).)
            Normally,  this  flag  is set to 1.  However, it is reset to the
            current value contained in the  file  /proc/sys/fs/suid_dumpable
            (which  by  default  has  the value 0), in the following circum-
            stances:
  • The process's effective user or group ID is changed.
  • The process's filesystem user or group ID is changed (see

credentials(7)).

  • The process executes (execve(2)) a set-user-ID or set-group-

ID program, resulting in a change of either the effective

               user ID or the effective group ID.
  • The process executes (execve(2)) a program that has file

capabilities (see capabilities(7)), but only if the permitted

               capabilities  gained  exceed  those already permitted for the
               process.
            Processes  that  are  not  dumpable  can  not  be  attached  via
            ptrace(2) PTRACE_ATTACH; see ptrace(2) for further details.
            If  a  process  is  not  dumpable, the ownership of files in the
            process's /proc/[pid] directory  is  affected  as  described  in
            proc(5).
     PR_GET_DUMPABLE (since Linux 2.3.20)
            Return (as the function result) the current state of the calling
            process's dumpable flag.
     PR_SET_ENDIAN (since Linux 2.6.18, PowerPC only)
            Set the endian-ness of the calling process to the value given in
            arg2,  which  should  be  one  of  the following: PR_ENDIAN_BIG,
            PR_ENDIAN_LITTLE, or PR_ENDIAN_PPC_LITTLE (PowerPC pseudo little
            endian).
     PR_GET_ENDIAN (since Linux 2.6.18, PowerPC only)
            Return  the  endian-ness of the calling process, in the location
            pointed to by (int *) arg2.
     PR_SET_FP_MODE (since Linux 4.0, only on MIPS)
            On the MIPS architecture, user-space code can be built using  an
            ABI  which  permits  linking with code that has more restrictive
            floating-point (FP) requirements.  For example, user-space  code
            may  be  built  to  target the O32 FPXX ABI and linked with code
            built for either one of the more restrictive FP32 or FP64  ABIs.
            When more restrictive code is linked in, the overall requirement
            for the process is to use the  more  restrictive  floating-point
            mode.
            Because the kernel has no means of knowing in advance which mode
            the process should be executed in, and  because  these  restric-
            tions   can  change  over  the  lifetime  of  the  process,  the
            PR_SET_FP_MODE operation is provided to  allow  control  of  the
            floating-point mode from user space.
            The  (unsigned  int)  arg2 argument is a bit mask describing the
            floating-point mode used:
            PR_FP_MODE_FR
                   When this bit is unset (so called FR=0 or FR0 mode),  the
                   32  floating-point registers are 32 bits wide, and 64-bit
                   registers are represented as a pair of  registers  (even-
                   and  odd-  numbered, with the even-numbered register con-
                   taining the lower 32 bits, and the odd-numbered  register
                   containing the higher 32 bits).
                   When  this  bit  is  set  (on supported hardware), the 32
                   floating-point registers are 64 bits wide (so called FR=1
                   or  FR1  mode).   Note  that  modern MIPS implementations
                   (MIPS R6 and newer) support FR=1 mode only.
                   Applications that use the O32 FP32 ABI can  operate  only
                   when  this  bit  is unset (FR=0; or they can be used with
                   FRE enabled, see below).  Applications that use  the  O32
                   FP64  ABI (and the O32 FP64A ABI, which exists to provide
                   the ability to  operate  with  existing  FP32  code;  see
                   below)  can  operate  only  when  this bit is set (FR=1).
                   Applications that use the O32 FPXX ABI can  operate  with
                   either FR=0 or FR=1.
            PR_FP_MODE_FRE
                   Enable  emulation  of  32-bit  floating-point mode.  When
                   this mode is enabled, it emulates  32-bit  floating-point
                   operations by raising a reserved-instruction exception on
                   every instruction that uses 32-bit formats and the kernel
                   then  handles  the instruction in software.  (The problem
                   lies in the discrepancy of handling  odd-numbered  regis-
                   ters  which are the high 32 bits of 64-bit registers with
                   even numbers in FR=0 mode and the lower 32-bit  parts  of
                   odd-numbered  64-bit  registers  in FR=1 mode.)  Enabling
                   this bit is necessary when code with  the  O32  FP32  ABI
                   should  operate with code with compatible the O32 FPXX or
