Everything is built from source and easily modifiable, powered by Buildroot

submodules/binutils-gdb/
submodules/buildroot/
submodules/gcc/
submodules/glibc/
submodules/linux/
submodules/qemu/

So you can modify source, rebuild and that's it, its in the VM.

E.g., let's hack the linux kernel:

asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
{
  pr_info("I'VE HACKED THE LINUX KERNEL!!!");

Rebuild Linux:

./build-linux

Rerun:

./run

And after boot we see:

<6>[    0.000000] I'VE HACKED THE LINUX KERNEL!!!

 Ancestors (5)