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PRCTL(2)                   Linux Programmer's ManualPRCTL(2)NAME       prctl - operations on a processSYNOPSIS       #include <sys/prctl.h>       int prctl(int option, unsigned long arg2, unsigned long arg3,                 unsigned long arg4, unsigned long arg5);DESCRIPTION       prctl()  is  called  with  a first argument describing what to do (with       values defined in <linux/prctl.h>), and further arguments with  a  sig-       nificance depending on the first one.  The first argument can be:       PR_CAP_AMBIENT (since Linux 4.3)              Reads  or  changes  the  ambient  capability  set of the calling              thread, according to the value of arg2, which must be one of the              following:              PR_CAP_AMBIENT_RAISE                     The  capability specified in arg3 is added to the ambient                     set.  The specified capability must already be present in                     both  the  permitted  and  the  inheritable  sets  of the                     process.   This  operation  is  not  permitted   if   the                     SECBIT_NO_CAP_AMBIENT_RAISE securebit is set.              PR_CAP_AMBIENT_LOWER                     The  capability specified in arg3 is removed from the am-                     bient set.              PR_CAP_AMBIENT_IS_SET                     The prctl() call returns 1 if the capability in  arg3  is                     in the ambient set and 0 if it is not.              PR_CAP_AMBIENT_CLEAR_ALL                     All  capabilities  will  be removed from the ambient set.                     This operation requires setting arg3 to zero.              In all of the above operations, arg4 and arg5 must be  specified              as 0.              Higher-level  interfaces  layered on top of the above operations              are provided in thelibcap(3) library in the form of cap_get_am-bient(3),cap_set_ambient(3), andcap_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/ca-              pability.h>.)  The capability bounding set dictates whether  the              process  can  receive  the capability through a file's permitted              capability set on a subsequent call toexecve(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   thelibcap(3)   library   in   the   form   ofcap_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   thelibcap(3)   library   in   the   form   ofcap_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 ofinit(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  towait(2) on the process to discover its termination sta-              tus.              The setting of the "child subreaper" attribute is not  inherited              by  children  created  byfork(2) andclone(2).  The setting is              preserved acrossexecve(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).  Someinit(1)  frame-              works  (e.g.,systemd(1)) employ a subreaper process for similar              reasons.       PR_GET_CHILD_SUBREAPER (since Linux 3.4)              Return the "child subreaper" setting of the caller, in the loca-              tion pointed to by (int *) arg2.       PR_SET_DUMPABLE (since Linux 2.3.20)              Set  the  state of the "dumpable" flag, which determines whether              core dumps are produced for the calling process upon delivery of              a signal whose default behavior is to produce a core dump.              In  kernels  up  to  and including 2.6.12, arg2 must be either 0              (SUID_DUMP_DISABLE,   process   is   not    dumpable)    or    1              (SUID_DUMP_USER,  process  is dumpable).  Between kernels 2.6.13              and 2.6.17, the value 2 was also permitted, which caused any bi-              nary which normally would not be dumped to be dumped readable by              root only; for security reasons, this feature has been  removed.              (See  also  the  description  of  /proc/sys/fs/suid_dumpable  inproc(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 (seecredentials(7)).              *  The process executes (execve(2)) a set-user-ID or  set-group-                 ID  program,  resulting  in  a change of either the effective                 user ID or the effective group ID.              *  The process executes (execve(2)) a program that has file  ca-                 pabilities  (seecapabilities(7)), but only if the permitted                 capabilities gained exceed those already  permitted  for  the                 process.              Processes  that  are  not  dumpable  can  not  be  attached  viaptrace(2) PTRACE_ATTACH; seeptrace(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 inproc(5).       PR_GET_DUMPABLE (since Linux 2.3.20)              Return (as the function result) the current state of the calling              process's dumpable flag.       PR_SET_ENDIAN (since Linux 2.6.18, PowerPC only)              Set the endian-ness of the calling process to the value given in              arg2, which should  be  one  of  the  following:  PR_ENDIAN_BIG,              PR_ENDIAN_LITTLE, or PR_ENDIAN_PPC_LITTLE (PowerPC pseudo little              endian).       PR_GET_ENDIAN (since Linux 2.6.18, PowerPC only)              Return the endian-ness of the calling process, in  the  location              pointed to by (int *) arg2.       