seccomp - operate on Secure Computing state of the process


   #include <linux/seccomp.h>
   #include <linux/filter.h>
   #include <linux/audit.h>
   #include <linux/signal.h>
   #include <sys/ptrace.h>

   int seccomp(unsigned int operation, unsigned int flags, void *args);


   The  seccomp()  system  call operates on the Secure Computing (seccomp)
   state of the calling process.

   Currently, Linux supports the following operation values:

          The only system calls that the calling thread  is  permitted  to
          make  are  read(2),  write(2), _exit(2) (but not exit_group(2)),
          and sigreturn(2).  Other system calls result in the delivery  of
          a  SIGKILL  signal.   Strict secure computing mode is useful for
          number-crunching applications that may need to execute untrusted
          byte code, perhaps obtained by reading from a pipe or socket.

          Note  that  although  the  calling  thread  can  no  longer call
          sigprocmask(2), it can use sigreturn(2)  to  block  all  signals
          apart  from  SIGKILL and SIGSTOP.  This means that alarm(2) (for
          example)  is  not  sufficient  for  restricting  the   process's
          execution  time.   Instead,  to  reliably terminate the process,
          SIGKILL must be used.  This can be done by using timer_create(2)
          with  SIGEV_SIGNAL  and  sigev_signo set to SIGKILL, or by using
          setrlimit(2) to set the hard limit for RLIMIT_CPU.

          This operation is available only if  the  kernel  is  configured
          with CONFIG_SECCOMP enabled.

          The value of flags must be 0, and args must be NULL.

          This operation is functionally identical to the call:


          The  system calls allowed are defined by a pointer to a Berkeley
          Packet Filter (BPF) passed via args.  This argument is a pointer
          to  a  struct sock_fprog; it can be designed to filter arbitrary
          system calls and  system  call  arguments.   If  the  filter  is
          invalid, seccomp() fails, returning EINVAL in errno.

          If  fork(2)  or  clone(2)  is  allowed  by the filter, any child
          processes will be constrained to the same system call filters as
          the  parent.  If execve(2) is allowed, the existing filters will
          be preserved across a call to execve(2).

          In order to use the  SECCOMP_SET_MODE_FILTER  operation,  either
          the  caller  must  have the CAP_SYS_ADMIN capability in its user
          namespace, or the thread must already have the no_new_privs  bit
          set.   If  that  bit  was not already set by an ancestor of this
          thread, the thread must make the following call:

              prctl(PR_SET_NO_NEW_PRIVS, 1);

          Otherwise, the SECCOMP_SET_MODE_FILTER operation will  fail  and
          return  EACCES  in  errno.   This  requirement  ensures  that an
          unprivileged process cannot apply a malicious  filter  and  then
          invoke   a   set-user-ID   or  other  privileged  program  using
          execve(2), thus potentially compromising that program.  (Such  a
          malicious  filter  might,  for  example, cause an attempt to use
          setuid(2) to set the caller's user IDs  to  non-zero  values  to
          instead return 0 without actually making the system call.  Thus,
          the program might be tricked into retaining superuser privileges
          in  circumstances  where  it  is  possible to influence it to do
          dangerous things because it did not actually drop privileges.)

          If prctl(2) or seccomp() is  allowed  by  the  attached  filter,
          further  filters  may  be  added.  This will increase evaluation
          time, but allows for further reduction  of  the  attack  surface
          during execution of a thread.

          The  SECCOMP_SET_MODE_FILTER  operation is available only if the
          kernel is configured with CONFIG_SECCOMP_FILTER enabled.

