Add Sandbox Using seccomp-bpf to Whitelist Allowed Syscalls

Filtering Syscalls with BPF

seccomp-bpf installs a bytecode program the kernel runs on every syscall. Allow read/write, deny execve, and untrusted code cannot spawn shells even with memory corruption.

Policy Shape

Typical sandbox:

• Load syscall number from seccomp data
• Compare against whitelist
• Return SECCOMP_RET_ALLOW or SECCOMP_RET_KILL

For example, a JSON parser sandbox might allow read, write, exit_group, brk, mmap and trap on socket.

Why This Matters in Production

seccomp-bpf is the exact mechanism behind Chrome's renderer sandbox, Docker's default seccomp profile, and systemd's SystemCallFilter=, all of which whitelist a minimal syscall set to shrink the kernel's attack surface for untrusted code. Getting the BPF comparison direction wrong here is a real security bug class: an inverted allow/deny check has shipped in real sandbox escapes, which is why filters are almost always reviewed line-by-line before merging.

Debugging Checklist

Before you call the implementation done, walk failure modes on purpose. Test empty structures, single-element edge cases, maximum concurrency, and errno paths that must not crash the program. OS code usually fails in production when happy-path tests pass but invariants break under contention or memory pressure.

Implementation Notes

Keep structures small and name fields after kernel counterparts when possible. That lets you read man pages and kernel source side by side while you work. Print observable events during development; remove noisy logs once tests pass reliably.

For This Exercise

You will write a seccomp-bpf filter, apply it with prctl, and demonstrate a blocked syscall. The task asks you to log the signal delivered when a forbidden syscall is attempted.