Use Linux kernel module

Kernel

 Posted on September 30, 2021  |  3 minutes  |  446 words  |  Iulian Costan

In Part 1 I wrote a simple Linux kernel module, called World, in Part 2 I configured that module, now it's time to use that module from within kernel space (another kernel module).

1. World module

First of all, let's review world.c module that was used in previous two parts.

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

  static char *whom = "World";

  void world_print(char* greet)
  {
      printk(KERN_INFO "%s, %s\n", greet, whom);
  }
  EXPORT_SYMBOL(world_print);

  static int __init world_init(void)
  {
      printk(KERN_INFO "Init, %s\n", whom);
      return 0;
  }
  module_init(world_init);

  static void __exit world_exit(void)
  {
      printk(KERN_INFO "Exit, %s\n", whom);
  }
  module_exit(world_exit);

  MODULE_AUTHOR("Iulian Costan <kernel@costan.ro>");
  MODULE_DESCRIPTION("World module that exports a symbol");
  MODULE_LICENSE("Dual BSD/GPL");

2. Hello module

Now, say hello to hello.c module :).

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

  static int howmany = 1;
  module_param(howmany, int, S_IRUGO);
  static char *greet = "Hello";
  module_param(greet, charp, S_IRUGO);

  extern void world_print(char* greet);

  static int __init hello_init(void)
  {
      printk(KERN_INFO "Init, Hello\n");
      int i;
      for (i = 0; i < howmany; i++) {
          world_print(greet);
      }
      return 0;
  }

  static void __exit hello_exit(void)
  {
      printk(KERN_INFO "Exit, Hello\n");
  }

  module_init(hello_init);
  module_exit(hello_exit);

  MODULE_AUTHOR("Iulian Costan <kernel@costan.ro");
  MODULE_DESCRIPTION("Hello module that depends on World module");
  MODULE_LICENSE("Dual BSD/GPL");

This time, init function is special because it iterates over module parameter and calls external function (symbol) world_print that is implemented in World module.

In addition to init, exit functions and macro calls we have two module parameters (that can be changed at compile, load or run time) and the external declaration for world_print function that will be imported from World module.

2.1 Build Hello

Execute make using the same Makefile/Kbuild files from Part 1 to compile and build hello.ko module file. Notice the depends line below, Hello module depends on World module and parm lines with two params: howmany and greet.

  modinfo hello.ko
filename:       /home/icostan/Projects/blog/content/post/hello.ko
license:        Dual BSD/GPL
description:    Hello module that depends on World module
author:         Iulian Costan <kernel@costan.ro
srcversion:     BB1B2CF1CD93B39F4366E4F
depends:        world
retpoline:      Y
name:           hello
vermagic:       5.14.8-arch1-1 SMP preempt mod_unload
parm:           howmany:int
parm:           greet:charp

2.2 Install Hello

As I mentioned before, during module install/load we can change the parameters as follows:

  echo 'Installing module...'
  sudo insmod ./hello.ko greet=Bonjour howmany=3

After successful installation,

  journalctl -k | grep Hello
Sep 30 08:55:28 drakarys kernel: Init, Hello

parameters are correctly set,

  cat /sys/module/hello/parameters/greet
  cat /sys/module/hello/parameters/howmany
Bonjour
3

and world_print function from World module was called 3 times, as expected.

  journalctl -k | grep Bonjour
Sep 30 08:55:28 drakarys kernel: Bonjour, World
Sep 30 08:55:28 drakarys kernel: Bonjour, World
Sep 30 08:55:28 drakarys kernel: Bonjour, World

And this is it… our useless Hello module calling World module on install but in next part we will talk about mode interesting stuff like device drivers or maybe ioctl and syscalls, let's see.

linux  kernel  module  c