no valid sudoers -- Read-only file system 解决方案

/etc/sudoers: syntax error near line
sudo: parse error in /etc/sudoers near line 25
sudo: no valid sudoers sources found, quitting
终极解决方案：

1. shift进入recovery模式
2. 选中root账号
3. chmod 666 /dev/null
   mount -o remount rw /
4. nano /etc/sudoers
恢复本文件内容并存盘。

# /etc/sudoers
#
# This file MUST be edited with the 'visudo' command as root.
#
# See the man page for details on how to write a sudoers file.
#

Defaults env_reset

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root ALL=(ALL) ALL

# Allow members of group sudo to execute any command after they have
# provided their password
# (Note that later entries override this, so you might need to move
# it further down)
%sudo ALL=(ALL) ALL
#
#includedir /etc/sudoers.d

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

参考：http://psychocats.net/ubuntu/fixsudo
http://my5622576.iteye.com/blog/662158