Runlinux

cd into a Linux kernel source tree, run one command, and get a running QEMU VM with BusyBox.

This is better: https://github.com/cirosantilli/linux-kernel-module-cheat

Based on: https://github.com/ivandavidov/minimal Merge back declined at: ivandavidov/minimal#20

Tested in Ubuntu 14.04 AMD64, QEMU 2.0.0.

Install

sudo apt-get install git qemu
sudo apt-get build-dep busybox linux-image-$(uname -r)
mkdir -p ~/bin
cd ~/bin
git clone --recursive https://github.com/cirosantilli/runlinux
echo 'PATH="$PATH:'$(pwd)'/runlinux"' >> ~/.bashrc
. ~/.bashrc

Examples

git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
cd linux
git checkout v4.2
runlinux

It takes a while the first time because things have to be built, but later runs will be faster.

Then hack the kernel source to your liking, and run:

runlinux

again to try it out.

Out-of-tree build with custom configuration:

export KBUILD_OUTPUT="$(pwd)/../build"
make menuconfig
runlinux

If an existing configuration is not found at KBUILD_OUTPUT, make defconfig is used. If found, it is used and left untouched.

Pass extra options to QEMU:

runlinux -- -bios ~/path/to/OVMF.fd

This for example uses the OVMF UEFI X64 r15214 instead of the default BIOS.

Run on real hardware

Generate a main.img file in your build directory:

runlinux -i

Insert an USB and determine its device (/dev/sdX):

sudo lsblk
sudo fdisk -l

Burn the image to the USB:

sudo dd if=main.img of=/dev/sdX

Then:

• insert the USB in a computer
• during boot, hit some special hardware dependant key, usually F12, Esc
• choose to boot from the USB

You can also ensure that the image works fine with:

qemu-system-x86_64 -enable-kvm -hda main.img

Tested on: ThinkPad T400.

Options

Custom initrd

n

If you just want to run your own root filesystem and ignore BusyBox completely, use:

runlinux -n /path/to/my/directory/
runlinux -n /path/to/my/init

If you pass it:

• a directory, the directory will be packed into a filesystem.

  The first thing that Linux runs is the /init executable of that directory. You usually want that to be an executable without dependencies that never exits.

• a file, it will be renamed to init and put at the root of the packed filesystem.

See this SO answer for more details on how to create your own simple initrd.

g

TODO this is currently broken: https://sourceware.org/bugzilla/show_bug.cgi?id=13984 but I've since found the workaround and just need to stop being lazy and implement it: http://stackoverflow.com/questions/11408041/how-to-debug-the-linux-kernel-with-gdb-and-qemu/33203642#33203642

Debug the kernel on GDB:

../runlinux/runlinux -g

This will set the CONFIG_DEBUG_INFO configuration and rebuild the kernel if necessary.

It runs QEMU on the background of the current shell, and opens GDB there.

You are now ready to debug, e.g.:

# Has to be hardware breakpoint. TODO why https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/901944/comments/12
hbreak start_kernel
list
continue