fix(core): keep no-follow reads protected where O_NOFOLLOW is missing - #10007

Closeout — cap-4 round on the 12:31Z review (single non-force push c067b924..47d0757fb to the fork branch):

• C1 FIXED (bdecd4c): reproduced at head — sessionService.rename.test.ts 10 failed | 9 passed. Root cause: the no-follow helper bound node:fs via namespace import; vitest externalizes CJS namespace imports with their own module copy, so the suite's vi.spyOn(fs, ...) never intercepted it. Fix keeps the helper spy-mockable. Suite now 19/19 (= the red required CI Test).
• C2 FIXED (84df828): inode-0 fail-closed refusal no longer carries code: 'ELOOP' — distinguishable from genuine symlink refusals, so consumer ELOOP handling (sessionArtifacts, workspace-registration-store, gitDiff) stops misfiring on legitimate files on inode-0 volumes (FAT/exFAT/SMB). The refusal posture itself (fix(core): fail closed on zero inode file cache #8290/fix(core): reject unverifiable validated read inodes #9857) unchanged. Note: spans 6 files (>3 ceiling) because the finding itself names 4 fix sites + test + index export; each hunk small and mechanical.
• S1 FIXED (55ce9d7): dropped the never-used speculative flags?/mode? params from the no-follow opens (no caller repo-wide).
• S3 FIXED (47d0757): plural readLastJsonStringFieldsSync symlink-refusal regression added; mutation red (plain fs.openSync revert leaks the secret).
• S2 DEFERRED to next round (untouched, within cap): async TOCTOU re-check pinning test — valid, bot probe design ready.

Verification: rename suite 19/19, no-follow-open 10/10, sessionStorageUtils 55/55, 411 passed across 6 directly-hit files, cli workspace-registration-store 26 passed; typecheck clean core/cli/acp-bridge; eslint+prettier clean. Not run locally: gitDiff fixture tests (host git 2.19 lacks git init -b; fails identically without these changes). Push note: this host's git-over-https to github.com was blocked, so the push went through the GitHub Git Data API with every tree/commit SHA verified byte-identical before the fork ref fast-forward.

Closeout round: cleared the last unresolved finding (deferred S2).

• 88d16cb50 pins the async TOCTOU identity re-check of openNoFollow: fs.promises.lstat is mocked with doctored stats (ino + 1, same prototype trick as the sync test) so the real opened handle's stat mismatches before; the test asserts the ELOOP rejection and exactly one handle.close().
• Mutation check: deleting the try { assertSameIdentity(...) } catch { await handle.close() } block fails exactly the new test (1 failed | 10 passed); restored code green. Suite 11/11.
• Pushed 47d0757fb..88d16cb50 (non-force); thread replied with SHA evidence + resolved. 0 unresolved now; review lane auto-triggered by the push.

The Ubuntu failure was PR-caused, not an infra flake: check:serve-fast-path-bundle found the ACP pre-listen closure pulling in the core barrel, including shell.ts, fzf, @iarna/toml, chokidar, and glob.

Fixed in 7354a1f2b by exporting the no-follow helper through a leaf core subpath and importing that subpath from sessionArtifacts.ts, keeping the ACP path off the eager barrel. The directly hit bundle-check suite passes (35 passed). Full core typecheck/build could not be reproduced in this existing worktree because its shared dependencies are stale/incomplete (missing OTel/fdir/etc.); no install was run. Fresh CI is now running on the new head.

CI note — Test (ubuntu-latest, Node 22.x) failure on run 32945995633 is an infra flake, not PR-caused:

• Log shows Test Files 65 passed (65) / Tests 1743 passed | 11 skipped — zero assertion failures.
• The only error is Error: [vitest-worker]: Timeout calling "onTaskUpdate" (vitest worker RPC hang in Unhandled Errors).
• CI's own deflake machinery classified it: deflake issue creation failed for #42; the rerun stands and it retries on the next flaky occurrence: 502 from GitHub.
  Conclusion: known flake TypeError: Cannot read properties of undefined (reading 'value') #42; rerun needed.

