Customize or disable Firecracker's seccomp filters: default per-thread filters, custom filters compiled with seccompiler-bin, and --no-seccomp
domain: github.com/firecracker-microvm/firecracker · 7 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Default behavior needs no action: seccomp filters allowing only the bare-minimum syscalls are compiled into the Firecracker binary at build time and installed per-thread — VMM (main) and VCPU threads right before executing guest code, API thread right before launching the HTTP server.
Inspect the default filter JSON under `resources/seccomp` in the repo; for a given release, the exact JSON filters used are included in that release's archive on the GitHub releases page.
For a custom filter, write a JSON policy file mapping the three thread categories (`vmm`, `api`, `vcpu`) to filters (one file per target arch-libc combination).
Compile it: `./seccompiler-bin --target-arch x86_64 --input-file x86_64_musl.json --output-file bpf_x86_64_musl` (arches: x86_64, aarch64; default output name seccomp_binary_filter.out; optional --split-output writes one raw .bpf file per thread for testing; --basic discards parameter checks and is deprecated).
Launch Firecracker with `--seccomp-filter <path-to-compiled-file>` to override the embedded default filters at startup.
To disable filtering entirely (prototyping only), launch with `--no-seccomp`.
Official docs: https://github.com/firecracker-microvm/firecracker/blob/main/docs/seccomp.md and https://github.com/firecracker-microvm/firecracker/blob/main/docs/seccompiler.md
Known gotchas
Debug binaries and experimental GNU targets ship with NO default seccomp filters installed — they are not intended for production.
Custom filters are dangerous: misconfiguration can abruptly terminate the process or silently disable the seccomp security boundary; the docs recommend defaults, and custom mitigation filters only as a tested, short-term fix.
Syscall sets differ between debug and release builds (e.g. debug assertions use fcntl(F_GETFD)), so a filter built for release may kill a debug binary.
Never use --no-seccomp in production.
seccompiler-bin is versioned and released in lockstep with Firecracker (same version number and support window) — don't mix versions; the user is responsible for filter file integrity (use checksums to avoid MITM).
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