Isolate Operators and Strip Comments in the Guard's Shell-Token Fallback

[ptr727]

Summary

When the guard's primary tokenizer cannot parse a command's quoting, most often because an apostrophe sits in a trailing comment, the fallback split each line plainly and saw no operators. A loop like while ! <check>; do sleep 30; done # the PR's checks then went unseen (#1635).

• The per-line fallback now isolates operators on any line that parses on its own. A line that does not parse falls back to plain splitting, as before.
• _strip_comments strips every comment over the whole command, and carries plain-quote state across lines as bash does. The stripped text is parsed before the per-line fallback runs.
• The strip is skipped wherever bash nests a context the scan does not model: a heredoc, ((, an extglob opener, backslashes, backticks, and $(, ${, $' and $".
• When the strip is used, develop's plain per-line tokens are appended after its tokens. The strip can then reveal a command the base fallback missed, but can never hide one it saw.
• _heredoc_opener keeps the base tokenizer, with no comment stripping.

The line flagged comment-added by the prose gate is the existing shlex commenters comment, moved unchanged into the shared _operator_lex helper.

Verification

• The self-test passes. It includes new cases for a quote spanning lines, arithmetic and extglob #, and a =~ regex #, each proven to fail without its fix.
• A differential fuzz against develop ran 30,000 generated commands per run. None went from deny to allow on the final head.
• Across all runs, bash with stubbed gh and git confirmed that every earlier deny-to-allow flip was a write inside a real comment. Bash never runs any of them.
• No deny appears that develop would not make and the previous head allowed. The remaining allow-to-deny cases are the An Apostrophe in a # Comment Defeats Requirement 7 Entirely #1635 fix: unbounded loops that develop missed.
• Three recorded local strict-review passes. The last found only a false deny in the per-line fallback, filed as Read a Multi-Line Quote Whole in the Guard's Token Fallback #1910.

Closes on promotion: #1635

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved shell command handling when quotes are malformed or comments appear, so operators and subsequent commands are interpreted more reliably.
  • Improved recognition of heredoc openers and wait loops with trailing comments, while preserving the behavior of quoted operators.

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📝 Walkthrough

Walkthrough

The shell write guard adds reusable tokenization helpers and optional comment stripping after parse failures. New self-test cases cover write detection and wait-loop classification across malformed quoting and comment contexts.

Changes

Shell Command Guard

Layer / File(s)	Summary
Comment-aware shell tokenization host-setup/agent-safety/claude/gh-write-guard.py	Shared lexer and fallback helpers support optional comment stripping after parse failures. Heredoc opener detection calls _shell_tokens with comment stripping disabled.
Write and wait-loop classification tests host-setup/agent-safety/claude/gh-write-guard.py	Self-test cases cover write detection in malformed-quote and comment contexts, and check bounded and unbounded wait-loop classification.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to 03410

The guard's new comment handling can still allow an unbounded wait loop when a trailing comment contains a backslash, and can wrongly block a timeout-bounded loop when a comment contains &. Resolve or confirm these cases before merging.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to 03410

A verified gap can let an unbounded shell wait pass the documented Bash safety check. The available comparison indicates that this particular gap predates the PR, while the changes improve detection for other trailing-comment cases. The evidenced exposure is a command on the local host, not a broader service boundary.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The independently evidenced exposure is a Bash hook invocation on the host. The available evidence does not establish cross-host, cross-tenant, or shared-store effects.

Security Findings and Attack Paths

• observed — The retained finding reports that a backslash in comment text can select a plain-splitting path that misses the separators of an executable unbounded wait, allowing the command past the wait check. The available comparison indicates this is an existing blind spot, not a bypass newly created by the PR; the full PR base was not independently hydrated.

Trust Boundaries and Controls

• observed — The control rejects a recognized sleeping while/until loop without an accepted bound. A reported allow decision does not produce the hook’s deny output.

Hardening Proposals

• proposed — Handle malformed quoting when comment stripping is skipped without relying solely on whitespace-split tokens for the wait-loop decision; preserve the distinction between executable operators and comment text.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: isolating shell operators and stripping comments in the guard's fallback tokenizer.
Docstring Coverage	✅ Passed	Docstring coverage is 85.71% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 1 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Copilot review overview

🟡 Changes recommended

The new comment-stripping logic can still miss unbounded loops when # appears after | inside [[ ... =~ ... ]] regex text, leaving a demonstrated requirement-7 bypass.

Review effort: Lite
Findings: 1

Open (1)

• Regex alternation can trigger premature comment stripping · New

What changed in this PR

This PR updates the gh-write-guard shell-token fallback so operator characters are isolated more reliably when the primary tokenizer fails (notably when an apostrophe appears in a trailing # comment), and adds comment-stripping + expanded self-test coverage to prevent unbounded-wait loops from being missed.

Changes:

• Add _strip_comments, _operator_lex, and improved fallback tokenization to better preserve operator boundaries even under malformed quoting.
• Combine comment-stripped operator tokens with the prior base fallback token stream to avoid hiding commands when # is actually literal text.
• Extend the self-test suite with cases covering multi-line quotes and # in contexts like arithmetic/extglob/regex.

File	Description
host-setup/​agent-safety/​claude/​gh-write-guard.py	Refines fallback shell tokenization and adds targeted self-tests to close an operator-detection bypass path.

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[ptr727]

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Actionable comments posted: 2

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In @host-setup/agent-safety/claude/gh-write-guard.py:
- Line 352: Update _COMMENT_SCAN_BAIL handling so a backslash inside a confirmed
shell comment does not abort comment stripping or prevent _loop_parts from
detecting an unbounded loop. Apply the bail check only to text outside confirmed
comments, while preserving operator isolation for the remaining loop text.
- Line 441: Update token handling around _operator_lex and _base_tokens so
_forks_out_of_reach checks fork operators only from comment-stripped command
tokens, not operators originating in comments. Preserve the base tokens for
other parsing while keeping the inherited timeout bound for the bounded loop.

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Reviewing files that changed from the base of the PR and between 0bdde8d and 0341099.

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• host-setup/agent-safety/claude/gh-write-guard.py

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