fix: Potential fix for code scanning alert no. 15: Uncontrolled data used in path expression

[skyoo2003]

Potential fix for https://github.com/skyoo2003/devcloud/security/code-scanning/15

Use a safer containment check in internal/services/s3/store.go by validating with filepath.Rel between canonical absolute paths, instead of strings.HasPrefix.

Best fix (without changing functionality):

1. In safePath, build candidate path with filepath.Join.
2. Canonicalize both fs.baseDir and candidate with filepath.Abs.
3. Compute rel, err := filepath.Rel(baseAbs, candAbs).
4. Reject if rel == ".." or starts with ".."+separator (or is absolute), otherwise accept.
5. Return canonical candidate path.

This preserves support for nested object keys while preventing traversal outside baseDir in a platform-correct way.

Suggested fixes powered by Copilot Autofix. Review carefully before merging.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Refactors FileStore.safePath to use absolute paths and filepath.Rel for secure, platform-correct containment checks instead of a fragile strings.HasPrefix prefix check, improving protection against path traversal while preserving existing behavior for nested object keys.

File-Level Changes

Change	Details	Files
Harden safePath path traversal protection by using absolute canonical paths and filepath.Rel-based containment checking instead of string prefix comparison.	Remove strings import and any reliance on strings.HasPrefix for validating resolved paths under the base directory. Resolve fs.baseDir to an absolute canonical baseAbs path, returning a wrapped error if resolution fails. Build the candidate path by joining baseAbs with the provided parts and canonicalize it with filepath.Abs, returning a wrapped error on failure. Compute the relative path from baseAbs to candidateAbs with filepath.Rel and reject candidates that escape the base directory (.., ..+separator prefixes, or absolute rel paths), returning a detailed traversal error. Return the canonical absolute candidate path on success, preserving support for nested object keys while enforcing directory containment.	internal/services/s3/store.go

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[sourcery-ai]

Hey - I've left some high level feedback:

• Consider computing and storing the absolute base directory once in NewFileStore (e.g., fs.baseAbs) instead of calling filepath.Abs(fs.baseDir) on every safePath invocation to avoid repeated work and potential inconsistencies if baseDir changes.
• The traversal check len(rel) >= 3 && rel[:3] == ".."+string(filepath.Separator) is a bit brittle; using strings.HasPrefix(rel, ".."+string(filepath.Separator)) would be clearer and avoid manual length/slice handling.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider computing and storing the absolute base directory once in `NewFileStore` (e.g., `fs.baseAbs`) instead of calling `filepath.Abs(fs.baseDir)` on every `safePath` invocation to avoid repeated work and potential inconsistencies if `baseDir` changes.
- The traversal check `len(rel) >= 3 && rel[:3] == ".."+string(filepath.Separator)` is a bit brittle; using `strings.HasPrefix(rel, ".."+string(filepath.Separator))` would be clearer and avoid manual length/slice handling.

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