How can I chroot into a filesystem with a different architechture?

Question

I'm trying to chroot into a Arch Linux ARM filesystem from x86_64.

I've seen that it's possible to do using static qemu by copying the binary into the chroot system:

$ cp /usr/bin/qemu-arm archarm-chroot/usr/bin    

But despite this I always get the following error:

chroot: failed to run command ‘/bin/bash’: Exec format error

I know this means that the architectures differ. Am I doing something wrong?

Answer 1

I use an ARM chroot from time to time: my phone runs Linux Deploy and the image dies now and then. I then copy it to my computer and examine the situation with chroot like this:

# This provides the qemu-arm-static binary
apt-get install qemu-user-static

# Mount my target filesystem on /mnt
mount -o loop fs.img /mnt

# Copy the static ARM binary that provides emulation
cp $(which qemu-arm-static) /mnt/usr/bin
# Or, more simply: cp /usr/bin/qemu-arm-static /mnt/usr/bin

# Finally chroot into /mnt, then run 'qemu-arm-static bash'
# This chroots; runs the emulator; and the emulator runs bash
chroot /mnt qemu-arm-static /bin/bash

Answer 2

IMPORTANT: Please look at the other answers. This is old and inaccurate answer.

You cannot chroot into different architecture. By chrooting, you are executing the binaries (from the chroot) on your architecture. Executing ARM binaries on x86 (and x86_64 in that matter) would lead to "Exec format error".

If you want to run binaries from different architecture you will need an Emulator. Qemu is a good candidate for this, but you will need to learn how to use it. This would involve creating RootFS and compiling a kernel for ARM. You will need a toolchain for compiling ARM binaries (and kernel) perhaps. One thing is for sure: Forget the chroot method, you cannot run binaries compiled for ARM on x86 (x86_64).

Edit: After the small talk with @UrichDangel, I realized, it should be possible to enter the chroot environment with qemu-user programs (qemu-arm in this case). Chroot should be executing qemu-arm compiled for your host architecture, then the qemu-arm can execute your /bin/sh (compiled for arm).

Answer 3

On Arch, Install qemu-user-static and binfmt-qemu-static from the AUR .

Then make sure to copy the qemu-*-static to the usr/bin/ directory in the thing you want to chroot to and then the chroot should work with something like chroot /mnt qemu-arm-static /bin/bash

Answer 4

I think the problem is that you should not copy qemu-arm but qemu-arm-static. This is a static compiled executable able to run from inside the chroot without any libraries.

You can also look in /proc/sys/fs/binfmt_misc if there exists a file qemu-arm. If not restart the service binfmt_support.

Answer 5

You can most definitely 'chroot' into a (mounted) filesystem meant for a different architecture and do some meaningful work, you just need the right tools.

Have a look at PRoot, which is a user-space implementation of chroot, mount --bind, and binfmt_misc: https://proot-me.github.io/

Together with QEMU's user mode emulators, you're all set.

Although you usually cannot perform a 'full' boot (i.e. starting init and services), it is good enough to run some binaries from their 'natural' place, with access to all their configuration files, including some that are bind-mounted from the 'host' system, etc.

Answer 6

I believe, for this OP, all he needed to do was configure binfmts, simply by running:

update-binfmts --enable qemu-arm

After running this, chroot into the arm filesystem would have been possible.

Answer 7

Adding to Luc's answer: you need to make sure the location of the interpreter is the same in the chroot as it is in the main file system. This is because the kernel detects the architecture of an executable and then uses the location of the interpreter as shown by update-binfmts --display to start it up. So the line

cp $(which qemu-arm-static) /mnt/usr/bin

should actually be

cp $(which qemu-arm-static) /mnt/$(which qemu-arm-static)

Otherwise, you may get "Not found"-errors inside your chroot as your kernel can't find the required interpreter, if the location of qemu-arm-static is not inside /usr/bin on your system.

Answer 8

sudo apt-get update
sudo apt-get install debootstrap qemu qemu-user-static
sudo qemu-debootstrap --arch armhf bionic armhf-chroot
sudo chroot armhf-chroot

uname -m 

Answer 9

I just ran into the same issue on Ubuntu. I did have binfmt configured and qemu-arm-static copied to the same chroot-ed path as on host system.

After an hour, I did set|grep bash on a host machine. I found that I had /bin/bash in two env variables: SHELL and SUDO_COMMAND. After replacing the variables, my chroot to ARM worked:

SHELL=/bin/sh SUDO_COMMAND=/bin/sh chroot hd