The script is at https://github.com/landley/toybox/blob/master/mkroot/mkroot.sh and the talk outline is at https://landley.net/talks/mkroot-2023.txt. This talk is a line by line explanation of what that script is doing and why, with a lot of backstory on the design decisions. Toybox is a BSD-licensed command line utility package written by an ex-maintainer of busybox, which has provided Android’s command line since Marshmallow.

You can try out the resulting binaries at https://landley.net/bin/mkroot/latest. The script to build the cross compilers from source is at https://github.com/landley/toybox/blob/master/scripts/mcm-buildall.sh (or use the binaries from https://landley.net/bin/toolchains) and a script that performs an automated smoketest on the resulting systems (to make sure they mount a block device and talk to the network and so on) is at https://github.com/landley/toybox/blob/master/mkroot/testroot.sh which also provides an example of how to run arbitrary automated loads under QEMU and examine the results.

The tiny systems are built from two source packages (linux and toybox), and pull in code from the toolchain’s C library (musl-libc for these toolchains), packaged into an initramfs. The script creates an init script, directory layout, and some /etc files, then builds and installs toybox’s command line utilities and packages the result into an initramfs. It then creates a kernel config for the chosen architecture, compiles Linux, and packages the result with a run-qemu.sh script to invoke the emulator.

The mkroot/packages directory provides examples of additional packages the script can download, cross compile, and add to the image. One of those packages builds busybox, if you prefer any of those utilities to toybox or would like to compare them side by side.