                   O32 FP64A ABIs (which require FR=1 FPU mode) or  when  it
                   is  executed  on  newer  hardware (MIPS R6 onwards) which
                   lacks FR=0 mode support when a binary with the  FP32  ABI
                   is used.
                   Note  that  this mode makes sense only when the FPU is in
                   64-bit mode (FR=1).
                   Note that the use of emulation inherently has a  signifi-
                   cant performance hit and should be avoided if possible.
            In  the  N32/N64 ABI, 64-bit floating-point mode is always used,
            so FPU emulation is not required and the FPU always operates  in
            FR=1 mode.
            This  option  is  mainly  intended for use by the dynamic linker
            (ld.so(8)).
            The arguments arg3, arg4, and arg5 are ignored.
     PR_GET_FP_MODE (since Linux 4.0, only on MIPS)
            Get the current floating-point  mode  (see  the  description  of
            PR_SET_FP_MODE for details).
            On  success,  the  call  returns a bit mask which represents the
            current floating-point mode.
            The arguments arg2, arg3, arg4, and arg5 are ignored.
     PR_SET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
            Set  floating-point  emulation  control  bits  to  arg2.    Pass
            PR_FPEMU_NOPRINT  to  silently  emulate floating-point operation
            accesses, or PR_FPEMU_SIGFPE to not emulate floating-point oper-
            ations and send SIGFPE instead.
     PR_GET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
            Return  floating-point  emulation  control bits, in the location
            pointed to by (int *) arg2.
     PR_SET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
            Set   floating-point   exception    mode    to    arg2.     Pass
            PR_FP_EXC_SW_ENABLE  to  use  FPEXC  for  FP  exception enables,
            PR_FP_EXC_DIV for floating-point divide by  zero,  PR_FP_EXC_OVF
            for  floating-point  overflow,  PR_FP_EXC_UND for floating-point
            underflow,  PR_FP_EXC_RES  for  floating-point  inexact  result,
            PR_FP_EXC_INV     for    floating-point    invalid    operation,
            PR_FP_EXC_DISABLED for FP exceptions disabled,  PR_FP_EXC_NONRE-
            COV for async nonrecoverable exception mode, PR_FP_EXC_ASYNC for
            async recoverable exception mode, PR_FP_EXC_PRECISE for  precise
            exception mode.
     PR_GET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
            Return floating-point exception mode, in the location pointed to
            by (int *) arg2.
     PR_SET_KEEPCAPS (since Linux 2.2.18)
            Set the state of the calling thread's "keep capabilities"  flag.
            The  effect  if this flag is described in capabilities(7).  arg2
            must be either 0 (clear the flag) or  1  (set  the  flag).   The
            "keep capabilities" value will be reset to 0 on subsequent calls
            to execve(2).
     PR_GET_KEEPCAPS (since Linux 2.2.18)
            Return (as the function result) the current state of the calling
            thread's  "keep  capabilities"  flag.  See capabilities(7) for a
            description of this flag.
     PR_MCE_KILL (since Linux 2.6.32)
            Set the machine check memory  corruption  kill  policy  for  the
            calling  thread.  If arg2 is PR_MCE_KILL_CLEAR, clear the thread
            memory corruption kill policy and use the  system-wide  default.
            (The system-wide default is defined by /proc/sys/vm/memory_fail-
            ure_early_kill; see proc(5).)  If arg2 is PR_MCE_KILL_SET, use a
            thread-specific  memory  corruption  kill policy.  In this case,
            arg3   defines   whether    the    policy    is    early    kill
            (PR_MCE_KILL_EARLY),  late  kill (PR_MCE_KILL_LATE), or the sys-
            tem-wide default (PR_MCE_KILL_DEFAULT).  Early kill  means  that
            the  thread  receives a SIGBUS signal as soon as hardware memory
            corruption is detected inside its address space.  In  late  kill
            mode,  the  process  is killed only when it accesses a corrupted
            page.  See sigaction(2) for more information on the SIGBUS  sig-
            nal.  The policy is inherited by children.  The remaining unused
            prctl() arguments must be zero for future compatibility.
     PR_MCE_KILL_GET (since Linux 2.6.32)
            Return the current per-process machine check kill  policy.   All
            unused prctl() arguments must be zero.
     PR_SET_MM (since Linux 3.3)
            Modify  certain kernel memory map descriptor fields of the call-
            ing process.  Usually these fields are set  by  the  kernel  and
            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 pro-
            vides a new value for the option.  The arg4 and  arg5  arguments
