LinuxFromScratch

https://www.linuxfromscratch.org/

Linux From Scratch (LFS) is a project that provides you with step-by-step instructions for building your own custom Linux system, entirely from source code.

List of the required downloads for LFS 11.3

   1 cd ~
   2 mkdir lfs
   3 cd lfs
   4 wget https://www.linuxfromscratch.org/lfs/downloads/stable/wget-list
   5 xargs -l wget < wget-list
   6 
   7 # wget --input-file=wget-list --continue --directory-prefix=$LFS/sources
   8 mkdir sources
   9 mv *patch  *xz *gz   *bz2  sources/

initramfs with static program

https://www.kernel.org/doc/html/latest/filesystems/ramfs-rootfs-initramfs.html#contents-of-initramfs

A good first step is to get initramfs to run a statically linked "hello world" program as init

sudo apt install qemu-system 

cd /tmp 
wget https://saimei.ftp.acc.umu.se/debian-cd/current/amd64/iso-cd/debian-12.1.0-amd64-netinst.iso
sudo mount -o loop /vagrant/debian-12.1.0-amd64-netinst.iso /mnt/iso

cat > hello.c << EOF
#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
  printf("Hello world!\n");
  sleep(999999999);
}
EOF
gcc -static hello.c -o init
echo init | cpio -o -H newc | gzip > test.cpio.gz

cp test.cpio.gz debiancd/install.amd/test.cpio.gz

cat > debiancd/isolinux/isolinux.cfg << EOF 
default bootcd
prompt 1
timeout 40

label bootcd
  kernel /install.amd/vmlinuz
  append initrd=/install.amd/test.cpio.gz vga=788
EOF

cd /tmp/
mkisofs -o /tmp/output2.iso  -R -l -L -D -b isolinux/isolinux.bin -c isolinux/boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -V "LFS_$(date +%Y%m%d)" /tmp/debiancd/ 
rm /vagrant/output2.iso
cp output2.iso /vagrant

qemu-system-x86_64 -kernel debiancd/install.amd/vmlinuz -initrd test.cpio.gz /dev/zero