fchroot 0.2.1

Funtoo franken-chroot tool.

fchroot

Introduction

fchroot, also known as "Franken-Chroot" or "Funtoo Chroot", is a utility that helps you to leverage the magic of QEMU to chroot into a non-native system. For example, on your x86-compatible 64-bit PC, it is possible to chroot into a 32-bit or 64-bit ARM environment and have it actually work.

This is accomplished by leveraging the "binfmt-misc" functionality built-in to the Linux kernel, combined with QEMU to perform emulation of non-native instruction sets. fchroot itself doesn't do any magic except provide a very easy-to-use mechanism to get this to work, typically requiring little or no manual configuration. You simply run fchroot just like chroot, and everything works. Well, I suppose that is a bit magical :) Here's an example of how things work with fchroot. Let's say you are on your 64-bit x86-64 PC, and want to enter a 64-bit ARM environment. Do this:

my64bitpc # tar xpvf stage3-arm64bit.tar.xz
my64bitpc # fchroot arm64-root
>>> arm-64bit frankenchroot B]...
#

That last # prompt may not look like anything special, but you are now inside a 64-bit ARM frankenchroot. A 64-bit ARM shell is providing that prompt to you, and if you run a standard Linux command, it will be running the 64-bit ARM variant of that command using QEMU emulation. Pretty cool stuff.

What fchroot Does For You

The fchroot command does lots of things automatically for you to make your franken-chroot experience as seamless as possible. First, note that fchroot is designed to have a similar calling convention to the standard Linux chroot command -- in fact, at the very end of doing all its magic, it passes all of its arguments to chroot to get you inside the frankenchroot. But fchroot does quite a bit of grunt-work to make the chroot work:

• fchroot will look inside the specified directory and attempt to auto-detect what kind of non-native environment it's dealing with. It will currently recognize arm-32bit and arm-64bit environments.
• fchroot will check to see if QEMU is available for the detected non-native architecture and abort with a (hopefully) useful error message if it is not.
• fchroot will use gcc to compile a special wrapper that will be used inside the chrooted environment, and store it in /usr/share/fchroot/wrappers so it is available for later use.
• fchroot will leverage Linux's binfmt_misc functionality and register a handler so that the kernel will know to use our wrapper (and QEMU) to run non-native binaries.
• fchroot will copy the necessary QEMU binary as well as its wrapper into /usr/local/bin/ inside the chrooted environment.
• fchroot will mount /proc, /dev (bind-mount) and /sys (bind-mount) within the chroot environment automatically if they are not already mounted.
• fchroot will copy the local system's /etc/resolv.conf to the chroot environment so that DNS resolution will work properly.
• fchroot will then execvp the chroot command to place you inside the franken-chroot environment.

Prerequisites

This section lists the prerequisites for getting fchroot running -- in other words, the things that you are responsible for ensuring are done. fchroot takes care of the rest.

Host and Emulated System

fchroot has been used on x86-compatible 64-bit systems, and currently supports arm-32bit and arm-64bit chrooted environments. It is relatively easy to add support for new native and emulated systems -- the code is designed to accept patches for new architectures -- but for now, be aware of the current architectures supported.

QEMU

QEMU will need to be installed with the aarch64 and arm user targets enabled. In addition, it will need to be compiled as a static binary. In Funtoo Linux and Gentoo Linux, this can be accomplished by adding the following configuration prior to running emerge qemu:

Add to /etc/make.conf:

QEMU_USER_TARGETS="aarch64 arm"

Add to /etc/portage/package.use:

app-emulation/qemu static-user
dev-libs/glib static-libs
sys-apps/attr static-libs
sys-libs/zlib static-libs
dev-libs/libpcre static-libs

binfmt_misc

In addition, you will need to ensure that binfmt_misc functionality is enabled in-kernel or as a module. By default, this is the case when using Funtoo Linux with its default debian-sources-lts kernel.

Setup

The easiest way to set up fchroot is to clone it directly from https://code.funtoo.org:

# git clone https://code.funtoo.org/bitbucket/scm/~drobbins/fchroot.git

You can then simply run fchroot directly from the git repository:

# fchroot/bin/fchroot /path/to/chroot

Acknowledgements

The fchroot command automates a process that was documented at the following locations:

• https://wiki.gentoo.org/wiki/Embedded_Handbook/General/Compiling_with_qemu_user_chroot
• https://github.com/sakaki-/gentoo-on-rpi3-64bit/wiki/Build-aarch64-Packages-on-your-PC%2C-with-User-Mode-QEMU-and-binfmt_misc

Many thanks to Sakaki and others who documented this process.

Contributing

To contribute, please visit https://code.funtoo.org/bitbucket/users/drobbins/repos/fchroot/browse and submit a pull request. For more information on how to submit pull requests on code.funtoo.org, see the following YouTube video: https://www.youtube.com/watch?v=V6PfB64oMWo

Author and Copyright

Copyright 2020 Daniel Robbins, Funtoo Solutions, Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.