CreatingKernelModuleNotes.md

Overview

Notes on creating device drivers and kernel modules

Reference

Books

• Linux Device Drivers, 3rd Edition. The free PDF version of this O'Reilly book by Greg Kroah-Hartman, et al, available here.

YouTube Videos

• Syscalls, Kernel vs. User Mode and Linux Kernel Source Code
• Linux kernel driver writing tutorial (USB) by Greg Kroah-Hartman

Terminology and Concepts

Refer to my KernelGlosssaryConcept document.

Important Directories and Files

Refer to my separate KernelDirectoriesAndFiles document

Tools

insmod

You use insmod to load a kernel module

sudo insmod mykrnlmod.ko

Assumes you are in the directory for mykrnlmod.o, otherwise provide the path

rmmod

You use rmmod to unload a kernel module

sudo rmmod mykrnlmod

Note: no extension is used here, as it was with insmod. You also do not need to provide a path if you are not in the directory.

modprobe

Adds and removes modules from the kernel. Provides more options than insmod and rmmod. Used in /etc/init.d/kmod to load kernel modules from /etc/modules (although mine was empty, so it may not do this anymore)

dmesg

The dmesg command allows you to see messages written inside the kernel ring/space. Very useful for viewing the output of your module.

To display the messages:

dmesg

To display the messages and then clear all existing messages:

sudo dmesg -c

lsmod

List modules, their size, how many times it's used and by what

lsmod | sort | less

custom shell script to list module files *.ko

#!/bin/bash
#
# find_all_modules.sh
# From Chapter 7 of Linux Kernel in a Nutshell
#
for i in `find /sys/ -name modalias -exec cat {} \;`; do
/sbin/modprobe --config /dev/null --show-depends $i ;
done | rev | cut -f 1 -d '/' | rev | sort -u

Compiling the Module

The compile process needs access to the kernel source tree. This is best done with a make file that has a reference to this directory. Here is an example of a simple make file to do this:

obj-m += mykrnlmod.o

KDIR=/home/<username>/source/kernel/linux-4.14

all:
	$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

clean:
	rm -rf *.o *.ko *.mod.* *.symvers *.order

Examples

Simple Example

This code simply prints a message on loading, and another message on unloading, they can be viewed using the dmesg command.

// mykrnlmod.c
//
//  Displays an initialization message upon loading the module, and an exit
//  message when unloading the module.  These messages are viewed by the
//  'dmesg' command without parameters.  To clear the kernel messages use
//  'sudo dmesg -c', which will display them before clearing them.
//
//   Load with 'sudo insmod mykrnlmod.ko'
//   Unload with 'sudo rmmod mykrnlmod'

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

static int my_init(void) {
    printk(KERN_INFO "mykernlmod: module loaded at 0x%p\n", my_init);
    return 0;
}

static void my_exit(void) {
    printk(KERN_INFO "mykernlmod: module unloaded from 0x%p\n", my_init);
}

module_init(my_init);
module_exit(my_exit);

MODULE_AUTHOR("Traeven");
MODULE_LICENSE("GPL v2");