Why do we need a kernel module?

Sometimes we need to carry out some privileged operation which is not available in Ring3. Linux kernel modules are a way to get hold of Ring0. Although linux provides a lot of APIs, but still the need arises sometimes for kernel modules.

A Linux kernel module is a piece of compiled binary code that is inserted directly into the Linux kernel, running at ring 0, the lowest and least protected ring of execution in the x86–64 processor. Code here runs completely unchecked but operates at incredible speed and has access to everything in the system.

Getting Started

Make a folder where you would put your kernel module code.

$ mkdir sample_module

Open up a file where you would write your main module code and write down the below sample contents. Suppose I name the file technicalityinside.c

#include<linux/init.h>
#include<linux/module.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Akash Panda");

static int technicalityinside_init(void) {
        printk(KERN_ALERT "Module loaded\n");
        return 0;
}

static void technicalityinside_exit(void) {
        printk(KERN_ALERT "Goodbye cruel world\n");
}

module_init(technicalityinside_init);
module_exit(technicalityinside_exit);

Now we have the simplest of all modules. Now let us understand what does it say line by line.

“includes” cover the required header files required for linux kernel module development.
There are different module licences available for MODULE_LICENSE:

• “GPL” [GNU Public License v2 or later]
• “GPL v2” [GNU Public License v2]
• “GPL and additional rights” [GNU Public License v2 rights and more]
• “Dual BSD/GPL” [GNU Public License v2or BSD license choice]
• “Dual MIT/GPL” [GNU Public License v2 or MIT license choice]
• “Dual MPL/GPL” [GNU Public License v2 or MPL license choice]

We define both the init (loading) and exit (unloading) functions as static and returning an int.

Please note that at the end of the file we have called module_init and module_exit functions. This gives us an opportunity to name the init and exit functions as we like.

Now let us look at writing Makefile

obj-m += technicalityinside.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

Once we are ready with Makefile, we can now run make to compile our module.

Now, once compiled, we are ready to insert the linux module and test it.

$ sudo insmod technicalityinside.ko

Once we have loaded the module, we can see the kernel log output

$ tail -4 /var/log/kern.log

Once we see our module is working, we can remove it by issuing rmmod command.
To learn how to load and unload kernel module, follow this article.