fix(link): invoke sh bare and surface non-UTF-8 ssh errors

[lidge-jun]

Summary

Fixes #6088. On a Windows OpenSSH server whose DefaultShell is PowerShell, joining as a Link Child failed with a misleading ssh output was not valid UTF-8. Two things combined:

1. quoteRemote single-quoted every argv element, including the command name. PowerShell parses a leading 'sh' as a string expression, so the next token '-c' is a parse error.
2. The Child's SSH runner decoded stderr with a fatal UTF-8 decoder. PowerShell's error text is often in a legacy code page (for example CP936), so the runner threw a generic decode error and the real diagnostic never reached sshFailureHint.

After this change:

• quoteRemote accepts exactly sh in command position and emits it bare. Every argument, including the -c script, stays single-quoted, and NUL stays rejected. Any other command name throws LinkSshArgumentError (an allowlist of one, so names like 1, . or -x that PowerShell would not dispatch cannot appear). Every current caller builds its argv through remoteOcxArgv, which already starts with sh.
• stderr is capped in bytes first and then decoded with replacement, so the existing sshFailureHint path can strip controls, redact OpenCodex secrets and URL queries, and cap the hint at 160 code points. Structured stdout (the version line, link issue JSON) stays strict UTF-8.

structure/remote-link.md and the Remote Link guide's troubleshooting text are updated. The guide does not broaden any Windows support claim.

Verification

• bun test tests/clients/link-ssh-argv.test.ts tests/server/link-management-routes.test.ts: 40 pass, 1 skip, 0 fail on macOS (36 pass before). The skipped case is the Windows-only PowerShell parser test.
• Red-green: the new argv and runner tests produced 4 failures against the old code and pass after the fix.
• New coverage: bare sh output and the rejected command forms; a POSIX case that executes the constructed remote string through /bin/sh -c and checks that argument bytes (quotes, newlines, non-ASCII, $(...), empty) arrive intact; invalid-UTF-8 (CP936) stderr producing a redacted, bounded hint while the key sent on stdin stays separate; invalid-UTF-8 stdout still failing with decode; the stderr byte cap applied before decoding.
• Windows-only: a test that runs powershell.exe and uses [System.Management.Automation.Language.Parser]::ParseInput on the constructed command, asserting no parse errors and a single CommandAst named sh. PR CI skips Windows shards, so I will dispatch the Cross-platform CI all lane on this exact head and confirm from the Windows log that this test ran and passed before [Bug]: SSH link mode fails as "ssh output was not valid UTF-8" when the remote's default shell is PowerShell (real error masked) #6088 is closed.
• bun run typecheck, bun run structure:check, bun run privacy:scan, docs-site build: pass. bun run test:changed from a same-commit throwaway checkout: result added below once the run completes. The local full suite was not run because seven release lanes share this machine and its test lock.

Known limit

The reporter verified the full join against a PowerShell 7 (pwsh) DefaultShell. Windows PowerShell 5.1 uses legacy native-argument passing, which does not escape embedded double quotes, and the -c script contains them (PATH="..."; exec ocx "$@"). That combination is untested here and may still fail. It would need either a quote-free script or a PowerShell-side escape, which is left as a follow-up.

Security review

Assets: link data keys, host identity, and error diagnostics. Entrypoints: locally constructed remote argv, and untrusted SSH stderr. The command position is now a fixed allowlist, and every argument remains single-quoted data for the invoked sh, so the change removes quoting from a constant and never from user input. Host-key policy, BatchMode, --key-stdin delivery and link admission are unchanged, and this PR does not touch join admission (#6076). stderr reaches the user only through the existing redacting, length-bounded hint, and it is never logged. Replacement decoding cannot widen what is shown, because it runs after the byte cap and before the same sanitizer.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-27T15:54:27.865494Z	a08c375	PR opened

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📝 Walkthrough

Walkthrough

The SSH link changes require sh as the remote command and quote its arguments individually. SSH stderr now supports non-fatal UTF-8 decoding, while stdout remains strict. Link error handling caps stderr bytes before decoding and documents the sanitized failure hint.

Changes

SSH link execution

Layer / File(s)	Summary
Remote command construction src/link/ssh-argv.ts, structure/remote-link.md, tests/clients/link-ssh-argv.test.ts	quoteRemote requires sh in command position and quotes the remaining arguments. Tests cover rejected command values, PowerShell parsing, and preservation of arguments containing special characters. See src/link/ssh-argv.ts:117, 180–186, structure/remote-link.md:7, and tests/clients/link-ssh-argv.test.ts:67–79, 133–162, 193–205.
SSH output decoding and failure hints src/link/ssh-runner.ts, structure/remote-link.md, docs-site/src/content/docs/guides/remote-link.md, tests/clients/link-ssh-argv.test.ts	SSH stderr uses replacement decoding; stdout remains strict UTF-8. Link error handling caps stderr bytes before decoding. Tests cover malformed stderr, failure-hint redaction and length, unchanged stdin bytes, strict stdout decoding, and the byte limit. The guide describes the sanitized hint. See src/link/ssh-runner.ts:96, 121, 153, 184–185, structure/remote-link.md:41, docs-site/src/content/docs/guides/remote-link.md:67, and tests/clients/link-ssh-argv.test.ts:226–266.

Priority: ➖ Normal

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

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: 🔵 Low · up to a08c3

The Windows SSH-link fix remains untested at the PowerShell-to-sh argument boundary. This is a bounded confidence gap rather than a demonstrated failure, so the change is mergeable with owner awareness.

Security Architecture Review

Security architecture risk: 🔵 Low · up to a08c3

The remote command remains restricted to sh, and error output remains bounded and sanitized before it is shown. The remaining uncertainty is whether arguments containing special characters reach sh unchanged when PowerShell is the remote shell.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The relevant trust boundary is an authorized Link operation invoking a command on its confirmed SSH host, then consuming that host's stderr as a bounded diagnostic. The visible change does not grant an arbitrary remote command name.

Trust Boundaries and Controls

• observed — The Windows-only test establishes that PowerShell parses the generated command as one sh invocation with simple arguments. A separate POSIX execution test checks special-character argument preservation; neither establishes native argument delivery through PowerShell for those inputs.
• observed — The management Link routes require dashboard or loopback-admin authorization rather than accepting the SSH operation without route authorization.

Resilience and Maintainability Implications

• inferred — Handled failures after a link ID is known have compensation paths, but the inspected source does not establish recovery if remote issuance commits and its response is lost before the client learns that ID. This is an unresolved lifecycle boundary, not a verified PR-introduced failure.

Hardening Proposals

• proposed — Before relying on PowerShell-backed Link execution for arguments containing untrusted text, verify the values received by native sh for apostrophes, PowerShell metacharacters, newlines, and empty arguments; parser success alone does not prove this boundary.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 28.57% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 3 files. (2 skipped: 2…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue [#6088] requires the SSH link command to work with a PowerShell default shell or to expose the remote failure instead of a UTF-8 decode error. src/link/ssh-argv.ts implements this in `quoteRem…
Out of Scope Changes check	✅ Passed	The changes stay within [#6088]. src/link/ssh-argv.ts changes remote command quoting and validation. src/link/ssh-runner.ts changes only SSH output decoding and bounded diagnostics. `tests/clients…
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the two main changes: invoking remote sh without quotes and surfacing SSH errors with non-UTF-8 output. It is concise and specific.

Full details: Docstring Coverage

Explanation

Docstring coverage is 28.57% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 3 files. (2 skipped: 2 unsupported.)

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[lidge-jun]

리뷰 · 우선순위 60 / 80

이 PR은 Windows OpenSSH의 기본 셸이 PowerShell일 때, 링크 Child로 붙는 명령이 실패하는 문제를 고쳐요. #6088이에요.

예전 원격 명령은 'sh' '-c' '...'처럼 명령 이름까지 작은따옴표로 감쌌어요. PowerShell은 앞에 따옴표가 붙은 단어를 명령이 아니라 문자열로 읽어요. 다음 칸 '-c'에서 파서가 막혀요. 그 오류 글은 중국어 Windows면 CP936 같은 예전 코드 페이지로 나와요. Child의 SSH 실행기는 stderr를 UTF-8로만 읽다가, 바이트가 하나라도 어긋나면 ssh output was not valid UTF-8을 던졌어요. 진짜 이유는 화면까지 못 갔어요.

이제는 quoteRemote가 명령 이름이 정확히 sh일 때만 통과시켜요. 그 자리만 따옴표 없이 sh로 내보내요. 나머지 인자는 예전처럼 작은따옴표로 감싸요. 널 문자가 있으면 거절해요. printf처럼 다른 이름은 LinkSshArgumentError예요. 지금 호출부는 전부 remoteOcxArgv라서 맨 앞이 sh예요. stderr는 바이트 상한을 먼저 자르고, 깨진 바이트는 대체 문자로 읽어요. 그 다음 기존 sshFailureHint가 제어 문자, 링크 키, URL 물음표 뒤를 지우고 160글자 안으로 줄여요. 버전 줄이나 link JSON 같은 stdout는 깨진 UTF-8이면 예전처럼 실패해요. 베이스는 dev예요.

tests/clients/link-ssh-argv.test.ts:148 - Windows 테스트는 PowerShell 파서에게 명령 이름이 sh인지만 물어요. sh를 실행하지 않고, 파서가 받은 인자 글도 비교하지 않아요. 따옴표, 줄바꿈, 빈 문자열이 남는 검사는 POSIX에서 /bin/sh -c로만 해요. Windows에 기본으로 있는 Windows PowerShell 5.1은 바깥 프로그램에 인자를 넘길 때 큰따옴표를 다시 만들어요. 원격 스크립트에는 "$PATH"와 "$@"가 들어 있어요. 파서는 통과해도 sh가 받은 스크립트는 깨질 수 있어요.

src/link/ssh-argv.ts:186 - 내보내는 명령 토큰은 sh 하나예요. PowerShell이 PATH에서 sh.exe를 찾아야 다음이 돌아요. #6088 재현 환경의 우회는 기본 셸을 C:\msys64\usr\bin\bash.exe로 바꾸는 거예요. 그 폴더는 보통 PowerShell PATH에 없어요. sh를 못 찾으면 접속은 그대로 실패하고, 이번 수정은 오류 문장만 보여 줘요.

src/link/ssh-argv.ts:145 - 주석은 아직 로그인 셸이 스크립트를 그대로 넘긴다고 적혀 있어요. 명령 이름은 이제 따옴표가 없어요. PowerShell이 인자를 다시 만들 수 있는 자리예요. structure/remote-link.md는 그 설명을 고쳤는데, 이 주석은 그대로예요.

메인테이너의 판단이 필요한 지점

완료의 기준을 정해 주세요. 오류 문장이 더 이상 가려지지 않으면 충분한지, PowerShell 기본 셸에서 Child 접속까지 되어야 하는지예요. PR 설명은 두 가지를 같이 말해요. #6088이 바라는 결과는 접속에 성공하거나, 실패하면 원격의 진짜 이유가 보이는 거예요. stderr를 대체 문자로 읽는 수정은 두 번째에 맞아요. 접속 성공은 sh가 PATH에 있고, Windows PowerShell 5.1이 "$@"를 깨지 않을 때만 맞아요.

너의 추천

stderr 디코드는 머지해도 돼요. stdout는 엄격한 UTF-8로 남아 있고, 힌트에서 키와 주소 물음표 뒤를 지우는 길과 바이트 상한은 테스트가 있어요. 키는 stdin으로 따로 가요.

#6088은 Windows에서 확인하기 전에 닫지 마세요. 기본 셸이 Windows PowerShell 5.1인 컴퓨터에서, 만든 원격 문자열을 powershell.exe가 실제로 실행하게 두고 ocx 인자 바이트가 남는지 봐 주세요. 파서 트리만 보면 부족해요. sh가 없으면 힌트에 그 문장이 나오는지만 확인하고, sh.exe를 찾는 일은 다음 수정으로 빼 주세요.

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Actionable comments posted: 1

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Inline comments:
In @src/link/ssh-argv.ts:
- Line 186: Add a Windows-only test for the command-generation path containing
the `index === 0` return: invoke the generated command through PowerShell with a
local `sh.exe` fixture and assert the received script and arguments preserve
embedded quotes, spaces, and an empty argument. Do not require a live SSH
endpoint; make the `sh.exe` prerequisite explicit if the supported Windows test
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• docs-site/src/content/docs/guides/remote-link.md
• src/link/ssh-argv.ts
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• tests/clients/link-ssh-argv.test.ts

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[lidge-jun]

This slice is integrated through the batch PR #6113 on current dev (6d64ea2), one commit per slice, so the three fixes need one CI cycle instead of three serial rebase cycles. The commit is tree-identical to this PR's head, plus the two Codex review fixes for #6108. I will close this PR once #6113 merges.

[lidge-jun]

Superseded by #6113, merged into dev as 2275680ab3. This slice landed unchanged as commit 9f50bddfe8.