tomoyo: Change pathname calculation for read-only filesystems.

Commit 5625f2e3 ("TOMOYO: Change pathname for non-rename()able
filesystems.") intended to be applied to filesystems where the content is
not controllable from the userspace (e.g. proc, sysfs, securityfs), based
on an assumption that such filesystems do not support rename() operation.

But it turned out that read-only filesystems also do not support rename()
operation despite the content is controllable from the userspace, and that
commit is annoying TOMOYO users who want to use e.g. squashfs as the root
filesystem due to use of local name which does not start with '/'.

Therefore, based on an assumption that filesystems which require the
device argument upon mount() request is an indication that the content
is controllable from the userspace, do not use local name if a filesystem
does not support rename() operation but requires the device argument upon
mount() request.
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jamorris@linux.microsoft.com>

security/tomoyo/realpath.c

@@ -295,7 +295,8 @@ char *tomoyo_realpath_from_path(const struct path *path)
 		 * or dentry without vfsmount.
 		 */
 		if (!path->mnt ||
-		    (!inode->i_op->rename))
+		    (!inode->i_op->rename &&
+		     !(sb->s_type->fs_flags & FS_REQUIRES_DEV)))
 			pos = tomoyo_get_local_path(path->dentry, buf,
 						    buf_len - 1);
 		/* Get absolute name for the rest. */