Linux Kernel Module Cheat

2.2.2.2. Your first kernel module hack

pr_info("hello init hacked\n");

and rebuild with:

./build-modules

Now there are two ways to test it out: the fast way, and the safe way.

The fast way is, without quitting or rebooting QEMU, just directly re-insert the module with:

insmod /mnt/9p/out_rootfs_overlay/lkmc/hello.ko

and the new pr_info message should now show on the terminal at the end of the boot.

This works because we have a 9P mount there setup by default, which mounts the host directory that contains the build outputs on the guest:

ls "$(./getvar out_rootfs_overlay_dir)"

The fast method is slightly risky because your previously insmodded buggy kernel module attempt might have corrupted the kernel memory, which could affect future runs.

Such failures are however unlikely, and you should be fine if you don’t see anything weird happening.

The safe way, is to fist quit QEMU, rebuild the modules, put them in the root filesystem, and then reboot:

./build-modules
./build-buildroot
./run --eval-after 'insmod hello.ko'

./build-buildroot is required after ./build-modules because it re-generates the root filesystem with the modules that we compiled at ./build-modules.

You can see that ./build does that as well, by running:

./build --dry-run

--eval-after is optional: you could just type insmod hello.ko in the terminal, but this makes it run automatically at the end of boot, and then drops you into a shell.

If the guest and host are the same arch, typically x86_64, you can speed up boot further with KVM:

./run --kvm

All of this put together makes the safe procedure acceptably fast for regular development as well.

It is also easy to GDB step debug kernel modules with our setup, see: Section 3.4, “GDB step debug kernel module”.