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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 in order to create a seccomp-based deny-list  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  deny-list, 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.

       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.)

       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.

EXAMPLE

       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

       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 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

       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_init(3),
       seccomp_load(3), seccomp_rule_add(3), and seccomp_export_bpf(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

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       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       https://www.kernel.org/doc/man-pages/.