--- name: "ceccomp" section: "1" platform: "linux" package: "ceccomp" version: "4.3.1-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/ceccomp" --- # ceccomp(1) A tool to analyze seccomp filters ## NAME ceccomp - A tool to analyze seccomp filters ## SYNOPSIS ``` usage: ceccomp [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*. - \-c, --color **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*. - \-a, --arch **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 x86\_64. - \-f, --format **FMT** Determines how `ceccomp` produces binary-format bpf code. Can be *hexfmt*, *hexline*, *raw* or *c-macro*. 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. **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. **Note**\ Since *version 4.3*, a new **FMT** *c-macro* is added to port seccomp rules to other architectures easily. | | | | --- | --- | | **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] [-e] [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. - \-c, --color **WHEN** Argument description can be found in asm - ASSEMBLE section. `disasm` may print more text in color including syntax highlighting for *TEXT*. - \-a, --arch **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 x86\_64. - \-e, --ebpf Treat *RAW* as eBPF insn bytes instead of cBPF filter bytes. This flag is designed to test whether eBPF-to-cBPF conversion is working or not. To use this function, please dump installed eBPF insns migrated from seccomp filters in a process via eBPF or a kernel module. Any input not generated by kernel may lead to undefined behavior. **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. - \-c, --color **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. - \-a, --arch **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. - \-c, --color **WHEN** Argument description can be found in asm - ASSEMBLE section. `trace` may print more text in color including syntax highlighting for *TEXT*. - \-o, --output **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`. - \-p, --pid **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. The pid here is actually process (thread) id, you can trace a thread or process. - \-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`. **Note**\ Since *version 4.2*, `dump` sub command is added as an alias of `trace`, to keep compatibility with `seccomp-tools`. | | | | --- | --- | | **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` or `ceccomp dump` | 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. ### capture - CAPTURE FILTERS WITH EBPF ``` ceccomp capture [-c WHEN] [-p PID] ``` Capture seccomp filters in global scope if **PID** not set. The eBPF program will trace `seccomp_check_filter` kernel function, reliably intercepting seccomp filters that the kernel will load, WITHOUT race conditions compared to trace - TRACE FILTER IN RUNTIME. This function is named "global capture" below. Linux 5.15 is the lowest recommended kernel version to run this function on *x86\_64*, other architecture may require higher kernel version to run this function. If **PID** is set, an eBPF program to extract installed seccomp filters in the corresponding process (actually thread) will be installed. Note that inside kernel, `pid` is referred thread id and `tgid` is referred to process id. The eBPF program directly pulls eBPF insns migrated from original seccomp filters out and converts them back to perform disasm - DISASSEMBLE. This function does not require `CONFIG_CHECKPOINT_RESTORE` kernel config, and is functional even if `ceccomp` itself is under seccomp. This function is named "capture pid" below. Linux 6.2 is the lowest kernel version to run this function, as `bpf_task_from_pid` was introduced then. **Important**\ When the kernel migrates seccomp filters, the conversion from cBPF to eBPF is lossy because several distinct cBPF instructions can be mapped to the same eBPF instruction sequence. As a result, the output from "capture pid" may not be identical to the original seccomp filter, even though it is functionally equivalent. Currently, compat task detection is only supported on *x86\_64* and *aarch64*. With this feature, `ceccomp` can identify whether a task is native (64-bit task on 64-bit kernel) or "compatible" (32-bit task on 64-bit kernel), so that `ceccomp` can pass the correct architecture token to libseccomp to query syscall name in captured seccomp filters. Note that eBPF utilizes asynchronous ring buffer to send back data to user-mode process, so if lots of seccomp filters are loaded at the same time, the buffer may run out and you need to see detailed errors via `bpftool prog trace`. Since we can’t send the errors back through full ring buffer, those overflowed filters are actually invisible to users without reading kernel messages. - \-c, --color **WHEN** Argument description can be found in asm - ASSEMBLE section. - \-p, --pid **PID** Which **process** (thread) to extract seccomp filter insns from. **Tip**\ All **PID** presented in either "global capture" or "capture pid" are **INITIAL NAMESPACE PID**, not PID in current namespace. It’s recommended that only run `ceccomp capture` in initial namespace. **Note**\ This module is optional, you can opt it out during configuration. This module require libbpf to work. **Note**\ This subcommand was introduced in *version 4.3*. ## 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`. **Caution**\ 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