fix(seatbelt): widen per-user Darwin temp grant to its container

[caarlos0]

📖 Description

On macOS, $TMPDIR is the per-user _CS_DARWIN_USER_TEMP_DIR (/var/folders/<a>/<b>/T/). Its siblings under the same per-user container — C (_CS_DARWIN_USER_CACHE_DIR) and 0 (misc) — are left unwritable when a sandbox is granted read-write access to only the temp leaf, so tools that stage under the per-user cache get denied.

This expands a read-write grant that targets a per-user Darwin temp/cache leaf into read-write grants for its three well-known per-user siblings (.../T, .../C, .../0) in the Seatbelt profile builder (write_filesystem_allow). Because the expansion lives in the profile builder, it applies to every consumer of the macOS backend (CLI, Node SDK, C#/FFI, Rust SDK) — none of them has to carry macOS /var/folders path knowledge.

The expansion is strictly guarded by a new darwin_temp_sibling_grants helper:

• It expands only when the path is exactly /var/folders/<a>/<b>/<leaf> with <leaf> one of T, C, or 0 (optionally under /private, the post-canonicalization form) and <a>/<b> are ordinary segments (never empty, ., or ..).
• It grants only those three siblings — never the enclosing /var/folders/<a>/<b> container itself, so no other directory under the container becomes writable and a caller keeps least-privilege access to everything outside the T/C/0 triad.
• A too-shallow path, a deeper path, a non-temp leaf, a ./.. traversal segment, or anything outside /var/folders yields no expansion — so an expansion can never reach the container root, /var/folders (every user's containers), or /.
• deniedPaths still override the expanded grants, since denies are emitted after allows (Seatbelt last-match-wins), so a caller can carve any single sibling back out.
• It is applied unconditionally in the platform-agnostic profile builder because a Seatbelt profile is always a macOS artifact regardless of build host — matching the existing guiAccess /private/var/folders grant.

The change only adds sibling grants; $TMPDIR (=.../T) itself was already granted, so no previously-working sandboxed command can break.

🔗 References

No linked issue. The macOS backend filesystem behavior is documented in docs/macos-support/seatbelt-backend.md, updated in this PR.

🔍 Validation

All on a macOS host (aarch64-apple-darwin), run from src/:

• cargo test -p seatbelt_common — 49 passed, incl. tests for the sibling expansion, non-per-user rejection (including ./.. traversal), a profile-integration test asserting all three siblings are granted while the bare container is not, and a deny-after-allow test proving a deniedPaths entry on a sibling still wins.
• cargo fmt -p seatbelt_common -- --check — clean.
• cargo clippy -p seatbelt_common --all-targets -- -D warnings — clean.

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)
• If this PR changes Cargo.lock, the dependency-feed-check check passes (no dependency change)

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR adjusts the macOS Seatbelt profile generator to broaden certain readwritePaths entries that target per-user Darwin temp/cache leaves under /var/folders/<a>/<b>/{T,C,0} so the enclosing per-user container is granted instead, preventing cache-related write denials in sandboxed tools. The change is implemented in the profile builder so it benefits all macOS backend consumers (CLI/SDKs/FFI) without duplicating Darwin path knowledge.

Changes:

• Widen readwritePaths grants for /var/folders/<a>/<b>/{T,C,0} (and /private/...) to /var/folders/<a>/<b> in the Seatbelt profile builder.
• Add unit tests validating widening behavior and non-matching cases.
• Document the widening behavior in the Seatbelt backend docs.

Show a summary per file

File	Description
src/backends/seatbelt/common/src/profile_builder.rs	Adds Darwin temp/cache leaf widening logic in write_filesystem_allow plus new helper + tests.
docs/macos-support/seatbelt-backend.md	Documents the readwritePaths widening behavior and its safety constraints.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Low

[caarlos0]

@huzaifa-d thank you, resolved.