selftests/filesystems: add failfs selftests

2026-07-27T15:18:00Z

Test the failfs semantics and both new entry points: - fchdir(FD_FAILFS_ROOT): * working directory lookups and getcwd() fail * other sentinels are rejected * the state is recoverable while the root is untouched - fchroot() with regular fds: * chroot parity * CAP_SYS_CHROOT required * ENOTDIR/EBADF/EINVAL checks - fchroot(FD_FAILFS_ROOT): * absolute lookups, stat, statfs and opens of the root including O_PATH fail with EOPNOTSUPP * dirfd-anchored I/O keeps working * ".." walks clamp at the top of the mount tree * /proc magic links resolve but can't be stat through * absolute symlinks fail while relative symlinks keep resolving - Unprivileged entry requires no_new_privs and is rejected for chrooted callers and for a shared fs_struct - entering makes the task count as chrooted so user namespace creation fails - Nothing can be mounted on top of failfs and OPEN_TREE_CLONE is rejected; the overmount test runs in a private mount namespace so a regression cannot touch the host root - setns() to a kept mount namespace fd restores root and working directory - The failfs root is inherited across fork() and absolute exec fails - Exec by fd of a dynamically linked binary fails on opening its absolute PT_INTERP interpreter The exec tests run the exec in a child so a wrongly successful exec cannot replace the test image and masquerade as a pass. Link: https://patch.msgid.link/20260724-work-failfs-v2-6-485dabbae185@kernel.org Signed-off-by: Christian Brauner (Amutable)

wireguard-linux/tools/testing/selftests/filesystems/failfs, branch stable

selftests/filesystems: add failfs selftests