ubuntu找回root密码，报错authentication token manipulation error

1).在启动项选择:Advaned options for Ubuntu. 
2).然后 选择:Ubuntu ,with Linux 3.13.0-24-generic (recovery mode),
3). 等待加载完成..你将看到"Recovery Menu (filesystem state :read-only)"
4).选择root              Drop to root shell prompt 就会进入单用户模式了..

然后是修改密码.
输入passwd user,回车后就出现 
 authentication token manipulation error.

passwd unchanged..

 
This is the result of trying to work on a file system while mounted as read-only.先改变一下可读模式，执行一下命令

mount -o rw,remount / .

再执行passwd root就可以修改了。