seccomp(2)



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

NAME
       seccomp - operate on Secure Computing state of the process

SYNOPSIS
       #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);

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

       Currently, Linux supports the following operation values:

       SECCOMP_SET_MODE_STRICT
              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 sig-
              procmask(2), it can use sigreturn(2) to block all signals  apart
              from  SIGKILL  and SIGSTOP.  This means that alarm(2) (for exam-
              ple) 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 setr-
              limit(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:

                  prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);

       SECCOMP_SET_MODE_FILTER
              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  in-
              valid, seccomp() fails, returning EINVAL in errno.

              If  fork(2) or clone(2) is allowed by the filter, any child pro-
              cesses 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 fails  and  re-
              turns EACCES in errno.  This requirement ensures that an unpriv-
              ileged 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 nonzero 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 be-
              cause it did not actually drop privileges.)

              If prctl(2) or seccomp() is allowed by the attached filter, fur-
              ther  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
              call:

                  prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args);

              The recognized flags are:

              SECCOMP_FILTER_FLAG_TSYNC
                     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
                     perspective.)

                     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.

              SECCOMP_FILTER_FLAG_LOG (since Linux 4.14)
                     All filter return actions except SECCOMP_RET_ALLOW should
                     be  logged.   An  administrator  may override this filter
                     flag by preventing specific actions from being logged via
                     the /proc/sys/kernel/seccomp/actions_logged file.

       SECCOMP_GET_ACTION_AVAIL (since Linux 4.14)
              Test to see if an action is supported by the kernel.  This oper-
              ation is helpful to confirm that the kernel knows of a more  re-
              cently  added  filter  return action since the kernel treats all
              unknown actions as SECCOMP_RET_KILL_PROCESS.

              The value of flags must be 0, and args must be a pointer  to  an
              unsigned 32-bit filter return action.

   Filters
       When  adding filters via SECCOMP_SET_MODE_FILTER, args points to a fil-
       ter 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 (and the  convention  being  used
       may  vary over the life of a process that uses execve(2) to execute bi-
       naries that employ the different conventions), it is usually  necessary
       to verify the value of the arch field.

       It is strongly recommended to use a whitelisting approach whenever pos-
       sible 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 alter-
       ing its meaning, leading to a blacklist bypass.  See also  Caveats  be-
       low.

       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-lan-
       guage 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.  (Proba-
       bly,  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 af-
       ter 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 surpris-
       ing 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 con-
       stant  SECCOMP_RET_ACTION_FULL)  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 exe-
       cution 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 action value of highest precedence (along with its  accompa-
       nying data) returned by execution of all of the filters.

       In  decreasing  order  of precedence, the action values that may be re-
       turned by a seccomp filter are:

       SECCOMP_RET_KILL_PROCESS (since Linux 4.14)
              This value results in immediate termination of the process, with
              a core dump.  The system call is not executed.  By contrast with
              SECCOMP_RET_KILL_THREAD below, all threads in the  thread  group
              are terminated.  (For a discussion of thread groups, see the de-
              scription of the CLONE_THREAD flag in clone(2).)

              The process terminates as though  killed  by  a  SIGSYS  signal.
              Even  if  a  signal  handler has been registered for SIGSYS, the
              handler will be ignored in this case and the process always ter-
              minates.   To  a  parent process that is waiting on this process
              (using waitpid(2) or similar), the returned wstatus  will  indi-
              cate that its child was terminated as though by a SIGSYS signal.

       SECCOMP_RET_KILL_THREAD (or SECCOMP_RET_KILL)
              This  value  results in immediate termination of the thread that
              made the system call.  The system call is not  executed.   Other
              threads in the same thread group will continue to execute.

              The  thread terminates as though killed by a SIGSYS signal.  See
              SECCOMP_RET_KILL_PROCESS above.

              Before Linux 4.11, any process terminated in this way would  not
              trigger  a  coredump  (even  though SIGSYS is documented in sig-
              nal(7) as having a default action of  termination  with  a  core
              dump).   Since  Linux  4.11, a single-threaded process will dump
              core if terminated in this way.

              With the addition of  SECCOMP_RET_KILL_PROCESS  in  Linux  4.14,
              SECCOMP_RET_KILL_THREAD   was   added  as  a  synonym  for  SEC-
              COMP_RET_KILL, in order to more clearly distinguish the two  ac-
              tions.

       SECCOMP_RET_TRAP
              This  value  results  in  the  kernel  sending a thread-directed
              SIGSYS signal to the triggering thread.  (The system call is not
              executed.)   Various  fields will be set in the siginfo_t struc-
              ture (see sigaction(2)) associated with signal:

              *  si_signo will contain SIGSYS.