PR_SET_FP_MODE (since Linux 4.0, only on MIPS)              On  the MIPS architecture, user-space code can be built using an              ABI which permits linking with code that  has  more  restrictive              floating-point  (FP) requirements.  For example, user-space code              may be built to target the O32 FPXX ABI  and  linked  with  code              built  for either one of the more restrictive FP32 or FP64 ABIs.              When more restrictive code is linked in, the overall requirement              for  the  process  is to use the more restrictive floating-point              mode.              Because the kernel has no means of knowing in advance which mode              the  process  should  be executed in, and because these restric-              tions  can  change  over  the  lifetime  of  the  process,   the              PR_SET_FP_MODE  operation  is  provided  to allow control of the              floating-point mode from user space.              The (unsigned int) arg2 argument is a bit  mask  describing  the              floating-point mode used:              PR_FP_MODE_FR                     When  this bit is unset (so called FR=0 or FR0 mode), the                     32 floating-point registers are 32 bits wide, and  64-bit                     registers  are  represented as a pair of registers (even-                     and odd- numbered, with the even-numbered  register  con-                     taining  the lower 32 bits, and the odd-numbered register                     containing the higher 32 bits).                     When this bit is set  (on  supported  hardware),  the  32                     floating-point registers are 64 bits wide (so called FR=1                     or FR1 mode).   Note  that  modern  MIPS  implementations                     (MIPS R6 and newer) support FR=1 mode only.                     Applications  that  use the O32 FP32 ABI can operate only                     when this bit is unset (FR=0; or they can  be  used  with                     FRE  enabled,  see below).  Applications that use the O32                     FP64 ABI (and the O32 FP64A ABI, which exists to  provide                     the  ability  to operate with existing FP32 code; see be-                     low) can operate only when this bit is set  (FR=1).   Ap-                     plications that use the O32 FPXX ABI can operate with ei-                     ther FR=0 or FR=1.              PR_FP_MODE_FRE                     Enable emulation of  32-bit  floating-point  mode.   When                     this  mode  is enabled, it emulates 32-bit floating-point                     operations by raising a reserved-instruction exception on                     every instruction that uses 32-bit formats and the kernel                     then handles the instruction in software.   (The  problem                     lies  in  the discrepancy of handling odd-numbered regis-                     ters which are the high 32 bits of 64-bit registers  with                     even  numbers  in FR=0 mode and the lower 32-bit parts of                     odd-numbered 64-bit registers in  FR=1  mode.)   Enabling                     this  bit  is  necessary  when code with the O32 FP32 ABI                     should operate with code with compatible the O32 FPXX  or                     O32  FP64A  ABIs (which require FR=1 FPU mode) or when it                     is executed on newer hardware  (MIPS  R6  onwards)  which                     lacks  FR=0  mode support when a binary with the FP32 ABI                     is used.                     Note that this mode makes sense only when the FPU  is  in                     64-bit mode (FR=1).                     Note  that the use of emulation inherently has a signifi-                     cant performance hit and should be avoided if possible.              In the N32/N64 ABI, 64-bit floating-point mode is  always  used,              so  FPU emulation is not required and the FPU always operates in              FR=1 mode.              This option is mainly intended for use  by  the  dynamic  linker              (ld.so(8)).              The arguments arg3, arg4, and arg5 are ignored.       PR_GET_FP_MODE (since Linux 4.0, only on MIPS)              Return  (as the function result) 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 of this flag is described incapabilities(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              toexecve(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.  Seecapabilities(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; seeproc(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.   Seesigaction(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  (as the function result) the current per-process machine              check kill policy.  All unused prctl() arguments must be zero.       PR_SET_MM (since Linux 3.3)              Modify certain kernel memory map descriptor fields of the  call-              ing process.  Usually these fields are set by the kernel and dy-              namic loader (seeld.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.              