archive: resolve absolute hardlink targets within extraction root (fixes #99)

[Yannik]

• fixes resolveHardlinkTarget rejects absolute hardlink targets — kaniko-built images fail to pull with "invalid hardlink target" since v0.3.0 (Docker 29.7.0) #99
• relates to / introduced in archive: resolve absolute symlinks and hard links within extraction root #93

The CVE-2026-17106 hardening rejects hardlink entries whose target is an absolute path. Some image builders (e.g. kaniko) write hardlink targets as absolute paths, so layers of such images fail to extract with "invalid hardlink target" since v0.3.0 (Docker 29.7.0).

Resolve absolute hardlink targets relative to the extraction root with chroot-like semantics, matching how absolute symlink targets are handled since 4f6cd58 and how pre-v0.3.0 extraction behaved. The root is stripped from the original linkname rather than the cleaned one, so targets like "/../victim" are not collapsed against "/" but keep failing the filepath.IsLocal check, and the resolved target then passes through resolveArchivePath confined to the os.Root extraction root.

Fix a regression introduced in v0.3.0 that caused archive extraction to reject
hardlinks with absolute targets, as produced by some image builders. Absolute
hardlink targets are now resolved relative to the extraction root while paths
that escape the root remain rejected.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.27%. Comparing base (216738e) to head (5cd0e30).
⚠️ Report is 70 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
- Coverage   65.81%   65.27%   -0.54%     
==========================================
  Files          42       44       +2     
  Lines        2039     2327     +288     
==========================================
+ Hits         1342     1519     +177     
- Misses        519      595      +76     
- Partials      178      213      +35     

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[thaJeztah]

Maybe we could use a table-test for this one, so that we can add other cases; asked my LLM to produce some that could be interesting;

/usr/bin/foo       -> usr/bin/foo
//usr/bin/foo      -> usr/bin/foo
/../victim         -> reject
/foo/../../victim  -> reject
/                  -> reject
../victim          -> reject

[thaJeztah]

Probably fine for a follow-up

[Copilot]

🟢 Ready to approve

The change aligns hardlink handling with existing absolute-path semantics for entries and symlinks while maintaining containment guarantees, and includes a targeted regression test.

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Pull request overview

This PR restores compatibility with container images that encode hardlink targets as absolute paths by resolving those targets relative to the extraction root (chroot-like semantics), while preserving existing safety checks to prevent path escapes.

Changes:

• Update hardlink-target resolution to accept absolute POSIX targets by stripping the leading / from the original linkname before validation.
• Add a regression test ensuring absolute hardlink targets extract correctly and produce a real hardlink (same inode).

File summaries

File	Description
archive.go	Resolves absolute hardlink targets relative to the extraction root while still rejecting escape attempts via filepath.IsLocal and resolveArchivePath.
archive_test.go	Adds coverage for extracting an absolute hardlink target and verifying it results in a true hardlink.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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[thaJeztah]

LGTM