Ubuntu 17.10 root users could not land a graphical interface

We need to modify two files /etc/pam.d/gdm-autologin /etc/pam.d/gdm-password, which will be
auth required pam_succeed_if.so user != root quiet_success
the line commented out.

# 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