            must be zero if unused.
            Since  Linux  3.10,  this  feature  is  available  all the time.
            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  exe-
                   cutable,  but  not writable or shareable (see mprotect(2)
                   and mmap(2) for more information).
            PR_SET_MM_END_CODE
                   Set the address below which the  program  text  can  run.
                   The  corresponding  memory area must be readable and exe-
                   cutable, but not writable or shareable.
            PR_SET_MM_START_DATA
                   Set the address above which initialized and uninitialized
                   (bss)  data  are  placed.   The corresponding memory area
                   must be readable and  writable,  but  not  executable  or
                   shareable.
            PR_SET_MM_END_DATA
                   Set the address below which initialized and uninitialized
                   (bss) data are placed.   The  corresponding  memory  area
                   must  be  readable  and  writable,  but not executable or
                   shareable.
            PR_SET_MM_START_STACK
                   Set the start address of the  stack.   The  corresponding
                   memory area must be readable and writable.
            PR_SET_MM_START_BRK
                   Set  the  address  above  which  the  program heap can be
                   expanded with brk(2) call.  The address must  be  greater
                   than  the ending address of the current program data seg-
                   ment.  In addition, the combined size  of  the  resulting
                   heap  and  the  size of the data segment can't exceed the
                   RLIMIT_DATA resource limit (see setrlimit(2)).
            PR_SET_MM_BRK
                   Set the current brk(2) value.  The requirements  for  the
                   address  are  the  same  as  for  the PR_SET_MM_START_BRK
                   option.
            The following options are available since Linux 3.5.
            PR_SET_MM_ARG_START
                   Set the address above which the program command  line  is
                   placed.
            PR_SET_MM_ARG_END
                   Set  the  address below which the program command line is
                   placed.
            PR_SET_MM_ENV_START
                   Set the address above which the  program  environment  is
                   placed.
            PR_SET_MM_ENV_END
                   Set  the  address  below which the program environment is
                   placed.
                   The    address    passed    with     PR_SET_MM_ARG_START,
                   PR_SET_MM_ARG_END,        PR_SET_MM_ENV_START,        and
                   PR_SET_MM_ENV_END should belong to a process stack  area.
                   Thus,  the  corresponding  memory  area must be readable,
                   writable, and (depending  on  the  kernel  configuration)
                   have the MAP_GROWSDOWN attribute set (see mmap(2)).
            PR_SET_MM_AUXV
                   Set  a  new  auxiliary  vector.  The arg3 argument should
                   provide the address of the vector.  The arg4 is the  size
                   of the vector.
            PR_SET_MM_EXE_FILE
                   Supersede  the /proc/pid/exe symbolic link with a new one
                   pointing to a new executable file identified by the  file
                   descriptor  provided in arg3 argument.  The file descrip-
                   tor should be obtained with a regular open(2) call.
                   To change the symbolic  link,  one  needs  to  unmap  all
                   existing executable memory areas, including those created
                   by the kernel itself (for example the kernel usually cre-
                   ates  at  least  one  executable  memory area for the ELF
                   .text section).
                   The second limitation is that  such  transitions  can  be
                   done  only  once  in  a  process  life time.  Any further
                   attempts will  be  rejected.   This  should  help  system
                   administrators  monitor unusual symbolic-link transitions
                   over all processes running on a system.
            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  eXten-
            sions (MPX) bounds tables.  The arg2, arg3, arg4, and arg5 argu-
            ments must be zero.
            MPX is  a  hardware-assisted  mechanism  for  performing  bounds
            checking on pointers.  It consists of a set of registers storing
            bounds information and a set  of  special  instruction  prefixes
            that  tell  the  CPU  on  which instructions it should do bounds
            enforcement.  There is a limited number of these  registers  and
            when there are more pointers than registers, their contents must
            be "spilled" into a set of  tables.   These  tables  are  called
