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# Conflicts: # .agents/notes/implemented/feature/2026-07-14-cross-family-fs-sandbox.i18n.yaml # .agents/notes/implemented/feature/2026-07-14-cross-family-fs-sandbox.md # .agents/notes/implemented/feature/2026-07-14-cross-family-fs-sandbox.zh.md # docs/capability-seams.md # docs/cordis-catalog/events.md # docs/cordis-catalog/services.md # docs/event-producer-consumer.md # docs/module-graph.md # docs/persistence-catalog.md # docs/rfc/INDEX.md # examples/acp-agent/README.md # examples/acp-agent/fs.cordis.snapshot.yml # examples/acp-agent/fs.cordis.yml # examples/acp-agent/tests/snapshots/escalation-approved/session.jsonl # examples/acp-agent/tests/snapshots/escalation-rejected/session.jsonl # examples/acp-agent/tests/snapshots/hook-cc-pretool-ask/session.jsonl # examples/acp-agent/tests/snapshots/permission-switching/session.jsonl # examples/acp-agent/tests/snapshots/workspace-context/system-prompt.expected.md # examples/acp-agent/tests/snapshots/workspace-context/tool-schemas.expected.json # examples/acp-agent/tests/snapshots/workspace-edit/system-prompt.expected.md # examples/acp-agent/tests/snapshots/workspace-edit/tool-schemas.expected.json # packages/bash/bash/src/index.ts # packages/bash/tool-bash/package.json # packages/bash/tool-bash/src/index.ts # packages/bash/tool-bash/tests/tools.spec.ts # packages/cordis/tool-cordis/src/api-catalog.ts # packages/fs/README.md # packages/fs/tool-fs/src/edit.ts # packages/fs/tool-fs/src/write.ts # packages/sandbox/README.md # pnpm-lock.yaml
34 lines
3.8 KiB
Markdown
34 lines
3.8 KiB
Markdown
# dsh-fs-sandbox — the sandbox-enforcing filesystem backend
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`SandboxedFileSystem` extends [`LocalFileSystem`](../fs-local/README.md) and registers as `ctx.fs`. It inherits every text-storage mechanic verbatim (resolve, stat, read/stream, list, the atomic write, the read-match-write edit critical section) and adds only a per-call MODE fence on `writeText`/`editText`. Reads always pass through — every mode permits reading.
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Loading it INSTEAD OF `dsh-fs-local`, together with a [`ctx.sandboxPolicy`](../../sandbox/sandbox-policy/README.md), is the whole swap; the model-facing tools (`dsh-tool-fs`) are untouched. Injects `sandboxPolicy` for the default mode and the `workspace-write` boundary root — the SAME policy home bash reads, so the two families never confine to different roots.
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## The fence
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The per-call mode is the tool-stamped effective mode (session override or escalation grant), falling back to the deployment default:
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- `read-only` — denies every mutation with the structured `FS_SANDBOX_DENIED`.
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- `workspace-write` — allows a mutation only when the target canonicalizes under a writable root: the workspace root plus the platform temp areas (`/tmp`, `os.tmpdir()`), the SAME set the Seatbelt profile grants, derived from the one [`writableRoots`](../../sandbox/README.md) function so the fs fence and the bash runner cannot drift. The target is re-canonicalized immediately before delegating, so an ancestor symlink swapped since the tool resolved it is caught.
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- `danger-full-access` — delegates unfenced.
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## Threat model: a policy fence, not a kernel boundary
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The fence is a check in TRUSTED code over a MODEL-CONTROLLED path — the operations are the seam's own (open, rename), only the target path is untrusted, so canonicalize-then-contain is the complete answer to this surface. This mirrors the `code-runtime` stance: containment, not a security boundary. Kernel-grade isolation of untrusted CODE stays `ctx.bash`'s job ([`dsh-bash-sandbox`](../../bash/bash-sandbox/README.md)). The residual TOCTOU (an ancestor symlink swapped between the containment re-check and the syscall) is narrowed by re-canonicalizing immediately before the write and is accepted for this threat model; a kernel-tight boundary needs `openat2`-class primitives not worth their portability cost here.
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A denial is a structured `FsError` (`FS_SANDBOX_DENIED`, carrying the effective mode) — no stderr text inference (unlike bash's kernel denials), because an in-process fence knows exactly what it refused. The model-facing `[sandbox: file access denied under <mode> mode]` marker and the one-approved-wider retry live in the tool layer (`dsh-tool-fs`), exactly as bash's do. See [the cross-family fs sandbox RFC](../../../.agents/notes/implemented/feature/2026-07-14-cross-family-fs-sandbox.md).
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## Model Experience
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Indirectly, through `dsh-tool-fs`, which renders this backend's `FS_SANDBOX_DENIED` refusals as the `[sandbox: file access denied under <mode> mode]` marker plus the same-turn escalation hint.
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#### KV Cache effect
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No direct invalidation; the named consumer owns any request-prefix changes.
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## Known Limitations and Deferred Work
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- **A policy fence, not a kernel boundary** — the check is trusted code over a model-controlled path, so the residual resolve-to-syscall TOCTOU is narrowed (by the in-place re-canonicalization) but not eliminated; adversarial host processes are out of scope. Kernel-grade isolation of untrusted code stays `ctx.bash`'s.
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- **Fence-vs-runner parity is derived, not asserted** — the writable set comes from `writableRoots`, shared with the Seatbelt profile and pinned by a parity test; a runner profile that changed its writable set without that function would drift.
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- **Requires `ctx.sandboxPolicy`** — the backend reads the default mode and workspace root from it and does not confine without it composed.
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