Hello World Module

  • In this program, you are going to learn

  • How to write a sample kernel module ?

  • How to write Makefile to compile driver module ?

  • How to check if module is loaded successfully ?

  • How to check if module is unloaded successfully ?

  • How to check prints from kernel modules ?

Topics in this section,

  • init_module function gets called on insertion of module using insmod command

  • cleanup_module function gets called on removal of module using rmmod command

 1#include <linux/module.h>
 2#include <linux/kernel.h>
 3
 4MODULE_LICENSE("GPL");
 5MODULE_AUTHOR("Linux_usr");
 6
 7int init_module(void)
 8{
 9	pr_info("Hello, world\n");
10
11	return 0;
12}
13
14void cleanup_module(void)
15{
16	pr_info("Good bye, world\n");
17}

  • In this Makefile,

    • obj-m indicates the build system to create a separate loadable .ko module

    • make -C informs the make command to look for master make file in the given path

    • $(shell uname -r) replaces the output of uname -r command

    • $(PWD) replaces it with Present Working Directory

1obj-m += hello.o
2
3all:
4	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
5clean:
6	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

 1$ pwd
 2basic_ldd/chapter1_basics/p1_basicModule
 3
 4
 5$ ls
 6hello.c  Makefile  README
 7
 8
 9$ make all
10make -C /lib/modules/6.2.0-32-generic/build M=/home/linux/basic_ldd/chapter1_basics/p1_basicModule modules
11make[1]: Entering directory /usr/src/linux-headers-6.2.0-32-generic
12  CC [M]  basic_ldd/chapter1_basics/p1_basicModule/hello.o
13  MODPOST basicl_ldd/chapter1_basics/p1_basicModule/Module.symvers
14  CC [M]  basicl_ldd/chapter1_basics/p1_basicModule/hello.mod.o
15  LD [M]  basicl_ldd/chapter1_basics/p1_basicModule/hello.ko
16  BTF [M] basicl_ldd/chapter1_basics/p1_basicModule/hello.ko
17make[1]: Leaving directory /usr/src/linux-headers-6.2.0-32-generic

 1$ ls -l
 2total 240
 3-rwxrwxrwx 1 linux linux    233 Sep 11 16:00 hello.c
 4-rw-rw-r-- 1 linux linux 108392 Sep 12 12:37 hello.ko
 5-rw-rw-r-- 1 linux linux    134 Sep 12 12:37 hello.mod
 6-rw-rw-r-- 1 linux linux    891 Sep 12 12:37 hello.mod.c
 7-rw-rw-r-- 1 linux linux  93536 Sep 12 12:37 hello.mod.o
 8-rw-rw-r-- 1 linux linux  16232 Sep 12 12:37 hello.o
 9-rwxrwxrwx 1 linux linux    154 Sep 11 16:00 Makefile
10-rw-rw-r-- 1 linux linux    134 Sep 12 12:37 modules.order
11-rw-rw-r-- 1 linux linux      0 Sep 12 12:37 Module.symvers
12-rwxrwxrwx 1 linux linux   1876 Sep 12 13:12 README

1$ modinfo hello.ko
2filename:       basic_ldd/chapter1_basics/p1_basicModule/hello.ko
3author:         Linux_usr
4license:        GPL
5srcversion:     CC6A6FD6CE9DAF87AC2E72B
6depends:
7retpoline:      Y
8name:           hello
9vermagic:       6.2.0-32-generic SMP preempt mod_unload modversions

1$ sudo insmod ./hello.ko

1$ dmesg
2[67117.450399] Hello, world

1$ lsmod | grep hello
2hello                  16384  0

1$ sudo rmmod hello

1$ dmesg
2[67136.900177] Good bye, world

API

Learning

MODULE_LICENSE

Tells the kernel what license our module is under

MODULE_AUTHOR

Declares the module’s author

pr_info

Prints an info-level message

Command

Learning

insmod

Command to insert a kernel module

rmmod

Command to remove a kernel module

lsmod

Command to display the status of modules in the Linux kernel

modinfo

Command to display the information about a Linux Kernel module

dmesg

Command to display kernel-related messages
See Also