Way back when, kernels used to be monolithic, meaning that adding new functionality required recompiling and installing it, followed by a reboot. Today, things are much easier. By using the kmod daemon (man 8 kmod), users are allowed to load and unload modules (i.e.: kernel object files) on demand, without all the fuss. These modules are C programs that must implement initialization and removal functions that are called automatically. Usually, these functions register / unregister other functions contained in your object with core kernel systems.

We can use lsmod to get a list of all present modules, and modinfo to obtain detailed information about a specific module.

$ lsmod
ecdh_generic           16384  1 bluetooth
 
$ modinfo ecdh_generic | grep description
description:    ECDH generic algorithm
 
$ modinfo bluetooth | grep description 
description:    Bluetooth Core ver 2.22

What we can understand from this is that the Elliptic Curve Diffie-Hellman module is 16384 bytes in size and is used by one other module, via the bluetooth ECDH helper. As you probably noticed, elixir.bootlin.com is a critical resource in navigating the kernel code.

Looking in the skel/01/ directory from our code skeleton, we will find a minimal build environment for our first module. Alas, compiling a kernel module differs from compiling a user space program. But just slightly: kernel-specific headers must be used, user space-specific libraries (e.g.: libc) are generally unavailable (so no printf()) and lastly, the same options that were used to compile the kernel itself must be specified. To this end, the kbuild system was introduced. As you can see, our Makefile invokes its correpsondent from the kernel source directory in /lib/modules/..., which in turn uses the configuration in our Kbuild file. The obj-m variable specifies the name of the final output object file (in this case, test.o). test-objs contains a sequence of dependent object files, so if you split your code across multiple sources, just add them to test-objs. If you have a single source, you can drop test-objs but the kbuild system will expect a test.c file to be present.

Now, let's compile our module, upload it into the kernel, and see what happens:

$ make
 
$ sudo insmod test.ko
$ sudo dmesg
...
[ 6348.461247] my-first-module: Hello world!
 
$ sudo rmmod test
$ sudo dmesg
...
[ 6348.461247] my-first-module: Hello world!
[ 6366.635090] my-first-module: Goodbye cruel, cruel world!

Here, we used insmod to upload a .ko kernel object file into the kernel proper and rmmod to remove it. dmesg is a tool that prints the kernel message buffer. Note that there are multiple log levels ranging from debug to emergency. pr_info() is the kernel's printf() variant that corresponds to one of the less urgent levels. dmesg can be configured to squelch messages under a certain level but depending on how your kernel was compiled, some of the more important messages will also be echoed to your terminal.