            "bounds  tables"  and the MPX prctl() operations control whether
            the kernel manages their allocation and freeing.
            When management is enabled, the kernel will take over allocation
            and  freeing of the bounds tables.  It does this by trapping the
            #BR exceptions that result at first use of missing bounds tables
            and  instead of delivering the exception to user space, it allo-
            cates the table and populates  the  bounds  directory  with  the
            location  of  the  new table.  For freeing, the kernel checks to
            see if bounds tables are present for memory which is  not  allo-
            cated, and frees them if so.
            Before  enabling  MPX management using PR_MPX_ENABLE_MANAGEMENT,
            the application must first have allocated  a  user-space  buffer
            for  the bounds directory and placed the location of that direc-
            tory in the bndcfgu register.
            These calls fail if the CPU or  kernel  does  not  support  MPX.
            Kernel  support  for MPX is enabled via the CONFIG_X86_INTEL_MPX
            configuration option.  You can check whether  the  CPU  supports
            MPX  by looking for the 'mpx' CPUID bit, like with the following
            command:
                 cat /proc/cpuinfo | grep ' mpx '
            A thread may not switch in or out of long  (64-bit)  mode  while
            MPX is enabled.
            All threads in a process are affected by these calls.
            The  child  of  a  fork(2) inherits the state of MPX management.
            During execve(2), MPX management is  reset  to  a  state  as  if
            PR_MPX_DISABLE_MANAGEMENT had been called.
            For further information on Intel MPX, see the kernel source file
            Documentation/x86/intel_mpx.txt.
     PR_SET_NAME (since Linux 2.6.9)
            Set the name of the calling thread, using the value in the loca-
            tion  pointed  to  by  (char *)  arg2.  The name can be up to 16
            bytes long, including the terminating null byte.  (If the length
            of  the  string, including the terminating null byte, exceeds 16
            bytes, the string is silently  truncated.)   This  is  the  same
            attribute   that   can  be  set  via  pthread_setname_np(3)  and
            retrieved using pthread_getname_np(3).  The attribute  is  like-
            wise accessible via /proc/self/task/[tid]/comm, where tid is the
            name of the calling thread.
     PR_GET_NAME (since Linux 2.6.11)
            Return the name of the calling thread, in the buffer pointed  to
            by  (char *)  arg2.   The buffer should allow space for up to 16
            bytes; the returned string will be null-terminated.
     PR_SET_NO_NEW_PRIVS (since Linux 3.5)
            Set the calling thread's no_new_privs bit to the value in  arg2.
            With  no_new_privs  set  to  1,  execve(2) promises not to grant
            privileges to do anything that could not have been done  without
            the  execve(2)  call (for example, rendering the set-user-ID and
            set-group-ID mode bits, and file  capabilities  non-functional).
            Once  set, this bit cannot be unset.  The setting of this bit is
            inherited by children created by fork(2) and clone(2), and  pre-
            served across execve(2).
            Since  Linux  4.10, the value of a thread's no_new_privs bit can
            be viewed via the NoNewPrivs  field  in  the  /proc/[pid]/status
            file.
            For  more  information,  see  the  kernel source file Documenta-
            tion/userspace-api/no_new_privs.rst        (or        Documenta-
            tion/prctl/no_new_privs.txt  before  Linux 4.13).  See also sec-
            comp(2).
     PR_GET_NO_NEW_PRIVS (since Linux 3.5)
            Return (as the function result) the value  of  the  no_new_privs
            bit  for the calling thread.  A value of 0 indicates the regular
            execve(2) behavior.  A value of 1 indicates execve(2) will oper-
            ate in the privilege-restricting mode described above.
     PR_SET_PDEATHSIG (since Linux 2.1.57)
            Set  the  parent  death  signal  of  the calling process to arg2
            (either a signal value in the range 1..maxsig, or 0  to  clear).
            This  is  the  signal that the calling process will get when its
            parent dies.  This value is cleared for the child of  a  fork(2)
            and  (since  Linux 2.4.36 / 2.6.23) when executing a set-user-ID
            or set-group-ID binary, or a binary that has associated capabil-
            ities  (see  capabilities(7)).   This  value is preserved across
            execve(2).
            Warning: the "parent" in this  case  is  considered  to  be  the
            thread  that  created  this process.  In other words, the signal
            will be sent when that  thread  terminates  (via,  for  example,
