CECCOMP(1)

NAME
ceccomp - A tool to analyze seccomp filters

SYNOPSIS

    usage: ceccomp <asm|disasm|emu|trace|probe|version|help> [FILE] [-q|--quiet]
                   [-f|--format FMT] [-a|--arch ARCH] [-p|--pid PID] [-s|--seize]
                   [-o|--output FILE] [-c|--color WHEN] ...

CONCEPT

Kernel use BPF filters to limit syscall rules, applied via seccomp or prctl syscall. For example, down below is a simple filter to block execve syscall in hex format:

    1: 20 00 00 00 00 00 00 00     $A = $syscall_nr
    2: 15 00 00 01 3b 00 00 00     if ($A != execve) goto 4
    3: 06 00 00 00 00 00 00 00     return KILL
    4: 06 00 00 00 00 00 ff 7f     return ALLOW

The part presented in hex is what kernel received, and ceccomp take it to disassemble back to human readable text. For instance the lineno in the left and statement in the right.

    Important

    Later I’ll use TEXT in short for BPF human readable text, and use RAW in short for BPF raw format, please keep that in mind.

DESCRIPTION

ceccomp have 5 main functions, basically it’s a C version of seccomp-tools, however, there are some breaking changes you need to know, which will be highlighted in each subcommand section.

  asm - ASSEMBLE

    ceccomp asm [-c WHEN] [-a ARCH] [-f FMT] [TEXT]

Assemble TEXT to RAW. Use it to embed hand written filter rules into C code or to see the original code of some TEXT.

WHEN
    Determines when to display warnings and errors in color. If the value is auto, ceccomp will display color when the output target is a "tty". Can be auto, never or always. The default value is auto.

ARCH
    Set to any architecture libseccomp supports. Will be used to determine the actual syscall number behind the name (for example, on x86_64, you could write "execve" instead of 59 like the basic example above). Your system arch will be taken if not set via uname. The default value on your system is i686.

    Note

    Since version 4.0, endianness is considered. If target endianness ARCH is different from machine endianness, the filters will be reversed (CODE and K) before outputting.

FMT
    Determines how ceccomp produces binary-format bpf code. Can be hexfmt, hexline or raw. You could find sample output in EXAMPLES section. The default value is hexline.

TEXT
    Take a optional filename to determine which file containing TEXT will be assembled. Will read from stdin if not set. - is treated as stdin.

    Important

    The assembly syntax was changed greatly since version 4.0, please checkout grammar reference below!

Please check out TEXT GRAMMAR REFERENCE section to see how to write a rule by hand. Some examples will be displayed in EXAMPLES section.
Command | Difference
seccomp-tools asm | Use its own grammar to assemble, a bit script like; can assemble invalid TEXT which will be rejected by kernel
ceccomp asm | You can just take disasm output to asm, no new grammar is needed to learn; take stdin as input by default

  disasm - DISASSEMBLE

    ceccomp disasm [-c WHEN] [-a ARCH] [RAW]

Disassemble RAW to TEXT. Use it to see what does a filter do if you could not access filter via trace and have to manually extract the filter out.

WHEN
    Argument description can be found in asm - ASSEMBLE section. disasm may print more text in color including syntax highlighting for TEXT.

ARCH
    Set to any architecture libseccomp supports. Will be used to determine how filtered syscall number in RAW filter is translated to syscall name (for example, on x86_64, the number 0x3b is translated to execve if is comparing syscall_nr, see the basic example above). The default value on your system is i686.

RAW
    A binary file with raw BPF codes. Takes stdin as input if not set. Treat - as stdin. The file is arch-revelent, so it may not be portable on different archs.

    Note

    Since version 4.0, endianness is considered. If target endianness ARCH is different from machine endianness, the filters will be reversed (CODE and K) before decoding.

    Note

    ceccomp will try to resolve syscall number under an arch ONLY IF that at that line, arch can be determined. On foreign arch (not equal to the arch you set), the foreign arch will be prepended to syscall name. You may notice that in some cases, seccomp-tools is able to resolve the name while ceccomp is not, that may be intended as the arch is not determined.

Command | Difference
seccomp-tools disasm | Disassembles in its format; never check if the filter is valid
ceccomp disasm | Disassembles in ceccomp format, and takes stdin as input by default; check arch strictly and always display foreign arch name

  emu - EMULATE

    ceccomp emu [-c WHEN] [-a ARCH] [-q] TEXT SYSCALL_NAME/SYSCALL_NR [ARGS[0] ARGS[1] ... ARGS[5] PC]

Emulate what will happen if syscall(SYSCALL_NR, ARGS[0], ARGS[1], ..., ARGS[5]) from PC is called following rules described in TEXT. Use it to see the result without actually running it in program or you don’t want to examine the filter rule manually. This subcommand can be used to automatically examining a filter.

WHEN
    Argument description can be found in asm - ASSEMBLE section. emu may print more text in color including syntax highlighting for TEXT and skipped statements.

