chroot allows you to “run a command or interactive shell with special root directory”, as the man page says. However, it is assumed that the second level root directory is built for the same CPU architecture. This causes a problem if you want to chroot into an ARM based image, for the Raspberry Pi, let’s say. qemu-arm-static, some “voodoo” and several tricks come to the rescue. The process is documented well at Sentry’s Tech Blog, and the original seems to be by Darrin Hodges.

After downloading and unzipping the image, it has to be mounted. There are a few ways to go about this, but I found the easiest was to use plain old mount with an offset. The typical RPi image file is a full disk image, as opposed to a single partition or ISO though. We are after the second partition, which in our case starts at sector 122880. (See this discussion for how to find the correct starting sector using fdisk).

mkdir /mnt/rpi
mount -o loop,offset=$(( 512 * 122880 )) 2014-01-07-wheezy-raspbian.img /mnt/rpi

Next we’ll copy a statically built QEMU binary for ARM to the mounted image. You might need to install QEMU on the host system first. Furthermore, we need to mount or bind the special system directories from the host to the chroot.

apt-get install qemu-user-static
cp /usr/bin/qemu-arm-static /mnt/rpi/usr/bin/

mount -o bind /dev /mnt/rpi/dev
mount -o bind /proc /mnt/rpi/proc
mount -o bind /sys /mnt/rpi/sys

Next comes the magic. This registers the ARM executable format with the QEMU static binary. Thus, the path to qemu-arm-static has to match where it is located on the host and slave systems (as far as I understand).

echo ':arm:M::\x7fELF\x01\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x28\x00:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/usr/bin/qemu-arm-static:' > /proc/sys/fs/binfmt_misc/register

Finally, it’s time for the moment of truth:

chroot /mnt/rpi

uname -a
Linux hrb 3.2.0-4-amd64 #1 SMP Debian 3.2.51-1 armv7l GNU/Linux

In some cases, the error “qemu: uncaught target signal 4 (Illegal instruction) – core dumped” occurs. User kinsa notes here that the lines of the file (i.e. on the slave, /mnt/rpi/etc/ has to be commented out (with a # in front).

Congratulations, you now have an ARM based chroot. What to do with it? Maybe install a few “missing” packages before copying over to one or more SD cards, set up the users, modify passwords, etc. Or take advantage of the CPU and memory of the host system or compile from source.

apt-get install htop tree ipython ipython3 gnuplot

As a final note, when done, you want to clean up the mount points.

umount /mnt/rpi/dev
umount /mnt/rpi/proc
umount /mnt/rpi/sys
umount /mnt/rpi