@qwen-code /triage

@qwen-code /triage

Merge conflict with main resolved by merging main into the branch (merge commit e2c80ed, plain non-force push — branch history preserved).

Conflict: packages/acp-bridge/src/sessionArtifacts.ts only. Main switched the workspace identity check to bigint stats (NTFS 64-bit file-id precision); this PR replaced the raw O_NOFOLLOW open with openNoFollow(). Resolution keeps both: bigint lstat for the pre-open identity, and the openNoFollow() helper for the open, so no-follow protection stays intact on platforms without O_NOFOLLOW (#8227).

Verification at the merge commit:

• core: no-follow-open.test.ts, sessionStorageUtils.test.ts, gitDiff.test.ts (127/127), backgroundShellRegistry.test.ts, sessionService.rename.test.ts — all pass
• acp-bridge: sessionArtifacts.test.ts — 131/131 pass
• cli: workspace-registration-store.test.ts — 27 pass / 1 skipped
• typecheck clean for core, acp-bridge, cli

Mergeable: now MERGEABLE (was CONFLICTING).

Note: reviewDecision shows CHANGES_REQUESTED, but the only CR reviews on this PR are two stale ones from qwen-code-ci-bot; the latest human review (@doudouOUC, 2026-08-26) found no blocking issues.

Local verification of PR #10007 — real environment, real symlinks

I built both arms locally (main at merge-base 2bd0ff9 vs. this PR at e2c80ed) and verified the claim end to end rather than only re-running the suite. Verdict up front: the fix does what it says, no blocking issue found. Two non-blocking observations are at the bottom, one of which is a one-line hardening I'd suggest before merge but would not hold the PR for.

How the Windows condition was reproduced

The interesting behaviour only exists where fs.constants.O_NOFOLLOW is undefined, so instead of mocking fs I removed the constant process-wide with an ESM loader hook (node --import) that resolves every node:fs / fs import to a shim: the real module, with exactly one property replaced. Everything else stays real — real files, real openat, real symlinks, the real bundled CLI. A control probe confirms a plain open still follows a symlink under the shim, i.e. only the flag is gone, not the kernel semantics.

Two more real-condition simulators were built for the fail-closed paths:

• an LD_PRELOAD shim intercepting the syscall(SYS_statx, …) wrapper libuv actually calls (glibc's statx() wrapper is bypassed by libuv, so the naive interposer does nothing) to make one directory subtree report st_ino == 0, the way FAT/exFAT does;
• the same interposer remapping two files' inodes to the NTFS shape — adjacent 64-bit ids above 2^53.

What the attack actually does, per arm

Both reproductions in the PR description hold on a real filesystem:

• Session metadata — a symlink planted at the session path redirects the read. On main under the Windows flag set the attacker-controlled title reaches the caller; with the PR it is refused. Same for the multi-field reader and for a symlink pointing outside the chats directory.
• Background-shell output tail — on main under the Windows flag set the symlink target's content arrives verbatim inside the <output-tail> element of the task notification, i.e. in model context. I used AWS_SECRET_ACCESS_KEY=wJalrXUtnFEMI/K7MDENG as the payload; with the PR the element degrades to <output-tail error="unreadable" />.
• Controls do not over-refuse — an ordinary regular file and a hard link to one are read normally in all four quadrants.

