Overview

Run ARM binaries from within a docker container on your x86_64 system. This can assist with cross compiling ARM systems from within docker, or a variety of other tasks.

Assumption

This assumes a docker host kernel with binfmt_misc support (module or built-in).

There are two pieces to this docker PoC. A data container (where we store our images) and the actual emulation system image.

Installation

1. add an ARM system image to data/images in this directory for later mounting on loopback interface (once we're in the docker container)

• alternatively you can grab a system tarball image and just cd into that (try one of these: http://archlinuxarm.org/platforms/armv7)

2. build the data image container (make inside data/) -- this stores your system images in a separate container than the running system

3. run the data container once (if you don't understand why, see https://docs.docker.com/userguide/dockervolumes/).

4. build the qemu-arm image (run make inside qemu-arm/).

Basically:

pushd data && make
docker run --name qemu-arm_data --entrypoint="/bin/true" -d qemu-arm-data
popd
pushd qemu-arm
make
popd
docker run --privileged -t -i --rm --volumes-from qemu-arm_data qemu-arm

In container, make sure binfmt is mounted (mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc)

Running

now in the docker container shell:

If mounting a system image:

mkdir -p /chroot
mount -o loop /data/images/<your image>.bin /chroot
cd /chroot
cp /usr/bin/qemu-arm-static /chroot/usr/bin/qemu-arm-static
chroot . /bin/sh

Otherwise just cd into your system image folder, copy qemu-arm-static into chroot/usr/bin and then chroot . /bin/sh.

Try running uname -a or running some files inside your chroot.

Troubleshooting

• Maybe your host docker kernel doesn't have binfmt_misc support. I build a custom boot2docker kernel for this here.

• Try checking update-binfmts --display to see if qemu-arm-static is registered

• Make sure binfmts is mounted (mount) - if not mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc

Related reading

• binfmt-misc - https://www.kernel.org/doc/Documentation/binfmt_misc.txt
• docker volumes - https://docs.docker.com/userguide/dockervolumes/
• https://wiki.debian.org/QemuUserEmulation
• Blog post by @neophob using this repo.