          When flags is 0, this operation is functionally identical to the

              prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args);

          The recognized flags are:

                 When  adding  a new filter, synchronize all other threads
                 of the calling process to the same seccomp  filter  tree.
                 A  "filter  tree" is the ordered list of filters attached
                 to a thread.  (Attaching identical  filters  in  separate
                 seccomp()  calls  results  in different filters from this

                 If any thread cannot synchronize to the same filter tree,
                 the call will not attach the new seccomp filter, and will
                 fail, returning the first thread  ID  found  that  cannot
                 synchronize.  Synchronization will fail if another thread
                 in the same process is in SECCOMP_MODE_STRICT  or  if  it
                 has  attached  new  seccomp  filters to itself, diverging
                 from the calling thread's filter tree.

   When adding filters  via  SECCOMP_SET_MODE_FILTER,  args  points  to  a
   filter program:

       struct sock_fprog {
           unsigned short      len;    /* Number of BPF instructions */
           struct sock_filter *filter; /* Pointer to array of
                                          BPF instructions */

   Each program must contain one or more BPF instructions:

       struct sock_filter {            /* Filter block */
           __u16 code;                 /* Actual filter code */
           __u8  jt;                   /* Jump true */
           __u8  jf;                   /* Jump false */
           __u32 k;                    /* Generic multiuse field */

   When executing the instructions, the BPF program operates on the system
   call information made available (i.e., use the BPF_ABS addressing mode)
   as a (read-only) buffer of the following form:

       struct seccomp_data {
           int   nr;                   /* System call number */
           __u32 arch;                 /* AUDIT_ARCH_* value
                                          (see <linux/audit.h>) */
           __u64 instruction_pointer;  /* CPU instruction pointer */
           __u64 args[6];              /* Up to 6 system call arguments */

   Because numbering of system calls varies between architectures and some
   architectures (e.g., x86-64) allow user-space code to use  the  calling
   conventions  of  multiple  architectures,  it  is  usually necessary to
   verify the value of the arch field.

   It is strongly recommended to  use  a  whitelisting  approach  whenever
   possible  because  such  an  approach  is  more  robust  and simple.  A
   blacklist will have to be  updated  whenever  a  potentially  dangerous
   system  call  is  added  (or  a  dangerous  flag or option if those are
   blacklisted), and it is often possible to alter the representation of a
   value without altering its meaning, leading to a blacklist bypass.

   The  arch  field is not unique for all calling conventions.  The x86-64
   ABI and the x32 ABI both use AUDIT_ARCH_X86_64 as arch, and they run on
   the  same  processors.   Instead, the mask __X32_SYSCALL_BIT is used on
   the system call number to tell the two ABIs apart.

   This means that in order to create a seccomp-based blacklist for system
   calls  performed  through  the  x86-64 ABI, it is necessary to not only
   check that arch equals AUDIT_ARCH_X86_64, but also to explicitly reject
   all system calls that contain __X32_SYSCALL_BIT in nr.

   The  instruction_pointer  field  provides  the  address of the machine-
   language instruction that performed the system  call.   This  might  be
   useful  in  conjunction  with  the  use  of /proc/[pid]/maps to perform
   checks based on which region (mapping) of the program made  the  system
   call.   (Probably,  it is wise to lock down the mmap(2) and mprotect(2)
   system calls to prevent the program from subverting such checks.)

   When checking values from args against a blacklist, keep in  mind  that
   arguments  are  often  silently  truncated  before being processed, but
   after the seccomp check.  For example, this happens if the i386 ABI  is
   used  on  an  x86-64 kernel: although the kernel will normally not look
   beyond the 32 lowest bits of the arguments,  the  values  of  the  full
   64-bit  registers  will  be  present  in  the  seccomp  data.   A  less
   surprising example is that if the x86-64  ABI  is  used  to  perform  a
   system  call  that  takes an argument of type int, the more-significant
   half of the argument register  is  ignored  by  the  system  call,  but
   visible in the seccomp data.

   A  seccomp  filter  returns a 32-bit value consisting of two parts: the
   most significant 16 bits (corresponding to  the  mask  defined  by  the
   constant  SECCOMP_RET_ACTION) contain one of the "action" values listed
   below;  the  least  significant  16-bits  (defined  by   the   constant
   SECCOMP_RET_DATA) are "data" to be associated with this return value.