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 (seemprotect(2)                     andmmap(2) for more information).              PR_SET_MM_END_CODE                     Set the address below which the  program  text  can  run.                     The  corresponding  memory area must be readable and exe-                     cutable, but not writable or shareable.              PR_SET_MM_START_DATA                     Set the address above which initialized and uninitialized                     (bss)  data  are  placed.   The corresponding memory area                     must be readable and  writable,  but  not  executable  or                     shareable.              PR_SET_MM_END_DATA                     Set the address below which initialized and uninitialized                     (bss) data are placed.   The  corresponding  memory  area                     must  be  readable  and  writable,  but not executable or                     shareable.              PR_SET_MM_START_STACK                     Set the start address of the  stack.   The  corresponding                     memory area must be readable and writable.              PR_SET_MM_START_BRK                     Set  the  address above which the program heap can be ex-                     panded withbrk(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 (seesetrlimit(2)).              PR_SET_MM_BRK                     Set the currentbrk(2) value.  The requirements  for  the                     address  are  the same as for the PR_SET_MM_START_BRK op-                     tion.              The following options are available since Linux 3.5.              PR_SET_MM_ARG_START                     Set the address above which the program command  line  is                     placed.              PR_SET_MM_ARG_END                     Set  the  address below which the program command line is                     placed.              PR_SET_MM_ENV_START                     Set the address above which the  program  environment  is                     placed.              PR_SET_MM_ENV_END                     Set  the  address  below which the program environment is                     placed.                     The    address    passed    with     PR_SET_MM_ARG_START,                     PR_SET_MM_ARG_END,        PR_SET_MM_ENV_START,        and                     PR_SET_MM_ENV_END should belong to a process stack  area.                     Thus,  the  corresponding  memory  area must be readable,                     writable, and (depending  on  the  kernel  configuration)                     have the MAP_GROWSDOWN attribute set (seemmap(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 regularopen(2) call.                     To change the symbolic link, one needs to unmap  all  ex-                     isting  executable  memory areas, including those created                     by the kernel itself (for example the kernel usually cre-                     ates  at  least  one  executable  memory area for the ELF                     .text section).                     In Linux 4.9 and earlier, the  PR_SET_MM_EXE_FILE  opera-                     tion  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 oper-                     ation 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  un-                     signed int.                     This  feature  is  available  only if the kernel is built                     with the CONFIG_CHECKPOINT_RESTORE option enabled.       PR_MPX_ENABLE_MANAGEMENT, PR_MPX_DISABLE_MANAGEMENT (since Linux 3.19)              Enable or disable kernel management of Memory Protection  eXten-              sions (MPX) bounds tables.  The arg2, arg3, arg4, and arg5 argu-              ments must be zero.              MPX is  a  hardware-assisted  mechanism  for  performing  bounds              checking on pointers.  It consists of a set of registers storing              bounds information and a set  of  special  instruction  prefixes              that  tell the CPU on which instructions it should do bounds en-              forcement.  There is a limited number  of  these  registers  and              when there are more pointers than registers, their contents must              be "spilled" into a set of  tables.   These  tables  are  called              "bounds  tables"  and the MPX prctl() operations control whether              the kernel manages their allocation and freeing.              When management is enabled, the kernel will take over allocation              and  freeing of the bounds tables.  It does this by trapping the              #BR exceptions that result at first use of missing bounds tables              and  instead of delivering the exception to user space, it allo-              cates the table and populates the bounds directory with the  lo-              cation  of the new table.  For freeing, the kernel checks to see              if bounds tables are present for memory which is not  allocated,              and frees them if so.              Before  enabling  MPX management using PR_MPX_ENABLE_MANAGEMENT,              the application must first have allocated  a  user-space  buffer              for  the bounds directory and placed the location of that direc-              tory in the bndcfgu register.              