Ubuntu 17.10 root 用户无法登陆图形界面

需要修改两个文件 /etc/pam.d/gdm-autologin /etc/pam.d/gdm-password，将其中的
auth required pam_succeed_if.so user != root quiet_success
行注释掉即可。

# grep root /etc/pam.d/gdm-password /etc/pam.d/gdm-autologin
/etc/pam.d/gdm-password:#auth   required        pam_succeed_if.so user != root quiet_success
/etc/pam.d/gdm-autologin:#auth  required        pam_succeed_if.so user != root quiet_success