I've built a root filesystem based off Debian Squeeze because I wanted to include all built (e.g. gcc) tools in.  However,  i was not able to boot the UML with this rootfs due to this error:

Kernel panic - not syncing: No init found.  Try passing init= option to kernel.

here is how I ran the UML:

./linux ubda=/nobackup/hxu2/uml/debian-root-fs mem=128M 

I also tried to add "init=/sbin/init" or "init=3" at the end, but got the same error. 

I believe I missed something when building the rootfs for the Init option. But what is it?  How should I set the init option in the rootfs?  please advise.