controlled-javascript-replay.md

Controlled JavaScript replay

run_controlled_replay executes only operator-selected extracted modules. It does not launch or drive the original application. Static inference, passive browser/Electron observation, isolated replay, and real application behavior remain different authorities.

Enable the ceiling

Replay is disabled by default. Configure exact source roots before REA starts:

export REA_JAVASCRIPT_REPLAY_ENABLED=true
export REA_JAVASCRIPT_REPLAY_ROOTS_JSON='["/absolute/extracted/modules"]'

The default trusted executables are the current Node.js runtime, /usr/bin/bwrap, /usr/bin/systemd-run, /usr/bin/systemctl, and /usr/bin/bash. Override them only with the corresponding REA_JAVASCRIPT_REPLAY_*_PATH variable. Configuration creates the administrator ceiling; it does not replace per-call approval. rea doctor reports disabled, available, and exact sandbox-probe failures without running module code. REA setup never installs these host components.

Plan, review, execute

Call the same CLI command or MCP tool twice. The first request uses mode: "plan". Its response commits module paths and digests, closed dependency aliases, cases, deterministic providers, runtime/backend identities, resource limits, no-network policy, private filesystems, and a plan_digest. Runtime commitments include the exact worker, seccomp filter, Node executable, ELF loader, and shared-library source/destination paths and SHA-256 digests.

The execute request must repeat the same manifest and include:

{
  "mode": "execute",
  "approved": true,
  "plan_digest": "<exact digest returned by plan>"
}

REA rebuilds the plan. Any changed module, stub, case, runtime, backend, limit, or export commitment returns plan_stale before worker admission.

Modules use esm or commonjs-factory. The latter accepts extracted Rspack factory syntax such as 123(module, exports, require) { ... } and implements the bounded require.d, require.r, require.n, and require.nmd helpers. Every import or require must map to a declared alias. There is no ambient package resolution.

Explicit cases can be combined with deterministic parser-boundaries, sanitizer-boundaries, or clipboard-boundaries generation. Supplying a right manifest runs both sides with fresh realms per case and reports equal, changed, or unknown comparisons.

Boundary and Evidence

The worker receives module bytes over bounded stdin and runs under a fresh Bubblewrap user/PID/network/IPC/UTS namespace, an architecture-checked seccomp filter, an empty mount root with a descriptor-backed read-only Node closure, private tmpfs, and a transient systemd user cgroup. Host and external network, host writes, process spawning, workers, native addons, inspector access, and ambient environment are unavailable. Wall time, memory, swap, tasks, CPU, case input, worker-protocol input, output, stderr, depth, and node counts are independently bounded. The parent also requires an exact worker response shape and authenticates every returned case ID, order, and input digest. Values are projected through own data descriptors only; Proxy values and accessors are rejected without invoking their traps or getters.

Returns, exceptions, serialization failures, denials, timeouts, OOMs, crashes, protocol failures, and cleanup state are observations with provider rea-javascript-replay and authority controlled-replay. An observation does not prove that the original renderer, preload, main process, browser, or remote service behaved identically.

Each left/right run is retained as its own observed source Evidence. A differential envelope is derived and links those source Evidence IDs, so the comparison never erases the underlying observations.

Optional reproducer_export is committed by the plan and requires both approved: true inside that object and separate evidence_write authority. The owner-only manifest is written only after complete sandbox cleanup. Source bytes are excluded unless include_sources: true was explicitly approved. An export failure is retained in the result and does not erase a completed replay observation.

Local real-artifact verification

npm run verify:replay uses source-owned parser and hostile fixtures. An operator can additionally supply a local ESM or Rspack/CommonJS-factory manifest without copying module source into the repository:

REA_REPLAY_INPUT_PATH=/absolute/replay-manifest.json npm run verify:replay

The verifier canonicalizes the manifest's module paths and prints only plan, module, and Evidence digests, case/comparison summaries, and cleanup state. This is the intended seam for local Notion-like parser/sanitizer benchmarks.