Driving the real bundled CLI, the session-metadata case is directly visible in /resume. A session-shaped symlink was planted in a real chats/ directory pointing at an attacker file outside it; a genuine session sits next to it as a control.

main, Windows flag set	PR #10007, Windows flag set

The attacker-controlled title PWNED-BY-SYMLINK-ATTACK is listed on main; with the PR it is gone and the entry falls back to its first user message, while the genuine session Refactor the billing module renders identically in both. For completeness, the same main build on native POSIX does not show it — confirming this was a Windows-only exposure, not a general regression:

Race behaviour

A separate attacker process renamed the read path back and forth between a regular file and a symlink while a reader hammered the metadata reader. Over roughly a million reads per arm:

I also tried to defeat the identity proof by getting the legitimate file's inode freed and reused by the attacker's payload: 1,072,354 attempts on ext4, zero inode-reuse hits — the allocator does not hand a just-freed inode straight back, so that theoretical gap is not reachable there.

POSIX is genuinely untouched

strace on both arms opening the same session file produces an identical sequence — one statx, then openat(AT_FDCWD, …, O_RDONLY|O_NOFOLLOW|O_CLOEXEC). No extra lstat/fstat, same flags. The "byte-for-byte unchanged on POSIX" claim is confirmed at the syscall level, not just by reading the code.

Suites, lint, typecheck, and a negative control

• core: 332 passed across no-follow-open, sessionStorageUtils, backgroundShellRegistry, gitDiff, readManyFiles, sessionService.rename. cli workspace-registration-store: 27 passed / 1 skipped. acp-bridge sessionArtifacts: 131 passed.
• ESLint --max-warnings 0 clean on all changed sources; tsc --noEmit exit 0 for core, cli and acp-bridge.
• Negative control: reverting only the six call sites to merge-base while keeping the PR's tests flips exactly 7 tests red (4 core, 1 cli, 2 acp-bridge) and nothing else. The new tests discriminate the fix rather than merely passing.

Non-blocking observations

N1 — the fallback's identity proof compares ino as a Number, and it only ever runs on Windows.
assertSameIdentity compares before.ino !== after.ino from plain lstat/fstat. Stats.ino is a double; on Windows it carries the 64-bit NTFS file id, and above 2^53 adjacent ids collapse onto the same Number. That is the same hazard sessionArtifacts.ts calls out in this very PR — "NTFS file ids are 64-bit and the Number spelling loses precision above 2^53, so two files created close together can round to the SAME numeric ino and defeat the swap check" — and solves with { bigint: true }. The fallback, which is the only identity check that runs on Windows, kept the Number spelling.

Remapping two files' inodes to that shape and re-running the race:

Four call sites, no other change; legitimate reads still succeed at the same rate. Whether real NTFS volumes commonly carry file ids past 2^53 depends on how far the volume's MFT sequence numbers have advanced, which I can't measure from Linux — but the precondition is the one this PR already treats as real elsewhere, and the fix is cheap. Not blocking, since relative to main (no protection at all on Windows) the PR is a strict improvement either way.

N2 — the gitDiff degradation comment is stronger than the behaviour.
openUntrackedForDiffRead says the fallback "cannot follow a symlink the gate did not already accept". Under a concurrent attacker on an ino-0 volume it can: 2,255 diff runs, 76 of which counted the symlink target's 40 lines instead of the 3-line file inside the worktree (0/2,287 with normal inodes, where the helper is enforced). The window is exactly the pre-#8227 one and the PR does not make it worse, so this is a wording nit rather than a defect — something like "the window is the same one that existed before #8227" would be accurate.

N3 — the two readManyFiles conversions are pure defence in depth.
Driving a real @-reference at a symlink through the bundled CLI, the file content never reaches the wire on either arm, on either flag set — an earlier realpath gate rejects it first. Correct to convert, but the observable attack-surface reduction in this PR comes from sessionStorageUtils and backgroundShellRegistry.

N4 — the fail-closed cost is real but bounded and visible.
On an ino-0 volume under the Windows flag set, a legitimate session file's title is no longer read (main returns it, the PR returns undefined), so /resume falls back to the first user message. Nothing crashes and POSIX is unaffected — the same probe on native POSIX still returns the title. This matches the posture the PR describes; flagging it only so the tradeoff is on the record.

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