Compile custom kernel modules

Build a sample kernel module as requested by various configurations at hardware and software level.

Before you begin

  • You installed the kernel-devel, gcc, and elfutils-libelf-devel packages. 
    # dnf install kernel-devel-$(uname -r) gcc elfutils-libelf-devel
  • You have root permissions.
  • You created the /root/testmodule/ directory where you compile the custom kernel module.

Procedure

  1. Create the /root/testmodule/test.c file with the following content. 
    #include <linux/module.h>
#include <linux/kernel.h>

int init_module(void)
    { printk("Hello World\n This is a test\n"); return 0; }

void cleanup_module(void)
    { printk("Good Bye World"); }

MODULE_LICENSE("GPL");

    The test.c file is a source file that provides the main functionality to the kernel module. The file has been created in a dedicated /root/testmodule/ directory for organizational purposes. After the module compilation, the /root/testmodule/ directory will contain multiple files.

    The test.c file includes from the system libraries:

    • The linux/kernel.h header file is necessary for the printk() function in the example code.
    • The linux/module.h file contains function declarations and macro definitions that are shared across several source files written in C programming language.
  2. Follow the init_module() and cleanup_module() functions to start and end the kernel logging function printk(), which prints text.
  3. Create the /root/testmodule/Makefile file with the following content. 
    obj-m := test.o

    The Makefile contains instructions for the compiler to produce an object file named test.o. The obj-m directive specifies that the resulting test.ko file is going to be compiled as a loadable kernel module. Alternatively, the obj-y directive can instruct to build test.ko as a built-in kernel module.

  4. Compile the kernel module: 
    # make -C /lib/modules/$(uname -r)/build M=/root/testmodule modules
     
    make: Entering directory '/usr/src/kernels/6.12.0-55.9.1.el10_0.x86_64'
  CC [M]  /root/testmodule/test.o
  MODPOST /root/testmodule/Module.symvers
  CC [M]  /root/testmodule/test.mod.o
  LD [M]  /root/testmodule/test.ko
  BTF [M] /root/testmodule/test.ko
Skipping BTF generation for /root/testmodule/test.ko due to unavailability of vmlinux
make: Leaving directory '/usr/src/kernels/6.12.0-55.9.1.el10_0.x86_64'

    The compiler creates an object file (test.o) for each source file (test.c) as an intermediate step before linking them together into the final kernel module (test.ko).

    After a successful compilation, /root/testmodule/ contains additional files that relate to the compiled custom kernel module. The compiled module itself is represented by the test.ko file.

Results

  1. Optional: check the contents of the /root/testmodule/ directory: 
    # ls -l /root/testmodule/
     
    total 152
-rw-r--r--. 1 root root    16 Jul 26 08:19 Makefile
-rw-r--r--. 1 root root    25 Jul 26 08:20 modules.order
-rw-r--r--. 1 root root     0 Jul 26 08:20 Module.symvers
-rw-r--r--. 1 root root   224 Jul 26 08:18 test.c
-rw-r--r--. 1 root root 62176 Jul 26 08:20 test.ko
-rw-r--r--. 1 root root    25 Jul 26 08:20 test.mod
-rw-r--r--. 1 root root   849 Jul 26 08:20 test.mod.c
-rw-r--r--. 1 root root 50936 Jul 26 08:20 test.mod.o
-rw-r--r--. 1 root root 12912 Jul 26 08:20 test.o
  2. Copy the kernel module to the /lib/modules/$(uname -r)/ directory: 
    # cp /root/testmodule/test.ko /lib/modules/$(uname -r)/
  3. