Tianyi Cui
2.7 KiB
Agent Note: Windows write-permission semantics — inherited DACLs, not mode bits
Status: implemented
The replacement-file decision in this record is superseded by Windows DACL preservation.
Problem
writeFileAtomic in @deepseek-ai/dsh-fs-local protects write-in-progress content with POSIX mode bits: the staging directory is created 0o700, the temp file is opened 0o600, and new files default to 0o600. On POSIX this keeps temporary content owner-only regardless of the parent directory's permissions.
Windows has no working equivalent behind the same API. Node's chmod there drives only the read-only attribute (every mode this package passes carries owner-write, so the calls are benign no-ops), and stat().mode reports synthetic 0o666/0o444 bits. The real security state is the file's DACL: a newly created file or directory inherits from its parent, while replacement needs the explicit handling owned by the superseding Agent Note.
Decision
New Windows files use directory inheritance rather than synthetic mode bits: the staging directory is created inside the target's parent directory (dirname(absolutePath)), so it and the temp file inherit the destination directory's DACL. Replacement files follow the stricter DACL preservation contract.
Tests assert mode bits on POSIX only. Native Windows coverage pins the package-owned replacement behavior; new-file inheritance remains an operating-system contract rather than a machine-specific ACL allowlist.
Alternatives considered
Explicit owner-only DACLs for new files. Rejected because they would break inheritance and surprise users whose project directories are deliberately shared. Replacement writes copy the target's existing DACL rather than inventing an owner-only policy.
Test-side ACL verification. A Get-Acl SID allowlist or icacls would verify Windows inheritance and the machine's %TEMP% ACL rather than package behavior; icacls also localizes well-known account names, making parsing locale-fragile.
Skip chmod on Windows. Platform-guarding benign no-op calls adds branches without changing behavior.
Consequences
POSIX keeps owner-only temp content regardless of the parent directory. A new Windows target inside a broadly accessible directory inherits that accessibility by design; a replacement retains the target's narrower DACL when one exists.
Mode preservation across a replace degenerates to a no-op on Windows: a writable file probes as 0o666, and replaying that through chmod leaves the read-only attribute clear. A read-only target cannot be replaced there because publication fails before the synthetic mode would matter.