(a) mount and (b) umount in util-linux 2.14.1, 2.17.2, and probably other versions allow local users to determine the existence of restricted directories by…

(a) mount and (b) umount in util-linux 2.14.1, 2.17.2, and probably other versions allow local users to determine the existence of restricted directories by (1) using the --guess-fstype command-line option or (2) attempting to mount a non-existent device, which generates different error messages depending on whether the directory exists.