SYSCALL_NAME/SYSCALL_NR
    If you set SYSCALL_NAME (like execve), it will be translated to SYSCALL_NR under ARCH first. Or else set SYSCALL_NR directly (like 59). Then the nr will be tested against the bpf filter to see the result of that syscall. This argument is NOT optional.

ARGS[0-5] and PC
    Register values when calling syscall. For example, on x86_64, these are equivalent to rdi, rsi, rdx, r10, r8, r9 and rip. Their default value is 0.

ARCH
    Argument description can be found in asm - ASSEMBLE section.

TEXT
    Take a filename to determine which file containing TEXT rule will be tested. Note that filename CAN NOT be ignored as ceccomp can not determine if a positional argument is syscall or filename. Use - to refer to stdin.

-q, --quiet
    Only print the eval result of the filter. For example, if last statement emulated is return KILL, then KILL is printed.
Command | Difference
seccomp-tools emu | Take a RAW as input
ceccomp emu | Take a TEXT as input and take stdin as input by default; set PC is possible

  trace - TRACE FILTER IN RUNTIME

    ceccomp trace [-c WHEN] [-q] [-o FILE] PROGRAM [program-args]
                  [-c WHEN] [-q] -p PID [-s]

The first line captures filters PROGRAM loads in runtime by tracing it; the second line extract seccomp filters from PID, or trace PID to capture subsequent seccomp filters; once fetched filters, print them in TEXT. You can only choose one of the two formats above. Use this if running the program is the simplest way to fetch bpf filters or a program with seccomp filters installed is waiting for input.

WHEN
    Argument description can be found in asm - ASSEMBLE section. trace may print more text in color including syntax highlighting for TEXT.

FILE
    May be useful when PROGRAM produces quite a lot output in stderr. ceccomp allow user to close stdin and stdout to limit PROGRAM input and output, so ceccomp use stderr to print messages when running PROGRAM, set FILE if you want to see TEXT in some other file. Treat - as stdout.

PROGRAM
    Set to the program you want to run, and program-args are its arguments just like running shell command exec PROGRAM program-args.

PID
    Set to the pid you want to inspect. PID is conflict with PROGRAM; you could either run a program dynamically or examine a pid in one command. Without -s flag, trace pid will try to extract seccomp filter in PID via ptrace(PTRACE_SECCOMP_GET_FILTER), which may not be available in some systems.

-s, --seize
    ONLY AVAILABLE FOR TRACE PID MODE. Set this flag will override trace pid behavior to attach to PID and keep tracing for seccomp filter loading like trace prog mode. This flag was introduced in version 4.0.

-q, --quiet
    Set to suppress the extra [INFO] prints when detect process forking, exiting or seccomp filter loading. This flag was introduced in version 4.0.

    Note

    To extract filters from PID, CAP_SYS_ADMIN is needed (without -s flag) and CAP_SYS_PTRACE may also be needed, the easiest way to acquire them is calling ceccomp with sudo.

    Note

    Since version 3.1, multiple process tracing is introduced, and when tracee forking/resolving/exiting, an extra INFO message is printed. You can discard it by running command like ceccomp trace -q PROG 2>/dev/null.

Command | Difference
seccomp-tools dump | Setting output format is possible; each filter can be output to a different file; killing PROGRAM once LIMIT times of filters loaded; wrapping PROGRAM in sh -c
ceccomp trace | All filters are output to a single file; never kill PROGRAM; PROGRAM is launched directly, so ./ is not needed; explicitly print when forking; able to attach to pid for dynamic seccomp filter capturing

  probe - TEST COMMON SYSCALLS INSTANTLY

    ceccomp probe [-c WHEN] [-o FILE] [-q] PROGRAM [program-args]

Run PROGRAM with program-args to captures FIRST seccomp filter, and then kill all children. Use it when a quick check against a program is needed, and detect potential seccomp rule issues.

All argument descriptions can be found in trace - TRACE FILTER IN RUNTIME section.

The output for this subcommand is the emulating result of common syscalls like execve, open and so on. If the filter itself is not capable of blocking syscalls, you could know that with a glance.

Typical output for this subcommand is described below, more detailed example could be found in EXAMPLES section.

    open      -> ALLOW
    read      -> ALLOW
    write     -> ALLOW
    execve    -> KILL
    execveat  -> KILL
    mmap      -> ALLOW
    mprotect  -> ALLOW
    openat    -> ALLOW
    sendfile  -> ALLOW
    ptrace    -> ERRNO(1)
    fork      -> ALLOW

    Note

    seccomp-tools don’t have this subcommand.

TEXT GRAMMAR REFERENCE

    Important

    The grammar changed greatly since version 4.0 as we refactored lexer for better human readability. The wrapper now prefixed by # as it’s a comment now. And line is replaced by label, so now lexer depends on label declaration to decide where to jump, instead of lineno in TEXT file.

A valid TEXT format is described in EBNF-like declaration here: If you have no interest to know what EBNF is, please keep reading for examples.

BPF ops which are not described below are banned by kernel.