These calls fail if the CPU or  kernel  does  not  support  MPX.              Kernel  support  for MPX is enabled via the CONFIG_X86_INTEL_MPX              configuration option.  You can check whether  the  CPU  supports              MPX  by looking for the 'mpx' CPUID bit, like with the following              command:                  cat /proc/cpuinfo | grep ' mpx '              A thread may not switch in or out of long  (64-bit)  mode  while              MPX is enabled.              All threads in a process are affected by these calls.              The  child  of  afork(2) inherits the state of MPX management.              Duringexecve(2), MPX management is  reset  to  a  state  as  if              PR_MPX_DISABLE_MANAGEMENT had been called.              For further information on Intel MPX, see the kernel source file              Documentation/x86/intel_mpx.txt.       PR_SET_NAME (since Linux 2.6.9)              Set the name of the calling thread, using the value in the loca-              tion  pointed  to  by  (char *)  arg2.  The name can be up to 16              bytes long, including the terminating null byte.  (If the length              of  the  string, including the terminating null byte, exceeds 16              bytes, the string is silently truncated.)  This is the same  at-              tribute  that can be set viapthread_setname_np(3) and retrieved              usingpthread_getname_np(3).  The attribute is likewise accessi-              ble via /proc/self/task/[tid]/comm, where tid is the name of the              calling thread.       PR_GET_NAME (since Linux 2.6.11)              Return the name of the calling thread, in the buffer pointed  to              by  (char *)  arg2.   The buffer should allow space for up to 16              bytes; the returned string will be null-terminated.       PR_SET_NO_NEW_PRIVS (since Linux 3.5)              Set the calling thread's no_new_privs 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 theexecve(2) call (for example, rendering the set-user-              ID and set-group-ID mode bits, and file  capabilities  non-func-              tional).   Once  set,  this the no_new_privs attribute cannot be              unset.  The setting of this attribute is inherited  by  children              created byfork(2) andclone(2), and preserved acrossexecve(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 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              attribute  for  the  calling thread.  A value of 0 indicates the              regularexecve(2) behavior.  A value of  1  indicatesexecve(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 (ei-              ther a signal value in the range  1..maxsig,  or  0  to  clear).              This  is  the  signal that the calling process will get when its              parent dies.              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 (seesignal(7)) and,              if  the  child installs a handler using thesigaction(2) SA_SIG-              INFO 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 afork(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  (seecapabilities(7));  otherwise,              this value is preserved acrossexecve(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/ker-              nel/yama/ptrace_scope).  When a "ptracer process ID"  is  passed              in  arg2,  the  caller is declaring that the ptracer process canptrace(2) the calling process as if it were a direct process an-              cestor.   Each  PR_SET_PTRACER  operation  replaces the previous              "ptracer process ID".  Employing PR_SET_PTRACER with arg2 set to              0  clears  the  caller's  "ptracer  process  ID".   If  arg2  is              PR_SET_PTRACER_ANY, the ptrace restrictions introduced  by  Yama              are effectively disabled for the calling process.              For  further  information, see the kernel source file Documenta-              tion/admin-guide/LSM/Yama.rst      (or       Documentation/secu-              rity/Yama.txt before Linux 4.13).       PR_SET_SECCOMP (since Linux 2.6.23)              Set  the secure computing (seccomp) mode for the calling thread,              to limit the available system calls.  The more recentseccomp(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 areread(2),write(2),_exit(2)              (but  notexit_group(2)), andsigreturn(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 permitfork(2), then the seccomp              mode is inherited by children created byfork(2);  ifexecve(2)              is  permitted,  then  the  seccomp  mode is preserved across ex-ecve(2).  If the filters permit prctl() calls,  then  additional              filters can be added; they are run in order until the first non-              allow result is seen.              For further information, see the kernel source  file  Documenta-              tion/userspace-api/seccomp_filter.rst       (or       Documenta-              tion/prctl/seccomp_filter.txt before Linux 4.13).       PR_GET_SECCOMP (since Linux 2.6.23)              Return (as the function result) the secure computing mode of the              calling  thread.  