    def test_check_safe_path(self):
        ret = disk_api._join_and_check_path_within_fs('/foo', 'etc',
                                                      'something.conf')
        self.assertEquals(ret, '/foo/etc/something.conf')
The software uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the software does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.