   If  multiple  filters exist, they are all executed, in reverse order of
   their addition to the filter tree---that is, the most recently  installed
   filter  is  executed first.  (Note that all filters will be called even
   if one of the earlier filters returns SECCOMP_RET_KILL.  This  is  done
   to  simplify  the  kernel  code  and  to provide a tiny speed-up in the
   execution of sets of filters by avoiding  a  check  for  this  uncommon
   case.)   The  return value for the evaluation of a given system call is
   the first-seen SECCOMP_RET_ACTION value of  highest  precedence  (along
   with  its  accompanying  data)  returned  by  execution  of  all of the

   In decreasing order of precedence, the values that may be returned by a
   seccomp filter are:

          This  value  results  in the process exiting immediately without
          executing the system call.  The  process  terminates  as  though
          killed by a SIGSYS signal (not SIGKILL).

          This  value results in the kernel sending a SIGSYS signal to the
          triggering process without executing the system  call.   Various
          fields will be set in the siginfo_t structure (see sigaction(2))
          associated with signal:

          *  si_signo will contain SIGSYS.

          *  si_call_addr  will  show  the  address  of  the  system  call

          *  si_syscall  and  si_arch  will indicate which system call was

          *  si_code will contain SYS_SECCOMP.

          *  si_errno will contain the  SECCOMP_RET_DATA  portion  of  the
             filter return value.

          The  program  counter will be as though the system call happened
          (i.e., it will not point to the system call  instruction).   The
          return  value  register  will  contain an architecture-dependent
          value; if resuming execution, set it  to  something  appropriate
          for  the  system  call.  (The architecture dependency is because
          replacing  it  with   ENOSYS   could   overwrite   some   useful

          This  value  results  in  the  SECCOMP_RET_DATA  portion  of the
          filter's return value being passed to user space  as  the  errno
          value without executing the system call.

          When  returned,  this  value will cause the kernel to attempt to
          notify a ptrace(2)-based tracer prior to  executing  the  system
          call.   If  there  is  no tracer present, the system call is not
          executed and returns a failure status with errno set to ENOSYS.

          A tracer will be notified if it  requests  PTRACE_O_TRACESECCOMP
          using ptrace(PTRACE_SETOPTIONS).  The tracer will be notified of
          a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion  of  the
          filter's  return  value  will  be  available  to  the tracer via

          The tracer can skip the system call by changing the system  call
          number  to  -1.  Alternatively, the tracer can change the system
          call requested by changing the system call  to  a  valid  system
          call  number.   If the tracer asks to skip the system call, then
          the system call will appear to return the value that the  tracer
          puts in the return value register.

          Before kernel 4.8, the seccomp check will not be run again after
          the tracer is notified.  (This means  that,  on  older  kernels,
          seccomp-based  sandboxes must not allow use of ptrace(2)---even of
          other sandboxed processes---without extreme care; ptracers can use
          this mechanism to escape from the seccomp sandbox.)

          This value results in the system call being executed.


   On     success,     seccomp()     returns     0.     On    error,    if
   SECCOMP_FILTER_FLAG_TSYNC was used, the return value is the ID  of  the
   thread  that  caused the synchronization failure.  (This ID is a kernel
   thread ID of the type returned by clone(2) and  gettid(2).)   On  other
   errors,  -1  is returned, and errno is set to indicate the cause of the


   seccomp() can fail for the following reasons:

          The caller did not have the CAP_SYS_ADMIN capability in its user
          namespace,   or   had   not   set   no_new_privs   before  using

   EFAULT args was not a valid address.

   EINVAL operation is  unknown;  or  flags  are  invalid  for  the  given

   EINVAL operation  included  BPF_ABS,  but  the specified offset was not
          aligned     to     a     32-bit     boundary     or     exceeded
          sizeof(struct seccomp_data).

