[Linux] Ubuntu crash by using gentoo to rescue

1. Load Gentoo iso into Option Drive of virtualbox

2. mount root file system tree
mount /dev/sda1 /tmp

3. mount device into ubuntu filesystem
mount -t proc none /tmp/proc/
mount –rbind /dev/ /tmp/dev/ (must use –rbind)
mount –rbind /sys/ /tmp/sys/

4. Switch to ubuntu file system
chroot /tmp

5. Start to edit ubuntu file system.