policy_fn: drop path strings, keep argv via sibling-thread freeze (#27)

[congwang-mk]

Summary

Closes the dynamic-policy half of #27 — policy_fn decisions on user-memory strings were racy because the kernel re-reads after the supervisor's Continue. This PR makes the supported subset of policy_fn safe by construction.

Two changes to the event surface:

1. Drop event.path and path_contains() from SyscallEvent, the FFI struct, and the Python dataclass. Path-based access control belongs in static Landlock rules (fs_read / fs_write / fs_deny), which are kernel-enforced and TOCTOU-immune. ctx.deny_path() stays for runtime additions.
2. Keep event.argv, but make it TOCTOU-safe. Before the supervisor sends Continue for an execve/execveat, it PTRACE_SEIZE + PTRACE_INTERRUPTs every sibling thread of the calling tid (new sibling_freeze module). The kernel's post-Continue re-read happens with no other writer running, binding the kernel's view of argv to exactly what the supervisor inspected.

The freeze has effectively zero observable cost: execve's de_thread step kills sibling threads anyway. Single-threaded callers (typical case) freeze nothing. On ptrace failure (e.g. YAMA blocking), the supervisor falls back to the prior Landlock-bounded behavior rather than refusing the syscall.

Why ptrace is the right answer here

Considered alternatives: BPF CLONE_THREAD deny (rejected — breaks multi-threaded users), thread-count gate (Threads==1 ⇒ safe ⇒ expose argv, else None — rejected as a leaky API), pure drop of argv (loses the feature). ptrace is the only option that keeps a clean argv: Option<Vec<String>> always-populated-for-execve API. sandlock already uses ptrace in checkpoint.rs, so this isn't a new dependency; ~120 LOC in a self-contained module.

Test plan

• cargo test --workspace — 182 tests pass, including restored test_policy_fn_execve_argv and test_policy_fn_deny_by_argv
• python3 -m pytest python/tests/test_policy_fn.py — 17 tests pass
• 2 new sibling_freeze unit tests
• CI on Linux x86_64 + arm64

Files

• crates/sandlock-core/src/sibling_freeze.rs (new) — freeze helper
• crates/sandlock-core/src/policy_fn.rs — drop path, drop path_contains, keep argv/argv_contains
• crates/sandlock-core/src/seccomp/notif.rs — stop reading path; freeze siblings before Continue on execve
• crates/sandlock-ffi/src/lib.rs — drop path field from sandlock_event_t
• python/src/sandlock/_sdk.py — mirror dataclass + ctypes changes
• README — rewrite the policy_fn section, document the TOCTOU note explicitly

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