输入命令：su -

输入root密码（此时密码不显示，直接输入）

输入命令:gedit /etc/pam.d/gdm&

在文本编辑器中注释掉"auth required pam_succeed_if.so user != root quiet"这一行

（在这一行前面加上"#",即改成#auth required pam_succeed_if.so user != root quiet）

保存后继续

输入命令:gedit /etc/pam.d/gdm-password&

同样地注释掉"auth required pam_succeed_if.so user != root quiet"这一行。

保存后退出 现在就能运用 root登录了

前提条件：解决gedit命令问题

The explanation of the problem is the following:

When you su to root from a terminal where you are logged in as another user the new "su-ed" user gonna inherit the environment variables from the parent shell user.

Example: let's say that for the user jack the DBUS_SESSION_BUS_ADDRESS environment variable is set to: "unix:abstract=/tmp/dbus-l5SiTFzmR8,guid=1af28c9d83400a896ef6268d4a7af59f"
Now if you open a terminal as jack and make a su to root the DBUS_SESSION_BUS_ADDRESS variable shall remain
as for the user jack. And here is the problem, exactly this specific env. variable (DBUS_SESSION_BUS_ADDRESS) generates that problem.
Gnome programs like gedit, nautilus . . . use the dbus protocol to communicate with gconf where application specific configurations are stored, and not just.
A program executed as root uses a different D-Bus session bus address compared when you execute that program as the jack user for instance. So the problem is that you switch to root, but the DBUS_SESSION_BUS_ADDRESS variable still points the the jack's dbus session bus address.

Solution:
The root user has a separate dbus session address too (located in /root/.dbus/session-bus/. . .)
So to use the root's dbus address when you are in a terminal and "su-ed" to root just clear the DBUS_SESSION_BUS_ADDRESS variable. like: export DBUS_SESSION_BUS_ADDRESS=""
A program that needs dbus communication with gconf for instance, checks that environment variable, and if it is empty it reads the dbus address from the users ~/.dbus/session-bus/ directory (the right place).
To make this permanent add the following line to the root users .bashrc file:
export DBUS_SESSION_BUS_ADDRESS=""
This means that every time when you su to root and the terminal is not a login shell (the case when you are logged in as jack and opened a terminal and executed su) the DBUS_SESSION_BUS_ADDRESS variable is cleared, so the applications gonna read the dbus address from the right place.

by the way you do not need to start a new dbus session with dbus-launch. Even if you do so but the applications still read the dbus address from the wrong place, the problem will persist.

就是在管理员用户的 .bashrc 文件尾部添加：

export DBUS_SESSION_BUS_ADDRESS=""

例如，编辑 ／root/.bashrc 文件，添加一行 export DBUS_SESSION_BUS_ADDRESS=“”

另外就是使用 su - root 代替su，sudo

其区别是su sudo 使用当前用户的环境变量，su - root会使用root用户的环境变量。