ubuntu on tiny210 boot up,but can't change to root?

lvjf

2013-10-18 05:24:11

From ftp,I downloaded the ubuntu image,and try it,it boot up ,and can login
with serial console.
but when I want to change to superuser with command "su -l",and input
password "fa",it show up error message"authentication failure".
  I have tried following method:
    1.edit /etc/shadow ,to reset root password.
    2.edit /etc/securetty, to add "s3c2410_serial0"
  but none of those methods works!