            pthread_exit(3)),  rather  than  after all of the threads in the
            parent process terminate.
     PR_GET_PDEATHSIG (since Linux 2.3.15)
            Return the current value of the parent process death signal,  in
            the location pointed to by (int *) arg2.
     PR_SET_PTRACER (since Linux 3.4)
            This is meaningful only when the Yama LSM is enabled and in mode
            1   ("restricted    ptrace",    visible    via    /proc/sys/ker-
            nel/yama/ptrace_scope).   When  a "ptracer process ID" is passed
            in arg2, the caller is declaring that the  ptracer  process  can
            ptrace(2)  the  calling  process  as if it were a direct process
            ancestor.  Each PR_SET_PTRACER operation replaces  the  previous
            "ptracer process ID".  Employing PR_SET_PTRACER with arg2 set to
            0  clears  the  caller's  "ptracer  process  ID".   If  arg2  is
            PR_SET_PTRACER_ANY,  the  ptrace restrictions introduced by Yama
            are effectively disabled for the calling process.
            For further information, see the kernel source  file  Documenta-
            tion/admin-guide/LSM/Yama.rst       (or      Documentation/secu-
            rity/Yama.txt before Linux 4.13).
     PR_SET_SECCOMP (since Linux 2.6.23)
            Set the secure computing (seccomp) mode for the calling  thread,
            to limit the available system calls.  The more recent seccomp(2)
            system  call  provides  a  superset  of  the  functionality   of
            PR_SET_SECCOMP.
            The  seccomp  mode is selected via arg2.  (The seccomp constants
            are defined in <linux/seccomp.h>.)
            With arg2 set to SECCOMP_MODE_STRICT, the only system calls that
            the  thread is permitted to make are read(2), write(2), _exit(2)
            (but not exit_group(2)), and sigreturn(2).  Other  system  calls
            result  in the delivery of a SIGKILL signal.  Strict secure com-
            puting mode is useful for number-crunching applications that may
            need to execute untrusted byte code, perhaps obtained by reading
            from a pipe or socket.  This operation is available only if  the
            kernel is configured with CONFIG_SECCOMP enabled.
            With arg2 set to SECCOMP_MODE_FILTER (since Linux 3.5), the sys-
            tem calls allowed are defined by a pointer to a Berkeley  Packet
            Filter  passed  in  arg3.   This argument is a pointer to struct
            sock_fprog; it can be designed to filter arbitrary system  calls
            and  system  call arguments.  This mode is available only if the
            kernel is configured with CONFIG_SECCOMP_FILTER enabled.
            If SECCOMP_MODE_FILTER filters permit fork(2), then the  seccomp
            mode  is  inherited by children created by fork(2); if execve(2)
            is  permitted,  then  the  seccomp  mode  is  preserved   across
            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/userspace-api/seccomp_filter.rst       (or       Documenta-
            tion/prctl/seccomp_filter.txt before Linux 4.13).
     PR_GET_SECCOMP (since Linux 2.6.23)
            Return (as the function result) the secure computing mode of the
            calling thread.  If the caller is not in secure computing  mode,
            this operation returns 0; if the caller is in strict secure com-
            puting mode, then the prctl() call will cause a  SIGKILL  signal
            to be sent to the process.  If the caller is in filter mode, and
            this system call is allowed by the seccomp filters,  it  returns
            2; otherwise, the process is killed with a SIGKILL signal.  This
            operation is available only if the  kernel  is  configured  with
            CONFIG_SECCOMP enabled.
            Since  Linux  3.8,  the  Seccomp field of the /proc/[pid]/status
            file provides a method of obtaining the same information,  with-
            out the risk that the process is killed; see proc(5).
     PR_SET_SECUREBITS (since Linux 2.6.26)
            Set  the  "securebits"  flags of the calling thread to the value
            supplied in arg2.  See capabilities(7).
     PR_GET_SECUREBITS (since Linux 2.6.26)
            Return (as the function result) the "securebits"  flags  of  the
            calling thread.  See capabilities(7).
     PR_SET_THP_DISABLE (since Linux 3.15)
            Set  the state of the "THP disable" flag for the calling thread.
            If arg2 has a nonzero value, the flag is set,  otherwise  it  is
            cleared.   Setting  this  flag  provides  a method for disabling
            transparent huge pages for jobs where the code cannot  be  modi-
            fied,  and  using a malloc hook with madvise(2) is not an option
            (i.e., statically allocated data).  The setting of the "THP dis-
            able"  flag  is  inherited by a child created via fork(2) and is
