path-util: Fix path_is_mount_point for files

Commits 27cc6f166 and f25afeb broke path_is_mount_point() for files (such as
/etc/machine-id → /run/machine-id bind mounts) as with the factorization of
fd_is_mount_point() we lost the parent directory. We cannot determine that from
an fd only as openat(fd, "..") only works for directory fds.

Change fd_is_mount_point() to behave like openat(): It now takes a file
descriptor of the containing directory, a file name in it, and flags (which can
be 0 or AT_SYMLINK_FOLLOW). Unlike name_to_handle_at() or openat(), fstatat()
only accepts the inverse flag AT_SYMLINK_NOFOLLOW and complains with EINVAL
about AT_SYMLINK_FOLLOW; so we need to transform the flags for that fallback.

Adjust rm_rf_children() accordingly (only other caller of fd_is_mount_point()
aside from path_is_mount_point()).

Add test cases for files, links, and file bind mounts (the latter will only
work when running as root). Split out a new test_path_is_mount_point() test
case function as it got significantly larger now.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/shared/rm-rf.c b/src/shared/rm-rf.c
index a89e8afc2a..bafd483be2 100644
--- a/src/shared/rm-rf.c
+++ b/src/shared/rm-rf.c
@@ -103,7 +103,7 @@ int rm_rf_children(int fd, RemoveFlags flags, struct stat *root_dev) {
                         }
 
                         /* Stop at mount points */
-                        r = fd_is_mount_point(subdir_fd);
+                        r = fd_is_mount_point(fd, de->d_name, 0);
                         if (r < 0) {
                                 if (ret == 0 && r != -ENOENT)
                                         ret = r;