Follow absolute sysroot links to host paths

[doanbaotrung]

Allow sysroot-staged symlinks with absolute targets to resolve through the normal host fallback path, while keeping relative symlink climb-outs blocked.

This fixes host bridge links such as Downloads -> /Users//Downloads returning ELOOP.

---

Summary by cubic

Allow absolute-target symlinks staged in the sysroot to resolve via the normal host fallback, fixing ELOOP on host bridge links like Downloads -> /Users/<user>/Downloads. Relative “..” climb-outs stay blocked.

• Bug Fixes
  • Follow sysroot absolute-target links to host paths when the host target exists; leave dangling links to resolve in the sysroot. Applies to absolute paths, dirfd/openat, trailing-slash directory opens, and chains where a relative link leads to an absolute-target link.
  • Block relative escapes (ELOOP) when a relative target would reach a host path; recheck dirfd-relative lookups against the sysroot.
  • Add a byte-exact link resolver to preserve Linux ENOTDIR behavior and prevent incorrect host fallback; update tests and stage /host-home-link and /home/muplar/Download to verify host bridge links (including trailing-slash directory opens).

^{Written for commit 5555c08. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 5 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="tests/test-sysroot-symlink-target.c">

<violation number="1" location="tests/test-sysroot-symlink-target.c:146">
P2: This new test resolves `Download/` to the host's `$HOME/Downloads` and FAILs the whole suite when that directory does not exist on the machine running CI (`~/.config`-less, headless, or freshly-provisioned macOS runners often have no Downloads). Unlike `/host-home-link` (target `$HOME`, which always exists), there is no guarantee `$HOME/Downloads` is present, so the opendir returns NULL and the regression guard fails spuriously. Recommend staging a guaranteed empty host dir (e.g. under `$tmpdir` sibling or `mktemp -d`) as the link target instead of depending on the caller's real Downloads folder.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

P2: This new test resolves Download/ to the host's $HOME/Downloads and FAILs the whole suite when that directory does not exist on the machine running CI (~/.config-less, headless, or freshly-provisioned macOS runners often have no Downloads). Unlike /host-home-link (target $HOME, which always exists), there is no guarantee $HOME/Downloads is present, so the opendir returns NULL and the regression guard fails spuriously. Recommend staging a guaranteed empty host dir (e.g. under $tmpdir sibling or mktemp -d) as the link target instead of depending on the caller's real Downloads folder.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At tests/test-sysroot-symlink-target.c, line 146:

<comment>This new test resolves `Download/` to the host's `$HOME/Downloads` and FAILs the whole suite when that directory does not exist on the machine running CI (`~/.config`-less, headless, or freshly-provisioned macOS runners often have no Downloads). Unlike `/host-home-link` (target `$HOME`, which always exists), there is no guarantee `$HOME/Downloads` is present, so the opendir returns NULL and the regression guard fails spuriously. Recommend staging a guaranteed empty host dir (e.g. under `$tmpdir` sibling or `mktemp -d`) as the link target instead of depending on the caller's real Downloads folder.</comment>

<file context>
@@ -121,6 +122,36 @@ int main(void)
+        DIR *dir;
+        if (chdir("/home/muplar") < 0) {
+            FAIL("chdir /home/muplar");
+        } else if ((dir = opendir("Download/")) != NULL) {
+            closedir(dir);
+            PASS();
</file context>

[doanbaotrung]

Addressed.

I changed the fixture in third_party/elfuse/mk/tests.mk so Download no longer points to $HOME/Downloads.

It now does:

mkdir -p "$$tmpdir/host-downloads"
ln -s "$$tmpdir/host-downloads" "$$tmpdir/home/muplar/Download"

So the test target is guaranteed to exist on CI, while still exercising the same guest-facing opendir("Download/") path.

Verified with:

make -C third_party/elfuse test-sysroot-symlink-target

[jserv]

Thank @doanbaotrung for contributing!