              *  si_call_addr will show the address of  the  system  call  in-
                 struction.

              *  si_syscall  and  si_arch  will indicate which system call was
                 attempted.

              *  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., the program counter will not point to the system call in-
              struction).  The return value register will contain an architec-
              ture-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
              information.)

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

       SECCOMP_RET_TRACE
              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 ex-
              ecuted 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
              PTRACE_GETEVENTMSG.

              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.)

       SECCOMP_RET_LOG (since Linux 4.14)
              This  value  results in the system call being executed after the
              filter return action is logged.  An administrator  may  override
              the  logging of this action via the /proc/sys/kernel/seccomp/ac-
              tions_logged file.

       SECCOMP_RET_ALLOW
              This value results in the system call being executed.

       If an action value other than one of the above is specified,  then  the
       filter  action  is  treated  as  either SECCOMP_RET_KILL_PROCESS (since
       Linux 4.14) or SECCOMP_RET_KILL_THREAD (in Linux 4.13 and earlier).

   /proc interfaces
       The files in the directory /proc/sys/kernel/seccomp provide  additional
       seccomp information and configuration:

       actions_avail (since Linux 4.14)
              A  read-only  ordered  list  of seccomp filter return actions in
              string form.  The ordering, from left-to-right, is in decreasing
              order  of  precedence.   The  list represents the set of seccomp
              filter return actions supported by the kernel.

       actions_logged (since Linux 4.14)
              A read-write ordered list of seccomp filter return actions  that
              are  allowed to be logged.  Writes to the file do not need to be
              in ordered form but reads from the file will be ordered  in  the
              same way as the actions_avail file.

              It  is  important  to note that the value of actions_logged does
              not prevent certain filter return actions from being logged when
              the  audit  subsystem is configured to audit a task.  If the ac-
              tion is not found in the actions_logged file, the final decision
              on  whether to audit the action for that task is ultimately left
              up to the audit subsystem to decide for all  filter  return  ac-
              tions other than SECCOMP_RET_ALLOW.

              The "allow" string is not accepted in the actions_logged file as
              it is not possible to log SECCOMP_RET_ALLOW actions.  Attempting
              to write "allow" to the file will fail with the error EINVAL.

   Audit logging of seccomp actions
       Since  Linux  4.14, the kernel provides the facility to log the actions
       returned by seccomp filters in the audit log.  The kernel makes the de-
       cision  to  log an action based on the action type,  whether or not the
       action is present in the actions_logged file, and whether kernel audit-
       ing  is  enabled (e.g., via the kernel boot option audit=1).  The rules
       are as follows:

       *  If the action is SECCOMP_RET_ALLOW, the action is not logged.

       *  Otherwise, if the action is either SECCOMP_RET_KILL_PROCESS or  SEC-
          COMP_RET_KILL_THREAD,  and that action appears in the actions_logged
          file, the action is logged.

       *  Otherwise, if the filter has  requested  logging  (the  SECCOMP_FIL-
          TER_FLAG_LOG  flag)  and  the  action  appears in the actions_logged
          file, the action is logged.

       *  Otherwise, if kernel auditing is enabled and the  process  is  being
          audited (autrace(8)), the action is logged.

       *  Otherwise, the action is not logged.

RETURN VALUE
       On   success,   seccomp()   returns   0.   On  error,  if  SECCOMP_FIL-
       TER_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 error.

ERRORS
       seccomp() can fail for the following reasons:

       EACCESS
              The caller did not have the CAP_SYS_ADMIN capability in its user
              namespace,  or  had  not  set  no_new_privs  before  using  SEC-
              COMP_SET_MODE_FILTER.

       EFAULT args was not a valid address.

       EINVAL operation  is unknown or is not supported by this kernel version
              or configuration.

       EINVAL The specified flags are invalid for the given operation.

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

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

       EINVAL operation specified SECCOMP_SET_MODE_FILTER, but the filter pro-
              gram pointed to by args was not valid or the length of the  fil-
              ter  program  was  zero or exceeded BPF_MAXINSNS (4096) instruc-
              tions.

       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  al-
              ready  existing  filter  program incurs an overhead penalty of 4
              instructions.

       EOPNOTSUPP
              operation specified  SECCOMP_GET_ACTION_AVAIL,  but  the  kernel
              does not support the filter return action specified by args.

