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Cross Platform ARM w/ Docker and quemu

2015-12-12 devops ubergarm

The goal is to automate building armhf (armv7) compatible Docker images. These images can be used for dev, build, test, and deploy to ensure a consistent user experience. The most common development, cloud, and build machines currently run x86-64 architecture hardware. This complicates working on software targeted at embedded arm platforms. With modern kernels that support binfmt misc and the right combination of qemu-XXX-static binary one can run arbitrary format Docker images on arbitrary host hardware.

This solution combines:

  • Host kernel support for binfmt misc
  • qemu-arm-static
  • Docker armhf images

Configure Host

There are a many articles out there on configuring binfmt / qemu / chroot to run ARM binaries, images, etc. Here are two common approaches.

The Clean Way

Assuming a newer Ubuntu or Debian system you can just install the necessary pacakges.

apt-get update && apt-get install -y --no-install-recommends \
        qemu-user-static \
update-binfmts --enable qemu-arm
update-binfmts --display qemu-arm

If everything looks good, skip ahead past the next section down to Test Host Setup.

The Dirty Way

If you want to keep it slim or manually configure everything this would be another approach.

Grab a host compatible qemu-arm-static binary and put it in place.

sudo curl -o /usr/bin/qemu-arm-static http://ubergarm.com/dre/qemu-arm-static
sudo chmod a+x /usr/bin/qemu-arm-static

Install binfmt module support if not already built into kernel.

grep BINFMT /usr/src/linux-headers-$(uname -r)/.config || zcat /proc/config.gz | grep BINFMT
sudo modprobe binfmt_misc
lsmod | grep BINFMT

Register qemu-arm

mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
echo ':qemu-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

Test Host Setup

Confirm that the host is setup correctly to run an arm binary.

qemu-arm-static -version
curl -O http://ubergarm.com/dre/hello-world-arm
chmod a+x hello-world-arm
file hello-world-arm
qemu-arm-static hello-world-arm

If all that looks good e.g. “Hello world!” both times then try Docker.

docker pull ioft/armhf-ubuntu:trusty
docker run --rm -it -v /usr/bin/qemu-arm-static:/usr/bin/qemu-arm-static ioft/armhf-ubuntu:trusty uname -a

You should get something like:

Linux 27393a16e4f8 3.13.0-68-generic #111-Ubuntu SMP Fri Nov 6 18:17:06 UTC 2015 armv7l armv7l armv7l GNU/Linux

as compared to the host uname -a:

Linux qemu-test 3.13.0-68-generic #111-Ubuntu SMP Fri Nov 6 18:17:06 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux


Not all arm images have an x86-64 qemu-arm-static sitting around in /usr/bin already. If it isn’t there, just volume mount the file from the host into place in the container as shown above. Until docker build -v works, I’ve gone ahead and created a base image which only adds qemu-arm-static to an otherwise ARM Docker Image to support Dockerfile’s and building Docker Images from an x86.

$ docker run --rm -it ubergarm/armhf-ubuntu:trusty file /usr/bin/qemu-arm-static


There you have it! Run, build, test, and dev ARM Docker Images right on your x86-64 host machine. Next up: bringing Continuous Integration to the embedded arm world!


I tested all of the above commands for just a few cents on a fresh clean DigitalOcean VPS. Use that referral link to get $10 credit if you want so try it out yourself. If you keep using it, they’ll eventually give me $25 credit. Thanks. Plug complete.


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