Create root filesystems for embedded systems

All is started from this link where a good boy shared some images in order to start to put hands on esotic architectures.

The bad thing in my opinion is that he doesn't explain how they have been generated, just in case someone need to customize something; in this post I want to show how to create root filesystems for all the necessary architectures using multistrap. If you want instead to use something more serious, here another post about buildroot, that allows you to do something similar.

Multistrap is a Debian tool used to create root filesystem: from the Debian's wiki

It was designed primarily for making root filesystems for foreign architecture embedded systems, but in fact can be used for many tasks where one might also use debootstrap.

Its main limitation compared to debootstrap is that it uses apt and dpkg directly so can only work on a debian system - debootstrap depends on nothing but shell, wget, binutils and thus can run pretty-much anywhere.

This is not a complete replacement for the images linked above since at the end of the procedure won't have a linux kernel to use to start QEMU but I think it's useful for someone with already a running instance.

Create the filesystem

The dependencies on a Debian system are

$ sudo apt-get install qemu multistrap qemu-user-static

since I am corageous I choose to create one root filesystem for the PowerPC architecture: save the content below as multistrap.conf

[General]
directory=target-rootfs
cleanup=true
noauth=true
unpack=true
debootstrap=Grip Net Utils
aptsources=Grip
#tarballname=rootfs.tar
#
[Grip]
noauth=true
packages=apt kmod lsof
source=http://emdebian.bytesatwork.ch/mirror/grip
keyring=emdebian-archive-keyring
suite=stable-grip

[Net]
#Basic packages to enable the networking
packages=netbase net-tools udev iproute iputils-ping ifupdown isc-dhcp-client ssh

[Utils]
#General purpose utilities
packages=locales adduser nano less wget dialog usbutils

Then we can create a root fs using multistrap (the -a flag is what sets the architecture)

$ sudo multistrap -a powerpc -f multistrap.conf -d /tmp/rootfs-ppc
$ sudo mount -o bind /dev/ /tmp/rootfs-ppc/dev
$ sudo cp /usr/bin/qemu-ppc-static /tmp/rootfs-ppc/usr/bin/
$ sudo LC_ALL=C LANGUAGE=C LANG=C chroot /tmp/rootfs-ppc/ dpkg --configure -a

select NO when asked to use dash as default shell. Now we can enter inside the system and how can see now you have a PowerPC architecture

$ sudo chroot /tmp/rootfs-ppc/ /bin/bash
root@host:/# uname -a
Linux antani 4.9.0-3-amd64 #1 SMP Debian 4.9.30-2 (2017-06-12) ppc GNU/Linux

If you want an internet connection remember to mount --bind the resolv.conf file inside the chroot.

If you want to create a real root filesystem for QEMU (or whatever) you can use the following command after installing the libguestfs-tools package:

$ sudo virt-make-fs --format=qcow2 --size=+200M /tmp/rootfs-ppc/ /tmp/rootfs.img

(see virt-make-fs man page for more informations).

Probably in a following up post I will write about using projects like OpenEmbedded or buildroot to have a complete and customizable running systems.

The following signatures couldn't be verified error

Depending on the system you are in, multistrap can fail with this error, from here the advise is to add above the line in /usr/sbin/multistrap that contains

$config_str .= " -o Apt::Get::AllowUnauthenticated=true"

this line

$config_str .= " -o Acquire::AllowInsecureRepositories=true";