UML Shell

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.

Usage

Build a UML kernel in the usual way.

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.

Change into the Kernel directory and run the mkumlfs script

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 Support

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.

Access the host filesystem if its available.

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

Dependencies

Currently they are fakeroot, busybox-static and cpio. All of these are available as packages of the same name on Debian/Ubuntu.