   EINVAL A  secure  computing  mode  has  already been set, and operation
          differs from the existing setting.

   EINVAL operation specified SECCOMP_SET_MODE_FILTER, but the kernel  was
          not built with CONFIG_SECCOMP_FILTER enabled.

   EINVAL operation  specified  SECCOMP_SET_MODE_FILTER,  but  the  filter
          program pointed to by args was not valid or the  length  of  the
          filter   program   was  zero  or  exceeded  BPF_MAXINSNS  (4096)

   ENOMEM Out of memory.

   ENOMEM The total length of all filter programs attached to the  calling
          thread  would  exceed  MAX_INSNS_PER_PATH  (32768) instructions.
          Note that for the  purposes  of  calculating  this  limit,  each
          already  existing filter program incurs an overhead penalty of 4

   ESRCH  Another thread caused a failure during thread sync, but  its  ID
          could not be determined.


   The seccomp() system call first appeared in Linux 3.17.


   The seccomp() system call is a nonstandard Linux extension.


   Rather  than hand-coding seccomp filters as shown in the example below,
   you may prefer to employ  the  libseccomp  library,  which  provides  a
   front-end for generating seccomp filters.

   The  Seccomp  field of the /proc/[pid]/status file provides a method of
   viewing the seccomp mode of a process; see proc(5).

   seccomp() provides a superset of  the  functionality  provided  by  the
   prctl(2) PR_SET_SECCOMP operation (which does not support flags).

   Since  Linux  4.4, the prctl(2) PTRACE_SECCOMP_GET_FILTER operation can
   be used to dump a process's seccomp filters.

   Seccomp-specific BPF details
   Note the following BPF details specific to seccomp filters:

   *  The BPF_H and BPF_B size modifiers are not supported: all operations
      must load and store (4-byte) words (BPF_W).

   *  To  access  the contents of the seccomp_data buffer, use the BPF_ABS
      addressing mode modifier.

   *  The BPF_LEN  addressing  mode  modifier  yields  an  immediate  mode
      operand whose value is the size of the seccomp_data buffer.


   The  program  below  accepts  four  or more arguments.  The first three
   arguments are a system call number, a numeric architecture  identifier,
   and  an error number.  The program uses these values to construct a BPF
   filter that is used at run time to perform the following checks:

   [1] If the program is not running on the  specified  architecture,  the
       BPF filter causes system calls to fail with the error ENOSYS.

   [2] If  the  program  attempts  to  execute  the  system  call with the
       specified number, the BPF filter causes the system  call  to  fail,
       with errno being set to the specified error number.

   The   remaining   command-line   arguments  specify  the  pathname  and
   additional arguments of a  program  that  the  example  program  should
   attempt  to execute using execv(3) (a library function that employs the
   execve(2) system call).  Some example runs of  the  program  are  shown

   First,  we display the architecture that we are running on (x86-64) and
   then construct a shell function that looks up system  call  numbers  on
   this architecture:

       $ uname -m
       $ syscall_nr() {
           cat /usr/src/linux/arch/x86/syscalls/syscall_64.tbl | \
           awk '$2 != "x32" && $3 == "'$1'" { print $1 }'

   When  the  BPF filter rejects a system call (case [2] above), it causes
   the system call to fail with the error number specified on the  command
   line.  In the experiments shown here, we'll use error number 99:

       $ errno 99
       EADDRNOTAVAIL 99 Cannot assign requested address

   In  the following example, we attempt to run the command whoami(1), but
   the BPF filter rejects the execve(2) system call, so that  the  command
   is not even executed:

       $ syscall_nr execve
       $ ./a.out
       Usage: ./a.out <syscall_nr> <arch> <errno> <prog> [<args>]
       Hint for <arch>: AUDIT_ARCH_I386: 0x40000003
                        AUDIT_ARCH_X86_64: 0xC000003E
       $ ./a.out 59 0xC000003E 99 /bin/whoami
       execv: Cannot assign requested address

