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Kernel Modules

Source: https://en.wikibooks.org/wiki/The_Linux_Kernel/Modules

The Linux kernel is a monolithic type kernel by design, meaning all required functionality for hardware operations are included in a single binary blob. However as of kernel version 3.0 (?) the it started external module support, this meant that if a user needed a said functionality he's not no more obliged to recompile the kernel from source "make menuconfig", all he's supposed to de is load or unload the module that adds support for the feature he wants to use.

Listing

In order to check what modules are loaded use lsmod, and since the list might be a bit lengthy grep is perfect helper is you already knew what you're looking for

lsmod | grep [MOD_NAME]

Loading (Inserting)

If the module you're after isn't there, insmod or Insert Module is the tool that will help you load the module.

insmod [MOD_NAME]

# equivalently
modprob [MOD_NAME]

Offloading (Removing)

If you no longer need the module to be loaded for whatever reason ( Security, Performance... ), use rmmod or Remove Module to unload it.

rmmod [MOD_NAME]

# equivalently
modprob -r [MOD_NAME]

Simple Kernel Module

Here is a simple kernel module source code in C, all it does is output to the kernel log TESTING MODULE\n when is starts and END OF TESTING MODULE\n when it's done.

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

MODULE_LICENCE("DUAL BSD/GPL");
static int test_init(void)
{
    printk(KERN_ALERT "TESTING MODULE\n");
    return 0;
}
static void test_end(void)
{
    printk(KERN_ALERT "END OF TESTING MODULE\n");
}
module_init(test_init);
module_exit(test_end);

The KERN_ALERT argument to printk set the priority of the job,

#define KERN_EMERG      "<0>"   /* system is unusable                   */
#define KERN_ALERT      "<1>"   /* action must be taken immediately     */
#define KERN_CRIT       "<2>"   /* critical conditions                  */
#define KERN_ERR        "<3>"   /* error conditions                     */
#define KERN_WARNING    "<4>"   /* warning conditions                   */
#define KERN_NOTICE     "<5>"   /* normal but significant condition     */
#define KERN_INFO       "<6>"   /* informational                        */
#define KERN_DEBUG      "<7>"   /* debug-level messages                 */

In order to compile the kernel module

Additional Info

The directory /lib/modules

The depmod tool is responsible for resolving module dependency before loading a module. So if a module calls function on another one it will make it a dependency for the first, in order to be loaded before the first one.

depmod

System Calls (Syscalls)

In order for user space processes Ring 3 to be able communicate to the kernel at Ring 0 an interface must be used, it's called System Calls or Syscalls.

Netem (Network Emulator)

https://wiki.linuxfoundation.org/networking/netem

This kernel module can be used to manipulate packets to emulate WAN (Delay, Jitter, Corruption, Duplications...etc)