While taking a course called "Advanced Linux: The Linux Kernel" on LinkedIn Learning I wanted to play around with making modules after finishing the sample module for the course. And here is what I came up with in a couple hours. I was honestly surprised I managed to get it working pretty smoothly. I used this great book, borrowed setting permissions from the TTY driver as explained here and modified it slightly when the kernel complained about NULL pointer dereference using this resource.

You need to have the Linux kernel development package installed. If you see a "build" directory under /lib/modules/<YOUR KERNEL VERSION>/ then you're good to go. If not, consult your distro-specific instruction on how to obtain the package.

Clone the repo, enter it and use

make -C /lib/modules/$(uname -r)/build M=$PWD modules

After succesful compilation, add the module using

sudo insmod isfri.ko

After the module has been added, you can use it by simply using cat on the new device

cat /dev/isfri

The module will then "respond" with a proper message according to the current day of the week:

• Friday: "IT IS!"
• Saturday: "You just missed it!"
• Sunday: "No, but it's still the weekend!"
• Any other day: "Nope."

If you want to remove the module from the kernel, simply use

sudo rmmod isfri