insmod hello.ko

Historically, josephmuia.ca has been a placeholder for links to my profile on other sites.

However, since I’ve begun writing a bit it seems customary to add a “Hello, World!” post.

Here it goes, in the form of a super simple Linux kernel module.

Linux kernel modules

Kernel modules are programs that extend the functionality of the kernel without having to reboot the system; they can be loaded and unloaded on demand.

The idea of writing code to run in the kernel is scary; with great power…

Fortunately, this module won’t do much other than log the iconic “Hello, World!” message.

Code

#include <linux/module.h> // Required by all modules.
#include <linux/kernel.h> // Required for kernel logging macros.
#include <linux/init.h> // Required for __init and __exit macros.

static int __init hello(void) {
  printk(KERN_INFO "Hello, World!\n");
  return 0;
}

static void __exit goodbye(void) {
  printk(KERN_INFO "Goodbye, cruel World!\n");
}

module_init(hello);
module_exit(goodbye);

The code above is cribbed from this guide.

A kernel module has two functions:

1. “init” which runs when the module is installed
2. “exit” which runs when the module is removed

Compiling

The below Makefile has targets to compile the module, producing hello.ko. Just run make.

obj-m += hello.o

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

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

I compiled and tested this module using kernel release 4.15.0-30-generic (uname -r).

Installing

Installing is as easy as sudo insmod ./hello.ko.

lsmod will read the contents of /proc/modules, displaying the currently loaded modules. If you’re following along you will see hello in the list after installing it.

Better yet, we can view our “Hello, World!” message in the kernel ring buffer with dmesg:

[ 3309.530533] Hello, World!

Removing

It’s been a slice, but it’s time to say goodbye. We can remove the module with sudo rmmod hello.

Again, we can see the output of printk() with dmesg:

[ 3319.331082] Goodbye, cruel World!