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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 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:

                  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 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 calling thread
              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 returns 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  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 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 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_FILTER_FLAG_SPEC_ALLOW (since Linux 4.17)
                     Disable Speculative Store Bypass mitigation.

       SECCOMP_GET_ACTION_AVAIL (since Linux 4.14)
              Test  to see if an action is supported by the kernel.  This operation is helpful to
              confirm that the kernel knows of a more recently 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 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 (and the convention being used may vary over the life of a process that uses
       execve(2)  to  execute  binaries  that  employ  the  different conventions), it is usually
       necessary to verify the value of the arch field.

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

       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 a policy must either deny all syscalls with __X32_SYSCALL_BIT or  it  must
       recognize  syscalls  with and without __X32_SYSCALL_BIT set.  A list of system calls to be
       denied based on nr that does not also contain nr values with __X32_SYSCALL_BIT set can  be
       bypassed by a malicious program that sets __X32_SYSCALL_BIT.

       Additionally, kernels prior to Linux 5.4 incorrectly permitted nr in the ranges 512-547 as
       well as the corresponding non-x32 syscalls ORed with __X32_SYSCALL_BIT.  For  example,  nr
       ==  521  and nr == (101 | __X32_SYSCALL_BIT) would result in invocations of ptrace(2) with
       potentially confused x32-vs-x86_64 semantics in the kernel.  Policies intended to work  on
       kernels  before  Linux  5.4 must ensure that they deny or otherwise correctly handle these
       system calls.  On Linux 5.4 and newer, such system calls will fail with the error  ENOSYS,
       without doing anything.

       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, 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_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 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 action value of highest precedence
       (along with its accompanying data) returned by execution of all of the filters.

       In decreasing order of precedence, the action values that may be  returned  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 description 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 terminates.  To a parent process that  is  waiting  on  this
              process  (using waitpid(2) or similar), the returned wstatus will indicate 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  signal(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 SECCOMP_RET_KILL, in order to
              more clearly distinguish the two actions.

              Note: the use of SECCOMP_RET_KILL_THREAD to kill a single thread in a multithreaded
              process  is  likely to leave the process in a permanently inconsistent and possibly
              corrupt state.

       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 structure (see sigaction(2)) associated with signal:

              *  si_signo will contain SIGSYS.

              *  si_call_addr will show the address of the system call instruction.

              *  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 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 information.)

       SECCOMP_RET_ERRNO
              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.

       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 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 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.)

              Note that a tracer process will not be notified if another filter returns an action
              value with a precedence greater than SECCOMP_RET_TRACE.

       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/actions_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 action 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   actions   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 decision to log an  action  based  on  the
       action type,  whether or not the action is present in the actions_logged file, and whether
       kernel auditing 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 SECCOMP_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_FILTER_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_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 error.

ERRORS

       seccomp() can fail for the following reasons:

       EACCES The  caller did not have the CAP_SYS_ADMIN capability in its user namespace, or had
              not set no_new_privs before using SECCOMP_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 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 filter program pointed  to  by
              args  was  not  valid  or  the  length  of  the filter program was zero or exceeded
              BPF_MAXINSNS (4096) instructions.

       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
              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  following
       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_gettime(2),   gettimeofday(2),   and
          time(2).   On such architectures, seccomp filtering for these system calls will have no
          effect.  (However, 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, applications 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:

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

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

          ·  Finally, the behavior of wrapper functions can change across  glibc  versions.   For
             example,  in  older  versions,  the  glibc  wrapper 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 filter for a system
       call other than might be expected.  Various manual pages  in  Section  2  provide  helpful
       details about the differences between 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 failures for legitimate operations that the
       application might need to perform.  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 operand whose value is
          the size of the seccomp_data buffer.

EXAMPLES

       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 below.

       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
       #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))

       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 deny-list 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 process */
               BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),
           };

           struct sock_fprog prog = {
               .len = ARRAY_SIZE(filter),
               .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

       bpfc(1),   strace(1),  bpf(2),  prctl(2),  ptrace(2),  sigaction(2),  proc(5),  signal(7),
       socket(7)

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

       The       kernel       source      files      Documentation/networking/filter.txt      and
       Documentation/userspace-api/seccomp_filter.rst (or  Documentation/prctl/seccomp_filter.txt
       before Linux 4.13).

       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
       ⟨http://www.tcpdump.org/papers/bpf-usenix93.pdf

COLOPHON

       This  page  is  part of release 5.10 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/.