# seccomp

## NAME



           seccomp - operate on Secure Computing state of the process
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <linux/seccomp.h>  **/* Definition of **SECCOMP_*** constants */
           **#include <linux/filter.h>   **/* Definition of **struct sock_fprog** */
           **#include <linux/audit.h>    **/* Definition of **AUDIT_*** constants */
           **#include <linux/signal.h>   **/* Definition of **SIG*** constants */
           **#include <sys/ptrace.h>     **/* Definition of **PTRACE_*** constants */
           **#include <sys/syscall.h>    **/* Definition of **SYS_*** constants */
           **#include <unistd.h>**
    
           **int syscall(SYS_seccomp, unsigned int** _operation_**, unsigned int** _flags_**,**
                       **void ***_args_**);**
    
           _Note_ : glibc provides no wrapper for **seccomp**(), necessitating the
           use of syscall(2).
    


## 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 termination of the calling thread, or
                  termination of the entire process with the **SIGKILL** signal
                  when there is only one thread.  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_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_NEW_LISTENER**(since Linux 5.0)
                         After successfully installing the filter program,
                         return a new user-space notification file
                         descriptor.  (The close-on-exec flag is set for the
                         file descriptor.)  When the filter returns
                         **SECCOMP_RET_USER_NOTIF** a notification will be sent
                         to this file descriptor.
    
                         At most one seccomp filter using the
                         **SECCOMP_FILTER_FLAG_NEW_LISTENER** flag can be
                         installed for a thread.
    
                         See seccomp_unotify(2) for further details.
    
                  **SECCOMP_FILTER_FLAG_SPEC_ALLOW**(since Linux 4.17)
                         Disable Speculative Store Bypass mitigation.
    
                  **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_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.
    
           **SECCOMP_GET_NOTIF_SIZES**(since Linux 5.0)
                  Get the sizes of the seccomp user-space notification
                  structures.  Since these structures may evolve and grow
                  over time, this command can be used to determine how much
                  memory to allocate for sending and receiving notifications.
    
                  The value of _flags_ must be 0, and _args_ must be a pointer to
                  a _struct seccomp_notif_sizes_ , which has the following form:
    
                  struct seccomp_notif_sizes
                      __u16 seccomp_notif;      /* Size of notification structure */
                      __u16 seccomp_notif_resp; /* Size of response structure */
                      __u16 seccomp_data;       /* Size of 'struct seccomp_data' */
                  };
    
                  See seccomp_unotify(2) for further details.
    
       **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_USER_NOTIF**(since Linux 5.0)
                  Forward the system call to an attached user-space
                  supervisor process to allow that process to decide what to
                  do with the system call.  If there is no attached
                  supervisor (either because the filter was not installed
                  with the **SECCOMP_FILTER_FLAG_NEW_LISTENER** flag or because
                  the file descriptor was closed), the filter returns **ENOSYS**
                  (similar to what happens when a filter returns
                  **SECCOMP_RET_TRACE** and there is no tracer).  See
                  seccomp_unotify(2) for further details.
    
                  Note that the supervisor process will not be notified if
                  another filter returns an action value with a precedence
                  greater than **SECCOMP_RET_USER_NOTIF**.
    
           **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 Linux 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
           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**.
    
           **EBUSY** While installing a new filter, the
                  **SECCOMP_FILTER_FLAG_NEW_LISTENER** flag was specified, but a
                  previous filter had already been installed with that flag.
    
           **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.
    


## STANDARDS



           Linux.
    


## 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:
    
           •  If the program is not running on the specified architecture,
              the BPF filter causes system calls to fail with the error
              **ENOSYS**.
    
           •  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 <linux/audit.h>
           #include <linux/filter.h>
           #include <linux/seccomp.h>
           #include <stddef.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/prctl.h>
           #include <sys/syscall.h>
           #include <unistd.h>
    
           #define X32_SYSCALL_BIT 0x40000000
           #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
    
           static int
           install_filter(int syscall_nr, unsigned 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 (syscall(SYS_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),
                                  strtoul(argv[2], NULL, 0),
                                  strtol(argv[3], NULL, 0)))
                   exit(EXIT_FAILURE);
    
               execv(argv[4], &argv[4]);
               perror("execv");
               exit(EXIT_FAILURE);
           }
    


* * *