Petter Reinholdtsen

Teaching vmdebootstrap to create Raspberry Pi SD card images
27th October 2013

The vmdebootstrap program is a a very nice system to create virtual machine images. It create a image file, add a partition table, mount it and run debootstrap in the mounted directory to create a Debian system on a stick. Yesterday, I decided to try to teach it how to make images for Raspberry Pi, as part of a plan to simplify the build system for the FreedomBox project. The FreedomBox project already uses vmdebootstrap for the virtualbox images, but its current build system made multistrap based system for Dreamplug images, and it is lacking support for Raspberry Pi.

Armed with the knowledge on how to build "foreign" (aka non-native architecture) chroots for Raspberry Pi, I dived into the vmdebootstrap code and adjusted it to be able to build armel images on my amd64 Debian laptop. I ended up giving vmdebootstrap five new options, allowing me to replicate the image creation process I use to make Debian Jessie based mesh node images for the Raspberry Pi. First, the --foreign /path/to/binfm_handler option tell vmdebootstrap to call debootstrap with --foreign and to copy the handler into the generated chroot before running the second stage. This allow vmdebootstrap to create armel images on an amd64 host. Next I added two new options --bootsize size and --boottype fstype to teach it to create a separate /boot/ partition with the given file system type, allowing me to create an image with a vfat partition for the /boot/ stuff. I also added a --variant variant option to allow me to create smaller images without the Debian base system packages installed. Finally, I added an option --no-extlinux to tell vmdebootstrap to not install extlinux as a boot loader. It is not needed on the Raspberry Pi and probably most other non-x86 architectures. The changes were accepted by the upstream author of vmdebootstrap yesterday and today, and is now available from the upstream project page.

To use it to build a Raspberry Pi image using Debian Jessie, first create a small script (the customize script) to add the non-free binary blob needed to boot the Raspberry Pi and the APT source list:

set -e # Exit on first error
cd "$rootdir"
cat <<EOF > etc/apt/sources.list
deb jessie main contrib non-free
# Install non-free binary blob needed to boot Raspberry Pi.  This
# install a kernel somewhere too.
wget \
    -O $rootdir/usr/bin/rpi-update
chmod a+x $rootdir/usr/bin/rpi-update
mkdir -p $rootdir/lib/modules
touch $rootdir/boot/start.elf
chroot $rootdir rpi-update

Next, fetch the latest vmdebootstrap script and call it like this to build the image:

sudo ./vmdebootstrap \
    --variant minbase \
    --arch armel \
    --distribution jessie \
    --mirror \
    --image test.img \
    --size 600M \
    --bootsize 64M \
    --boottype vfat \
    --log-level debug \
    --verbose \
    --no-kernel \
    --no-extlinux \
    --root-password raspberry \
    --hostname raspberrypi \
    --foreign /usr/bin/qemu-arm-static \
    --customize `pwd`/customize \
    --package netbase \
    --package git-core \
    --package binutils \
    --package ca-certificates \
    --package wget \
    --package kmod

The list of packages being installed are the ones needed by rpi-update to make the image bootable on the Raspberry Pi, with the exception of netbase, which is needed by debootstrap to find /etc/hosts with the minbase variant. I really wish there was a way to set up an Raspberry Pi using only packages in the Debian archive, but that is not possible as far as I know, because it boots from the GPU using a non-free binary blob.

The build host need debootstrap, kpartx and qemu-user-static and probably a few others installed. I have not checked the complete build dependency list.

The resulting image will not use the hardware floating point unit on the Raspberry PI, because the armel architecture in Debian is not optimized for that use. So the images created will be a bit slower than Raspbian based images.

Tags: debian, english, freedombox, mesh network.

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