nyx/tools/sb-trace

sb-trace seeds

This directory holds per-capability allowlist seeds for the macOS sandbox-exec deny-default rollout.

What the seeds are

Each .allow file is a fragment of sandbox-exec profile syntax (one or more (allow ...) directives, plus comments). At runtime, src/dynamic/sandbox/process_macos.rs::profile_path consults the NYX_SB_DENY_DEFAULT environment variable; when set, it locates the seed for the active capability, rewrites the baked profile's (allow default) directive to (deny default), and appends the seed body verbatim. Sandbox-exec resolves later directives over earlier ones, so the appended allow rules stack on top of the deny baseline.

The splice path lives in process_macos.rs::splice_deny_default; it is pure, unit-tested, and a no-op when the seed for a capability is missing. Misconfiguration cannot brick the sandbox-exec backend.

How the seeds get generated

Run tools/sb-trace.sh from a macOS host that has the interpreters on $PATH. The script materialises each .sb profile with (allow default) rewritten to (deny default), runs each per-language probe under sandbox-exec, queries log show --predicate 'eventMessage CONTAINS "(<pid>) deny"' for the kernel deny records the probe triggered, converts each deny line into the matching (allow ...) rule, appends it to the profile, and re-runs the probe. The loop stops when an iteration produces no new denies (the probe ran cleanly under the accumulated allows) or when the kernel's per-tuple dedup window swallows every remaining record.

The PID-targeted log query sidesteps the dedup window: each iteration's probe runs as a new process with a fresh PID, so the kernel emits a fresh deny record even when the operation tuple repeats. The older (trace "<file>") mechanism is silently ignored on macOS 26+ and is no longer used.

Output:

tools/sb-trace/<cap>.allow         (committed after hand-review)

After a run, hand-review each .allow seed before committing. The emitted seeds usually need two passes:

1. Replace host-specific literal paths with regex matches. For instance /Users/eli/.pyenv/versions/3.11/lib/python3.11/... should become a regex anchored on ^/Users/[^/]+/\\.pyenv/.
2. Group related rules onto one (allow <op> a b c ...) directive when the targets share semantics.

The parser logic that turns one deny line into one allow rule is exercised in CI via tests/sb_trace_script.rs, which invokes tools/sb-trace.sh --selftest — a mode that runs the parser against canned input and exits non-zero on any mismatch.

Activating a seed at runtime

Set both env vars before invoking nyx:

export NYX_SB_DENY_DEFAULT=1
export NYX_SB_SEED_DIR="$(pwd)/tools/sb-trace"

The seed dir defaults to tools/sb-trace/ relative to the workspace root, so the second env var is only needed when running outside the workspace.

The runtime splice is opt-in. Production builds leave the baked (allow default) body intact unless the operator flips the env var.

Verifying a seed end-to-end

The smoke test deny_default_seed_loads_under_strict in tests/sandbox_hardening_macos.rs exercises the splice through the production call site. It writes a synthetic seed to a tempdir, points NYX_SB_SEED_DIR at it, calls profile_path, and asserts the materialised file contains both (deny default) and the synthetic seed body.

For a real-host smoke test against a generated seed, run:

NYX_SB_DENY_DEFAULT=1 \
NYX_SB_SEED_DIR="$(pwd)/tools/sb-trace" \
cargo nextest run --features dynamic --test sandbox_hardening_macos

When every cap profile has a seed that lets the python3 / node cold-start clear, the macOS strict-mode acceptance row in .github/workflows/dynamic.yml flips from "ships (allow default)" to "ships deny-default by default" — that's the closing condition for the Phase 18 follow-up.