With a compiled kernel image, it can be useful to load your own initramfs:
- make it as small as possible considering the features you need
- create a minimal test environment for the kernel image
- mount a remote filesystem as the root
- your own reason
Anyways, with the official documentation help,
let's create an initramfs.
We need to gen_init_cpio tool that can found in the kernel repository:
gen_init_cpio.c needs to be compiled:
wget -O gen_init_cpio.c 'https://raw.githubusercontent.com/torvalds/linux/v6.7/usr/gen_init_cpio.c'
gcc -o gen_init_cpio gen_init_cpio.c
Now it is ready for use.
Let's create an hello world init program that the kernel can execute to show that everything works correctly:
printf '%s\n' '#include <stdio.h>' '#include <unistd.h>' 'int main() { printf("Hello world!\n"); sleep(999999999); }' | gcc -fpic -static -xc - -o init
Here is a basic initramfs description, we give it to gen_init_cpio directly:
./gen_init_cpio -t 0 -c - > initramfs.cpio <<EOF
# this is a file containing newline separated entries that
# describe the files to be included in the initramfs archive:
# # a comment
# file <name> <location> <mode> <uid> <gid> [<hard links>]
# dir <name> <mode> <uid> <gid>
# nod <name> <mode> <uid> <gid> <dev_type> <maj> <min>
# slink <name> <target> <mode> <uid> <gid>
# pipe <name> <mode> <uid> <gid>
# sock <name> <mode> <uid> <gid>
# Root directory
dir . 1755 0 0
# common directories
dir proc 1775 0 0
dir dev 1775 0 0
dir bin 1775 0 0
dir sys 1775 0 0
dir run 1775 0 0
dir mnt 1775 0 0
dir etc 1775 0 0
# symlink /sbin -> /bin
slink bin /sbin 1744 0 0
# common devices
dir dev/pts 1775 0 0
dir dev/shm 1775 0 0
dir dev/udev 1775 0 0
nod dev/zero 1666 0 0 c 1 5
nod dev/null 1666 0 0 c 1 3
nod dev/console 1622 0 0 c 5 1
nod dev/loop0 1644 0 0 b 7 0
nod dev/ttyS0 1666 0 0 c 4 64
nod dev/tty 1666 0 0 c 5 0
nod dev/ptmx 1666 0 0 c 5 2
nod dev/random 1444 0 0 c 1 8
nod dev/urandom 1444 0 0 c 1 9
slink dev/core /proc/kcore 1777 0 0
slink dev/fd /proc/self/fd 1777 0 0
slink dev/stdout /proc/self/fd/1 1777 0 0
slink dev/stdin /proc/self/fd/0 1777 0 0
slink dev/stderr /proc/self/fd/2 1777 0 0
# include previously compiled init
# make sure the path is /init
file /init init 0755 0 0
EOF
The line starting with file takes an external file as a second parameter, and that's how we include the init program in the archive.
Note this is rootless, we don't need to re-create the filesystem tree locally or use fakeroot to create special file in the archive.
If you're following along, you now have created initramfs.cpio archive!
With our initramfs we just need a linux kernel to be able to run it in qemu:
qemu-system-${HOSTTYPE} -kernel vmlinuz-6.7.5-060705-generic -initrd initramfs.cpio -append "console=ttyS0" -cpu max -m 128M -nographic -serial mon:stdio -nodefaults
You can find pre-compiled kernel image in ubuntu repository
Happy hacking!