The detached-tree X-mount.subdir path arrived in 2.42 (ae19f7546); our libmount
has no AT_SYMLINK_NOFOLLOW, no api->subdir in hook_mount.c and no 6.15 logic.
Upstream shipped 2.41.6 the same day as 2.42.3 with the other three CVEs and
deliberately not this one. Debian marks trixie vulnerable but also bookworm
2.38.1, which predates the path entirely.
https://security-tracker.debian.org/tracker/CVE-2026-78409