            preserved across execve(2).
     PR_TASK_PERF_EVENTS_DISABLE (since Linux 2.6.31)
            Disable  all  performance  counters  attached  to  the   calling
            process, regardless of whether the counters were created by this
            process or another process.  Performance counters created by the
            calling  process  for  other processes are unaffected.  For more
            information on performance counters, see the Linux kernel source
            file tools/perf/design.txt.
            Originally    called    PR_TASK_PERF_COUNTERS_DISABLE;   renamed
            (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.   If  the  nanosecond
            value  supplied in arg2 is greater than zero, then the "current"
            value is set to this value.  If arg2 is less than  or  equal  to
            zero,  the  "current"  timer  slack  is  reset  to  the thread's
            "default" timer slack value.
            The "current" timer slack is used by the kernel to  group  timer
            expirations  for  the  calling  thread  that  are  close  to one
            another; as a consequence, timer expirations for the thread  may
            be  up  to  the  specified  number of nanoseconds late (but will
            never expire early).  Grouping timer expirations can help reduce
            system power consumption by minimizing CPU wake-ups.
            The  timer  expirations affected by timer slack are those set by
            select(2),   pselect(2),   poll(2),   ppoll(2),   epoll_wait(2),
            epoll_pwait(2),  clock_nanosleep(2),  nanosleep(2), and futex(2)
            (and thus the library functions implemented via futexes, includ-
            ing    pthread_cond_timedwait(3),    pthread_mutex_timedlock(3),
            pthread_rwlock_timedrdlock(3),    pthread_rwlock_timedwrlock(3),
            and sem_timedwait(3)).
            Timer slack is not applied to threads that are scheduled under a
            real-time scheduling policy (see sched_setscheduler(2)).
            When a new thread is created, the two  timer  slack  values  are
            made  the  same  as  the "current" value of the creating thread.
            Thereafter, a thread can adjust its "current" timer slack  value
            via  PR_SET_TIMERSLACK.   The  "default" value can't be changed.
            The timer slack values of init (PID 1), the ancestor of all pro-
            cesses,  are  50,000  nanoseconds  (50 microseconds).  The timer
            slack values are preserved across execve(2).
            Since Linux 4.6, the "current" timer slack value of any  process
            can  be  examined  and  changed  via the file /proc/[pid]/timer-
            slack_ns.  See proc(5).
     PR_GET_TIMERSLACK (since Linux 2.6.28)
            Return (as the function result) the "current" timer slack  value
            of the calling thread.
     PR_SET_TIMING (since Linux 2.6.0-test4)
            Set  whether  to  use  (normal, traditional) statistical process
            timing or accurate timestamp-based process  timing,  by  passing
            PR_TIMING_STATISTICAL  or  PR_TIMING_TIMESTAMP to arg2.  PR_TIM-
            ING_TIMESTAMP is not currently implemented  (attempting  to  set
            this mode will yield the error EINVAL).
     PR_GET_TIMING (since Linux 2.6.0-test4)
            Return  (as  the function result) which process timing method is
            currently in use.
     PR_SET_TSC (since Linux 2.6.26, x86 only)
            Set the state of the  flag  determining  whether  the  timestamp
            counter  can be read by the process.  Pass PR_TSC_ENABLE to arg2
            to allow it to be read, or PR_TSC_SIGSEGV to generate a  SIGSEGV
            when the process tries to read the timestamp counter.
     PR_GET_TSC (since Linux 2.6.26, x86 only)
            Return  the  state of the flag determining whether the timestamp
            counter can be read, in the location pointed to by (int *) arg2.
     PR_SET_UNALIGN
            (Only  on: ia64, since Linux 2.3.48; parisc, since Linux 2.6.15;
            PowerPC, since Linux 2.6.18;  Alpha,  since  Linux  2.6.22;  sh,
            since Linux 2.6.34; tile, since Linux 3.12) Set unaligned access
            control bits to arg2.  Pass PR_UNALIGN_NOPRINT to  silently  fix
            up  unaligned  user  accesses,  or PR_UNALIGN_SIGBUS to generate
            SIGBUS on unaligned user access.  Alpha also supports  an  addi-
            tional  flag with the value of 4 and no corresponding named con-
            stant, which instructs kernel to not fix up  unaligned  accesses
            (it  is analogous to providing the UAC_NOFIX flag in SSI_NVPAIRS
            operation of the setsysinfo() system call on Tru64).
     PR_GET_UNALIGN
            (see PR_SET_UNALIGN for information on  versions  and  architec-
            tures)  Return  unaligned  access  control bits, in the location
            pointed to by (unsigned int *) arg2.