   In  the  next example, the BPF filter rejects the write(2) system call,
   so that, although it is successfully started, the whoami(1) command  is
   not able to write output:

       $ syscall_nr write
       $ ./a.out 1 0xC000003E 99 /bin/whoami

   In  the final example, the BPF filter rejects a system call that is not
   used by the whoami(1) command, so it is able  to  successfully  execute
   and produce output:

       $ syscall_nr preadv
       $ ./a.out 295 0xC000003E 99 /bin/whoami

   Program source
   #include <errno.h>
   #include <stddef.h>
   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <linux/audit.h>
   #include <linux/filter.h>
   #include <linux/seccomp.h>
   #include <sys/prctl.h>

   #define X32_SYSCALL_BIT 0x40000000

   static int
   install_filter(int syscall_nr, int t_arch, int f_errno)
       unsigned int upper_nr_limit = 0xffffffff;

       /* Assume that AUDIT_ARCH_X86_64 means the normal x86-64 ABI */
       if (t_arch == AUDIT_ARCH_X86_64)
           upper_nr_limit = X32_SYSCALL_BIT - 1;

       struct sock_filter filter[] = {
           /* [0] Load architecture from 'seccomp_data' buffer into
                  accumulator */
           BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                    (offsetof(struct seccomp_data, arch))),

           /* [1] Jump forward 5 instructions if architecture does not
                  match 't_arch' */
           BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, t_arch, 0, 5),

           /* [2] Load system call number from 'seccomp_data' buffer into
                  accumulator */
           BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                    (offsetof(struct seccomp_data, nr))),

           /* [3] Check ABI - only needed for x86-64 in blacklist use
                  cases.  Use JGT instead of checking against the bit
                  mask to avoid having to reload the syscall number. */
           BPF_JUMP(BPF_JMP | BPF_JGT | BPF_K, upper_nr_limit, 3, 0),

           /* [4] Jump forward 1 instruction if system call number
                  does not match 'syscall_nr' */
           BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, syscall_nr, 0, 1),

           /* [5] Matching architecture and system call: don't execute
               the system call, and return 'f_errno' in 'errno' */
           BPF_STMT(BPF_RET | BPF_K,
                    SECCOMP_RET_ERRNO | (f_errno & SECCOMP_RET_DATA)),

           /* [6] Destination of system call number mismatch: allow other
                  system calls */

           /* [7] Destination of architecture mismatch: kill process */

       struct sock_fprog prog = {
           .len = (unsigned short) (sizeof(filter) / sizeof(filter[0])),
           .filter = filter,

       if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog)) {
           return 1;

       return 0;

   main(int argc, char **argv)
       if (argc < 5) {
           fprintf(stderr, "Usage: "
                   "%s <syscall_nr> <arch> <errno> <prog> [<args>]\n"
                   "Hint for <arch>: AUDIT_ARCH_I386: 0x%X\n"
                   "                 AUDIT_ARCH_X86_64: 0x%X\n"
                   "\n", argv[0], AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);

       if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {

       if (install_filter(strtol(argv[1], NULL, 0),
                          strtol(argv[2], NULL, 0),
                          strtol(argv[3], NULL, 0)))

       execv(argv[4], &argv[4]);


   bpf(2),   prctl(2),   ptrace(2),   sigaction(2),   proc(5),  signal(7),

   Various    pages    from    the    libseccomp    library,    including:
   scmp_sys_resolver(1),         seccomp_init(3),         seccomp_load(3),
   seccomp_rule_add(3), and seccomp_export_bpf(3).

   The  kernel  source   files   Documentation/networking/filter.txt   and

   McCanne,  S.  and  Jacobson,  V.  (1992)  The  BSD Packet Filter: A New
   Architecture for User-level Packet Capture, Proceedings of  the  USENIX
   Winter 1993 Conference


   This  page  is  part of release 4.09 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


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