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

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

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

NOTES
       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 ptrace(2) PTRACE_SECCOMP_GET_FILTER operation  can
       be used to dump a process's seccomp filters.

   Architecture support for seccomp BPF
       Architecture support for seccomp BPF filtering is available on the fol-
       lowing architectures:

       *  x86-64, i386, x32 (since Linux 3.5)
       *  ARM (since Linux 3.8)
       *  s390 (since Linux 3.8)
       *  MIPS (since Linux 3.16)
       *  ARM-64 (since Linux 3.19)
       *  PowerPC (since Linux 4.3)
       *  Tile (since Linux 4.3)
       *  PA-RISC (since Linux 4.6)

   Caveats
       There are various subtleties to consider when applying seccomp  filters
       to a program, including the following:

       *  Some traditional system calls have user-space implementations in the
          vdso(7) on many architectures.  Notable examples include  clock_get-
          time(2),  gettimeofday(2), and time(2).  On such architectures, sec-
          comp filtering for these system calls will have  no  effect.   (How-
          ever,  there  are  cases  where the vdso(7) implementations may fall
          back to invoking the true system call, in which case seccomp filters
          would see the system call.)

       *  Seccomp  filtering is based on system call numbers.  However, appli-
          cations typically do not directly invoke system calls,  but  instead
          call  wrapper  functions  in  the C library which in turn invoke the
          system calls.  Consequently, one must be aware of the following:

          o  The glibc wrappers for some traditional system calls may actually
             employ  system calls with different names in the kernel.  For ex-
             ample,  the  exit(2)  wrapper  function  actually   employs   the
             exit_group(2) system call, and the fork(2) wrapper function actu-
             ally calls clone(2).

          o  The behavior of wrapper functions may vary across  architectures,
             according  to  the range of system calls provided on those archi-
             tectures.  In other words, the same wrapper function  may  invoke
             different system calls on different architectures.

          o  Finally,  the  behavior  of  wrapper  functions can change across
             glibc versions.  For example, in older versions, the glibc  wrap-
             per  function  for  open(2)  invoked  the system call of the same
             name, but starting in glibc 2.26, the implementation switched  to
             calling openat(2) on all architectures.

       The consequence of the above points is that it may be necessary to fil-
       ter for a system call other than might  be  expected.   Various  manual
       pages  in  Section  2 provide helpful details about the differences be-
       tween wrapper functions and the underlying system calls in  subsections
       entitled C library/kernel differences.

       Furthermore,  note  that  the application of seccomp filters even risks
       causing bugs in an application, when the filters cause unexpected fail-
       ures  for legitimate operations that the application might need to per-
       form.  Such bugs may not easily be discovered when testing the  seccomp
       filters if the bugs occur in rarely used application code paths.

   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 oper-
          and whose value is the size of the seccomp_data buffer.

EXAMPLE
       The program below accepts four or more arguments.  The first three  ar-
       guments  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  speci-
           fied  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  addi-
       tional  arguments  of a program that the example program should attempt
       to execute using execv(3) (a library  function  that  employs  the  ex-
       ecve(2)  system  call).  Some example runs of the program are shown be-
       low.

       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
           x86_64
           $ 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
           59
           $ ./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
           1
           $ ./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
           295
           $ ./a.out 295 0xC000003E 99 /bin/whoami
           cecilia

   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
              (in the x32 ABI, all system calls have bit 30 set in the
              'nr' field, meaning the numbers are >= X32_SYSCALL_BIT) */
           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 BPF_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 */
               BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),

               /* [7] Destination of architecture mismatch: kill task */
               BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL),
           };

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

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

           return 0;
       }

       int
       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);
               exit(EXIT_FAILURE);
           }

           if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
               perror("prctl");
               exit(EXIT_FAILURE);
           }

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

           execv(argv[4], &argv[4]);
           perror("execv");
           exit(EXIT_FAILURE);
       }

SEE ALSO
       strace(1), bpf(2), prctl(2),  ptrace(2),  sigaction(2),  proc(5),  sig-
       nal(7), socket(7)

       Various  pages  from  the  libseccomp  library, including: scmp_sys_re-
       solver(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 Docu-
       mentation/userspace-api/seccomp_filter.rst (or Documentation/prctl/sec-
       comp_filter.txt before Linux 4.13).

       McCanne, S. and Jacobson, V. (1992) The BSD Packet Filter: A New Archi-
       tecture for User-level Packet Capture, Proceedings of the USENIX Winter
       1993 Conference <http://www.tcpdump.org/papers/bpf-usenix93.pdf>

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                        SECCOMP(2)

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