RETURN VALUE

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

ERRORS

     EACCES option  is  PR_SET_SECCOMP  and arg2 is SECCOMP_MODE_FILTER, but
            the process does not have the CAP_SYS_ADMIN  capability  or  has
            not  set  the  no_new_privs  attribute  (see  the  discussion of
            PR_SET_NO_NEW_PRIVS above).
     EACCES option is PR_SET_MM, and arg3 is PR_SET_MM_EXE_FILE, the file is
            not executable.
     EBADF  option  is  PR_SET_MM,  arg3 is PR_SET_MM_EXE_FILE, and the file
            descriptor passed in arg4 is not valid.
     EBUSY  option is PR_SET_MM, arg3 is PR_SET_MM_EXE_FILE,  and  this  the
            second  attempt to change the /proc/pid/exe symbolic link, which
            is prohibited.
     EFAULT arg2 is an invalid address.
     EFAULT option is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER, the  sys-
            tem was built with CONFIG_SECCOMP_FILTER, and arg3 is an invalid
            address.
     EINVAL The value of option is not recognized.
     EINVAL option is  PR_MCE_KILL  or  PR_MCE_KILL_GET  or  PR_SET_MM,  and
            unused prctl() arguments were not specified as zero.
     EINVAL arg2 is not valid value for this option.
     EINVAL option  is  PR_SET_SECCOMP or PR_GET_SECCOMP, and the kernel was
            not configured with CONFIG_SECCOMP.
     EINVAL option is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER,  and  the
            kernel was not configured with CONFIG_SECCOMP_FILTER.
     EINVAL option is PR_SET_MM, and one of the following is true
  • arg4 or arg5 is nonzero;
  • arg3 is greater than TASK_SIZE (the limit on the size of the

user address space for this architecture);