  Comment and Label

ceccomp disasm displays a lot of things, but some of them are optional for asm.

    #Label  CODE  JT   JF      K
    #---------------------------------
     L0001: 0x06 0x00 0x00 0x7fff0000 return ALLOW
    #---------------------------------

Any text after # will be discarded by asm like some script languages.

Empty lines are accepted.

Label declaration is an identifier at the beginning of line and suffixed by : like L0001. An identifier is a string starts with alpha and contains with only alphanumeric characters and underscore _. Label is only necessary if it’s the destination of goto, these redundant labels added by disasm are for readability. E.g. in if ($A == 0) goto somewhere, somewhere is a label and must be declared after the statement. Label declaration can take a line separately, or be put in front of statement.

The CODE, JT, JF and K value generated by disasm will be discarded by asm, asm only parse the effective statement after K.

    Note

    There are some slight difference between ceccomp disasm and seccomp-tools disasm, down below is a general example. And some statements are different, so don’t pipe seccomp-tools output to ceccomp blindly.

    line  CODE  JT   JF      K
    =================================
    0000: 0x06 0x00 0x00 0x7fff0000  return ALLOW

  Assignment

A can be set to seccomp attributes directly. But X can not be assigned with seccomp attributes directly due to kernel limit.

    $A = $arch
    $A = $syscall_nr

To assign A with those 64-bit long fields, low_ or high_ prefix is needed.

    $A = $low_pc
    $A = $high_pc
    $A = $low_args[0]
    $A = $high_args[0]
    ...
    $A = $low_args[5]
    $A = $high_args[5]

A special attribute is sizeof(struct seccomp_data), that can be assigned to A or X directly.

    $A = $scmp_data_len
    $X = $scmp_data_len

Temporary memory is 32-bit, to access them, you could use hex or dec as index. Both A and X is assignable. Assigning immediate values to A or X accepts any format of number if you imply the correct base by "0x" or "0b".

    $X = $mem[0]
    $A = $mem[0xf]
    $A = $mem[15] # both hex and dec index are OK
    $A = 0
    $X = 0x3b
    $A = 0b1111
    $A = 0333

You could also assign X to A or in the reverse order. Assign X or A to temporary memory is definitely okay.

    $A = $X
    $X = $A
    $mem[3] = $X
    $mem[0x4] = $A

  Arithmetic Operations

Various operations can be applied to A.

    $A += 30
    $A -= 4
    $A *= 9
    $A /= 1
    $A &= 7
    $A >>= 6

The right value can be X.

    $A &= $X
    $A |= $X
    $A ^= $X
    $A <<= $X

And there is a way to negativate A.

    $A = -$A

  Jump Downwards If ...

Unconditional jump:

    goto L3

Jump if:

    if ($A == execve) goto L3
    if ($A != 1234) goto L4
    if ($A & $X) goto L5
    if !($A & 7) goto L6
    if ($A <= $X) goto L7

If true jump to ... if false jump to...:

    if ($A > $X) goto L3, else goto L4
    if ($A >= 4567) goto L5, else goto L6

ONLY in conditions, you CAN replace number with syscall name or arch name. In example above, 0x3b is replaced by execve. All the syscall name will be resolved to syscall number under your selected arch. If you want to resolve a syscall name in foreign arch (not equal to your selected arch), please prepend a arch and dot. For example, your arch is x86_64, and you are writing aarch64 rules, then please write like:

    if ($A == aarch64.read) goto 5

Note that if you manually set arch to aarch64 with -a aarch64, you can omit aarch64. in statement.

  Return Code

Return value of register A:

    return $A

Or return a immediate value, with extra field in (). Actions including TRACE, TRAP and ERRNO accept an extra field, without (), they are treated as action(0).

    return 0x13371337
    return KILL
    return KILL_PROCESS
    return TRAP(123)
    return ERRNO(0)
    return TRACE
    return TRACE(3)
    return LOG
    return NOTIFY

  Short Example

The following TEXT is valid for asm, which blocks execve and execveat for amd64 syscalls:

    $A = $syscall_nr
    if ($A == execve) goto forbid
    if ($A == execveat) goto forbid
    return ALLOW
    forbid: return KILL

RESTRICTIONS

Ceccomp asm put some restrictions on TEXT for better performance.

    1. '\0' must not be found in TEXT since it’s a text file.

    2. A line must be shorter than 384 bytes.

    3. A TEXT file must have less than 16384 lines.

    4. A TEXT file must be smaller than 4 MiB.

And for both asm and disasm, effective statements (that can be encoded or decoded into BPF) must be less or equal than 4096, this is enforced by kernel.

A fun fact about ceccomp asm: any basic ANSI color in TEXT file, e.g., \x1b[31m, will be discarded when processing.

EXAMPLES

Manpage can not display images, so please check out html version of this page to see examples.

REPO

Visit to find the code. Pull Requests and Issues are welcome!

Copyright © 2025-present, distributed under GPLv3 or later.

AUTHORS

dbgbgtf

RocketDev