Linux Kernel Module Cheat

7.1. Replace init

To have more control over the system, you can replace BusyBox’s init with your own.

The most direct way to replace init with our own is to just use the init= command line parameter directly:

./run --kernel-cli 'init=/lkmc/count.sh'

This just counts every second forever and does not give you a shell.

This method is not very flexible however, as it is hard to reliably pass multiple commands and command line arguments to the init with it, as explained at: Section 7.4, “Init environment”.

For this reason, we have created a more robust helper method with the --eval option:

./run --eval 'echo "asdf qwer";insmod hello.ko;./linux/poweroff.out'

It is basically a shortcut for:

./run --kernel-cli 'init=/lkmc/eval_base64.sh - lkmc_eval="insmod hello.ko;./linux/poweroff.out"'

Source: rootfs_overlay/lkmc/eval_base64.sh.

This allows quoting and newlines by base64 encoding on host, and decoding on guest, see: Section 17.3.1, “Kernel command line parameters escaping”.

It also automatically chooses between init= and rcinit= for you, see: Section 7.3, “Path to init”

--eval replaces BusyBox' init completely, which makes things more minimal, but also has has the following consequences:

/etc/fstab mounts are not done, notably /proc and /sys, test it out with:
```
./run --eval 'echo asdf;ls /proc;ls /sys;echo qwer'
```
no shell is launched at the end of boot for you to interact with the system. You could explicitly add a sh at the end of your commands however:
```
./run --eval 'echo hello;sh'
```

The best way to overcome those limitations is to use: Section 7.2, “Run command at the end of BusyBox init”

If the script is large, you can add it to a gitignored file and pass that to --eval as in:

echo '
cd /lkmc
insmod hello.ko
./linux/poweroff.out
' > data/gitignore.sh
./run --eval "$(cat data/gitignore.sh)"

or add it to a file to the root filesystem guest and rebuild:

echo '#!/bin/sh
cd /lkmc
insmod hello.ko
./linux/poweroff.out
' > rootfs_overlay/lkmc/gitignore.sh
chmod +x rootfs_overlay/lkmc/gitignore.sh
./build-buildroot
./run --kernel-cli 'init=/lkmc/gitignore.sh'

Remember that if your init returns, the kernel will panic, there are just two non-panic possibilities:

run forever in a loop or long sleep
poweroff the machine