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

   1 sudo apt install qemu-system genisoimage
   2 
   3 cd /tmp 
   4 wget https://saimei.ftp.acc.umu.se/debian-cd/current/amd64/iso-cd/debian-12.1.0-amd64-netinst.iso
   5 sudo mount -o loop /vagrant/debian-12.1.0-amd64-netinst.iso /mnt/iso
   6 
   7 # reuse isolinux from debian iso
   8 mkdir /tmp/debiancd 
   9 mkdir /tmp/debiancd/isolinux 
  10 mkdir /tmp/debiancd/install.amd 
  11 cd /mnt/iso
  12 cp isolinux/isolinux.cfg isolinux/*c32 isolinux/menu.cfg isolinux/txt.cfg /tmp/debiancd/isolinux 
  13 cp install.amd/initrd.gz install.amd/vmlinuz /tmp/debiancd/install.amd/
  14 cp /mnt/iso/isolinux/isolinux.bin /tmp/debiancd/isolinux/
  15 
  16 # isolinux.cfg
  17 cat > debiancd/isolinux/isolinux.cfg << EOF 
  18 default bootcd
  19 prompt 1
  20 timeout 40
  21 
  22 label bootcd
  23   kernel /install.amd/vmlinuz
  24   append initrd=/install.amd/test.cpio.gz vga=788
  25 EOF
  26 
  27 # hello.c
  28 cat > hello.c << EOF
  29 #include <stdio.h>
  30 #include <unistd.h>
  31 
  32 int main(int argc, char *argv[])
  33 {
  34   printf("Hello world!\n");
  35   sleep(999999999);
  36 }
  37 EOF
  38 gcc -static hello.c -o init
  39 
  40 # build cpio initramfs
  41 echo init | cpio -o -H newc | gzip > test.cpio.gz
  42 cp test.cpio.gz debiancd/install.amd/test.cpio.gz
  43 
  44 cd /tmp/
  45 mkisofs -o /tmp/output.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/ 
  46 rm /vagrant/output.iso
  47 cp output.iso /vagrant
  48 
  49 # the output.iso can be launched in VirtualBox booting the iso in a VM Other Linux, Linux 64 bit 
  50 
  51 # test with qemu
  52 qemu-system-x86_64  -kernel debiancd/install.amd/vmlinuz -initrd test.cpio.gz /dev/zero