feat(fs): Add file-backed symlinks with stat virtualization for Linux #22
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…inux This commit implements a dual symlink representation system for Linux filesystems (overlayfs and passthrough) to support stat virtualization on symlinks. ## Overview On Linux, many filesystems don't support extended attributes on symlinks, which prevents us from virtualizing uid/gid/mode. This implementation provides two symlink representations: 1. **Regular symlinks** - Traditional symlinks (when stat override not needed) 2. **File-backed symlinks** - Regular files storing the link target as content, with the actual file type (S_IFLNK) stored in the override xattr ## Key Changes ### Implementation (overlayfs & passthrough) - Modified symlink creation to always use file-backed representation on Linux - Symlink target is stored as file content (up to PATH_MAX bytes) - File type S_IFLNK is preserved in the override_stat xattr - Added proper readlink support for file-backed symlinks - Fixed readlink to use /proc/self/fd/ for O_PATH file descriptors ### Platform Differences - **Linux**: Uses file-backed symlinks to support xattr-based stat virtualization - **macOS**: Continues using regular symlinks with xattr support ### Test Updates - Comprehensive test updates with platform-specific verification - Added detailed checks for xattr content and file representation - Enhanced tests to verify copy-up behavior, nested symlinks, and edge cases - All tests properly handle both symlink representations ## Technical Details File-backed symlinks: - Created as regular files with mode 0600 - Content: symlink target path - xattr format: "uid:gid:mode" where mode includes S_IFLNK - Transparent to VFS layer - appears as regular symlinks to applications This approach ensures consistent behavior across different filesystem types while maintaining full stat virtualization capabilities. Fixes symlink stat virtualization on Linux filesystems that don't support xattrs on symlinks.
- Use symlink_metadata() instead of exists() to check symlink existence since exists() follows symlinks and returns false for broken symlinks - Add XATTR_NOFOLLOW flag when reading xattrs from symlinks on macOS to read from the symlink itself rather than its target These changes ensure the tests properly validate symlink behavior on macOS where xattrs can be set directly on symlinks.
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Summary
This PR implements a dual symlink representation system for Linux filesystems (overlayfs and passthrough) to enable stat virtualization on symlinks. This solves the limitation where many Linux filesystems don't support extended attributes on symlinks.
Problem
On Linux, most filesystems (ext4, btrfs, etc.) don't support extended attributes on symlinks. This prevents us from virtualizing uid/gid/mode on symlinks, which is required for proper container filesystem isolation.
Solution
Implemented a file-backed symlink representation that:
Implementation Details
Core Changes
uid:gid:modepreserves the S_IFLNK file type/proc/self/fd/for O_PATH file descriptorsPlatform Behavior
Test Coverage