fs: fix user path of nested backing files

backing_file_open() derives the path to be stored in the new backing
file from user_file->f_path.  This is incorrect when user_file itself
is a backing file, which is the case for nested stacking filesystems,
e.g. overlayfs mounts where the lowerdir of one overlayfs is the merged
directory of another.  Since commit def3ae83da ("fs: store real path
instead of fake path in backing file f_path") the f_path of a backing
file holds the real path of the intermediate layer, not the path that
the user opened.

Commit 924577e4f6 ("ovl: Fix nested backing file paths") fixed this
for such configurations by passing file_user_path() from
ovl_open_realfile().  However, commit 6af36aeb14 ("lsm: add
backing_file LSM hooks") changed the first argument of
backing_file_open() from the user path back to the user file and
derived the path from user_file->f_path again, silently re-introducing
the problem.

As a result, files mapped through a nested overlayfs show the wrong
path in /proc/<pid>/maps and in perf/ftrace mmap records.  For example,
with two nested overlayfs mounts:

  mkdir -p /ovl/{lower,upper,work,merged} /ovl/nested
  echo hello > /ovl/lower/foo
  mount -t overlay overlay \
	-o lowerdir=/ovl/lower,upperdir=/ovl/upper,workdir=/ovl/work \
	/ovl/merged
  # at least two lowerdirs are needed when upperdir is nonexistent
  mount -t overlay overlay \
	-o lowerdir=/ovl/merged:/ovl/lower /ovl/nested

mapping /ovl/nested/foo shows a disconnected path instead of the user
path:

  # readlink /proc/self/fd/3
  /ovl/nested/foo
  # grep foo /proc/self/maps
  7f6e2c100000-7f6e2c101000 r--s 00000000 00:24 15813027 /foo

The bogus path is derived from the f_path of the intermediate backing
file, whose mount is a private clone that d_path() cannot resolve.

Fix this by using file_user_path(), which returns the outermost
user-visible path for backing files and falls back to
&user_file->f_path for regular files.  This restores the behavior of
commit 924577e4f6 ("ovl: Fix nested backing file paths") for
overlayfs and also fixes the same problem for the other
backing_file_open() callers, fuse passthrough and erofs ishare, when
their user file is itself a backing file.

backing_tmpfile_open() has the same pattern but is not affected: it is
only called by ovl_create_tmpfile() for the upper layer, and another
overlayfs is rejected as upperdir by the DCACHE_OP_REAL check in
ovl_mount_dir_check(), so its user_file can never be a backing file.

Fixes: 6af36aeb14 ("lsm: add backing_file LSM hooks")
Cc: stable@vger.kernel.org
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
Link: https://patch.msgid.link/20260804034204.3487077-1-libaokun@linux.alibaba.com
Tested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/backing-file.c

@@ -35,7 +35,7 @@ struct file *backing_file_open(const struct file *user_file, int flags,
 			       const struct path *real_path,
 			       const struct cred *cred)
 {
-	const struct path *user_path = &user_file->f_path;
+	const struct path *user_path = file_user_path(user_file);
 	struct file *f;
 	int error;