The usual root images used alongside UML are unsuitable for kernel hacking because they waste too much time and resources booting up a full Linux system.
All I need is a shell to hack in.
UML shell is a small utility to boot a user mode linux kernel and run a shell (busybox, currently) inside it. If its built in, hostfs can be used to mount the host filesystem and use the files from there.
Eg. make ARCH=um
This will build the UML image and modules. You could use the provided
kernel-config-example as your .config to have a minimal kernel suitable for
hacking and debugging.
REBUILD=1 /path/to/mkumlfs.sh
This will install the modules, packages, initscript, generate the initramfs, boot up the kernel and start a busybox shell.
GDB=1 /path/to/mkumlfs.sh
This will start the kernel under gdb. Set your breakpoints, etc. and the hit
r. Some gdb commands necessary for running UML are read from gdbcommands.txt
in the uml-sh installation directory.
mkumlfs.sh will attempt to mount the host filesystem using hostfs if
supported by the kernel. You can do it yourself as follows:
mkdir /mnt
mount -t hostfs none /mnt
Currently they are fakeroot, busybox-static and cpio. All of these are available as packages of the same name on Debian/Ubuntu.