So, for the past couple of weeks I've been working on creating a Fedora Core 3 disk image for UML. In the end, the process ended up being quite straight-forward, but in a case of "a little bit of knowledge can be a dangerous thing," I kept screwing it up.
Pretty much, all you have to do is to create an ext2 loopback disk image as suggested in the UML docs, and then copy a running fedora core system (using cp -r -d --parents --preserve=all or whatever) into it (obviously, you should avoid copying the disk image into itself). That's pretty much it. Labeling the disk image as /1 doesn't seem to be enough to get UML to mount the disk image properly as its root file system, so you do have to modify the /etc/fstab file so that the root file system is mapped to /dev/ubda (for some reason, the virtual drive appears as /dev/ubda instead of /dev/ubd/0 like in other images) using an ext2 format (unless you compiled ext3 support into the kernel). It's probably a good idea to enable networking in the kernel as well by turning on these kernel options:
UML Network Devices/Virtual network device
UML Network Devices/TUN/TAP transport
Then, you can just boot up UML into the image you created. You don't have to change the Kudzu configuration, change the networking to use something other than eth0, or anything. It just works. Below are the things I tried to do, which ended up being completely unnecessary:
Although Fedora Core 3 intially boots with a small initrd RAM disk image as its root file system before switching to using the hard disk as its root, this initrd is only for loading additional kernel modules. Since User Mode Linux has most options compiled directly into the kernel, it is not necessary to use an initrd image with UML. Also, it is not necessary to create ubd devices in /dev since Fedora seems to do that automatically. And hostfs mounting /sbin or /bin doesn't work out too well. You can mostly get away with hostfs mounting /usr but you have to prevent xfs from starting up (simply remove /etc/init.d/xfs) because I think it tries writing stuff into /usr, which won't work because on the host-side, UML won't have sufficient permissions to write there. I also didn't bother properly installing uml-tools, meaning that UML couldn't use the port-mapper to bring up the virtual console xterms. Since Fedora Core 3 requires you to log-in through a virtual console, you need to get the port-mapper working properly (I had to modify UML to find the port-mapper in my home directory instead of its default location). Also, Virtual Console #7 and #8 won't appear or won't show anything, so don't think the boot-up has failed if you don't see anything there. And that pretty much summarizes many, many days of mistakes.