If the caller is not in secure computing mode,              this operation returns 0; if the caller is in strict secure com-              puting  mode,  then the prctl() call will cause a SIGKILL signal              to be sent to the process.  If the caller is in filter mode, and              this  system  call is allowed by the seccomp filters, it returns              2; otherwise, the process is killed with a SIGKILL signal.  This              operation  is  available  only  if the kernel is configured with              CONFIG_SECCOMP enabled.              Since Linux 3.8, the Seccomp  field  of  the  /proc/[pid]/status              file  provides a method of obtaining the same information, with-              out the risk that the process is killed; seeproc(5).       PR_SET_SECUREBITS (since Linux 2.6.26)              Set the "securebits" flags of the calling thread  to  the  value              supplied in arg2.  Seecapabilities(7).       PR_GET_SECUREBITS (since Linux 2.6.26)              Return  (as  the  function result) the "securebits" flags of the              calling thread.  Seecapabilities(7).       PR_GET_SPECULATION_CTRL (since Linux 4.17)              Return (as the function result) 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_SPECU-                     LATION_CTRL              PR_SPEC_ENABLE                     The speculation feature is enabled,  mitigation  is  dis-                     abled.              PR_SPEC_DISABLE                     The  speculation  feature  is disabled, mitigation is en-                     abled              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  specula-              tion misfeature.              If  PR_SPEC_PRCTL is set, then per-thread control of the mitiga-              tion is available.  If not set, prctl() for the speculation mis-              feature will fail.              The  arg3, arg4, and arg5 arguments must be specified as 0; oth-              erwise 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 ar-              gument is used to hand in the control value, which is one of the              following:              PR_SPEC_ENABLE                     The  speculation  feature  is enabled, mitigation is dis-                     abled.              PR_SPEC_DISABLE                     The speculation feature is disabled,  mitigation  is  en-                     abled              PR_SPEC_FORCE_DISABLE                     Same  as  PR_SPEC_DISABLE but cannot be undone.  A subse-                     quent 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() call              failing with the error ENXIO.  For further details, see the ker-              nel 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  modi-              fied,  and  using a malloc hook withmadvise(2) is not an option              (i.e., statically allocated data).  The setting of the "THP dis-              able"  flag  is  inherited by a child created viafork(2) and is              preserved acrossexecve(2).       PR_TASK_PERF_EVENTS_DISABLE (since Linux 2.6.31)              Disable  all  performance  counters  attached  to  the   calling              process, regardless of whether the counters were created by this              process or another process.  Performance counters created by the              calling  process  for  other processes are unaffected.  For more              information on performance counters, see the Linux kernel source              file tools/perf/design.txt.              Originally  called  PR_TASK_PERF_COUNTERS_DISABLE;  renamed (re-              taining the same numerical value) in Linux 2.6.32.       PR_TASK_PERF_EVENTS_ENABLE (since Linux 2.6.31)              The converse of PR_TASK_PERF_EVENTS_DISABLE; enable  performance              counters attached to the calling process.              Originally called PR_TASK_PERF_COUNTERS_ENABLE; renamed in Linux              2.6.32.       PR_GET_THP_DISABLE (since Linux 3.15)              Return (as 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)              Return the clear_child_tid address set byset_tid_address(2) and              theclone(2) CLONE_CHILD_CLEARTID flag, in the location pointed              to by (int **) arg2.  This feature is available only if the ker-              nel  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 ker-              nel 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  an-              other; as a consequence, timer expirations for the thread may be              up to the specified number of nanoseconds late (but  will  never              expire  early).  Grouping timer expirations can help reduce sys-              tem power consumption by minimizing CPU wake-ups.              The timer expirations affected by timer slack are those  set  byselect(2),pselect(2),poll(2),ppoll(2),epoll_wait(2),epoll_pwait(2),clock_nanosleep(2),nanosleep(2),  andfutex(2)              (and thus the library functions implemented via futexes, includ-              ingpthread_cond_timedwait(3),pthread_mutex_timedlock(3),pthread_rwlock_timedrdlock(3),pthread_rwlock_timedwrlock(3),              andsem_timedwait(3)).              Timer slack is not applied to threads that are scheduled under a              real-time scheduling policy (seesched_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  value is inherited by a child created viafork(2), and is              preserved acrossexecve(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.  