  • arg2 is PR_SET_MM_START_CODE, PR_SET_MM_END_CODE,

PR_SET_MM_START_DATA, PR_SET_MM_END_DATA, or

               PR_SET_MM_START_STACK, and the permissions of the correspond-
               ing memory area are not as required;
  • arg2 is PR_SET_MM_START_BRK or PR_SET_MM_BRK, and arg3 is

less than or equal to the end of the data segment or speci-

               fies  a value that would cause the RLIMIT_DATA resource limit
               to be exceeded.
     EINVAL option is PR_SET_PTRACER and arg2 is not 0,  PR_SET_PTRACER_ANY,
            or the PID of an existing process.
     EINVAL option  is  PR_SET_PDEATHSIG and arg2 is not a valid signal num-
            ber.
     EINVAL option is PR_SET_DUMPABLE and arg2 is neither  SUID_DUMP_DISABLE
            nor SUID_DUMP_USER.
     EINVAL option is PR_SET_TIMING and arg2 is not PR_TIMING_STATISTICAL.
     EINVAL option  is  PR_SET_NO_NEW_PRIVS  and  arg2  is not equal to 1 or
            arg3, arg4, or arg5 is nonzero.
     EINVAL option is PR_GET_NO_NEW_PRIVS and arg2, arg3, arg4, or  arg5  is
            nonzero.
     EINVAL option is PR_SET_THP_DISABLE and arg3, arg4, or arg5 is nonzero.
     EINVAL option is PR_GET_THP_DISABLE and arg2, arg3, arg4,  or  arg5  is
            nonzero.
     EINVAL option is PR_CAP_AMBIENT and an unused argument (arg4, arg5, or,
            in the case of PR_CAP_AMBIENT_CLEAR_ALL, arg3)  is  nonzero;  or
            arg2  has  an  invalid  value;  or arg2 is PR_CAP_AMBIENT_LOWER,
            PR_CAP_AMBIENT_RAISE, or PR_CAP_AMBIENT_IS_SET and arg3 does not
            specify a valid capability.
     ENXIO  option was PR_MPX_ENABLE_MANAGEMENT or PR_MPX_DISABLE_MANAGEMENT
            and the kernel or the  CPU  does  not  support  MPX  management.
            Check that the kernel and processor have MPX support.
     EOPNOTSUPP
            option  is PR_SET_FP_MODE and arg2 has an invalid or unsupported
            value.
     EPERM  option is PR_SET_SECUREBITS, and the caller does  not  have  the
            CAP_SETPCAP  capability,  or  tried to unset a "locked" flag, or
            tried to set a flag whose corresponding locked flag was set (see
            capabilities(7)).
     EPERM  option      is     PR_SET_KEEPCAPS,     and     the     caller's
            SECBIT_KEEP_CAPS_LOCKED flag is set (see capabilities(7)).
     EPERM  option is PR_CAPBSET_DROP, and the  caller  does  not  have  the
            CAP_SETPCAP capability.
     EPERM  option   is   PR_SET_MM,  and  the  caller  does  not  have  the
            CAP_SYS_RESOURCE capability.
     EPERM  option is PR_CAP_AMBIENT and arg2 is  PR_CAP_AMBIENT_RAISE,  but
            either  the  capability  specified in arg3 is not present in the
            process's permitted and  inheritable  capability  sets,  or  the
            PR_CAP_AMBIENT_LOWER securebit has been set.

VERSIONS

     The prctl() system call was introduced in Linux 2.1.57.

CONFORMING TO

     This  call  is  Linux-specific.   IRIX  has a prctl() system call (also
     introduced in Linux 2.1.44 as irix_prctl  on  the  MIPS  architecture),
     with prototype
         ptrdiff_t prctl(int option, int arg2, int arg3);
     and  options  to  get the maximum number of processes per user, get the
     maximum number of processors the calling  process  can  use,  find  out
     whether  a specified process is currently blocked, get or set the maxi-
     mum stack size, and so on.

SEE ALSO

     signal(2), core(5)

COLOPHON

     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
     https://www.kernel.org/doc/man-pages/.

Linux 2018-02-02 PRCTL(2)

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