Seeproc(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_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)              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_FP_MODE,   PR_GET_KEEPCAPS,       PR_GET_NO_NEW_PRIVS, PR_GET_THP_DISABLE,  PR_CAPBSET_READ,  PR_GET_TIM-       ING,   PR_GET_TIMERSLACK,  PR_GET_SECUREBITS,  PR_GET_SPECULATION_CTRL,       PR_MCE_KILL_GET, PR_CAP_AMBIENT+PR_CAP_AMBIENT_IS_SET, and (if  it  re-       turns)  PR_GET_SECCOMP  return  the nonnegative values described above.       All other option values return 0 on success.  On error, -1 is returned,       and errno is set appropriately.ERRORS       EACCES option  is  PR_SET_SECCOMP  and arg2 is SECCOMP_MODE_FILTER, but              the process does not have the CAP_SYS_ADMIN  capability  or  has              not  set  the  no_new_privs  attribute  (see  the  discussion of              PR_SET_NO_NEW_PRIVS above).       EACCES option is PR_SET_MM, and arg3 is PR_SET_MM_EXE_FILE, the file is              not executable.       EBADF  option  is  PR_SET_MM,  arg3 is PR_SET_MM_EXE_FILE, and the file              descriptor passed in arg4 is not valid.       EBUSY  option is PR_SET_MM, arg3 is PR_SET_MM_EXE_FILE,  and  this  the              second  attempt to change the /proc/pid/exe symbolic link, which              is prohibited.       EFAULT arg2 is an invalid address.       EFAULT option is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER, the  sys-              tem was built with CONFIG_SECCOMP_FILTER, and arg3 is an invalid              address.       EINVAL The value of option is not recognized.       EINVAL option is PR_MCE_KILL or PR_MCE_KILL_GET or PR_SET_MM,  and  un-              used prctl() arguments were not specified as zero.       EINVAL arg2 is not valid value for this option.       EINVAL option  is  PR_SET_SECCOMP or PR_GET_SECCOMP, and the kernel was              not configured with CONFIG_SECCOMP.       EINVAL option is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER,  and  the              kernel was not configured with CONFIG_SECCOMP_FILTER.       EINVAL option is PR_SET_MM, and one of the following is true              *  arg4 or arg5 is nonzero;              *  arg3  is greater than TASK_SIZE (the limit on the size of the                 user address space for this architecture);              *  arg2     is     PR_SET_MM_START_CODE,     PR_SET_MM_END_CODE,                 PR_SET_MM_START_DATA,          PR_SET_MM_END_DATA,         or                 PR_SET_MM_START_STACK, and the permissions of the correspond-                 ing memory area are not as required;              *  arg2  is  PR_SET_MM_START_BRK  or  PR_SET_MM_BRK, and arg3 is                 less than or equal to the end of the data segment  or  speci-                 fies  a value that would cause the RLIMIT_DATA resource limit                 to be exceeded.       EINVAL option is PR_SET_PTRACER and arg2 is not 0,  PR_SET_PTRACER_ANY,              or the PID of an existing process.       EINVAL option  is  PR_SET_PDEATHSIG and arg2 is not a valid signal num-              ber.       EINVAL option is PR_SET_DUMPABLE and arg2 is neither  SUID_DUMP_DISABLE              nor SUID_DUMP_USER.       EINVAL option is PR_SET_TIMING and arg2 is not PR_TIMING_STATISTICAL.       EINVAL option  is  PR_SET_NO_NEW_PRIVS  and  arg2  is not equal to 1 or              arg3, arg4, or arg5 is nonzero.       EINVAL option is PR_GET_NO_NEW_PRIVS and arg2, arg3, arg4, or  arg5  is              nonzero.       EINVAL option is PR_SET_THP_DISABLE and arg3, arg4, or arg5 is nonzero.       EINVAL option  is  PR_GET_THP_DISABLE  and arg2, arg3, arg4, or arg5 is              nonzero.       EINVAL option is PR_CAP_AMBIENT and an unused argument (arg4, arg5, or,              in  the  case  of PR_CAP_AMBIENT_CLEAR_ALL, arg3) is nonzero; or              arg2 has an invalid  value;  or  arg2  is  PR_CAP_AMBIENT_LOWER,              PR_CAP_AMBIENT_RAISE, or PR_CAP_AMBIENT_IS_SET and arg3 does not              specify a valid capability.       EINVAL option was  PR_GET_SPECULATION_CTRL  or  PR_SET_SPECULATION_CTRL              and unused arguments to prctl() are not 0.       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 (seecapabilities(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 (seecapabilities(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.VERSIONS       The prctl() system call was introduced in Linux 2.1.57.CONFORMING TO       This call is Linux-specific.  IRIX has a prctl() system call (also  in-       troduced  in Linux 2.1.44 as irix_prctl on the MIPS architecture), with       prototype           ptrdiff_t prctl(int option, int arg2, int arg3);       and options to get the maximum number of processes per  user,  get  the       maximum  number  of  processors  the  calling process can use, find out       whether a specified process is currently blocked, get or set the  maxi-       mum stack size, and so on.SEE ALSOsignal(2),core(5)COLOPHON       This  page  is  part of release 5.05 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                             2019-08-02PRCTL(2)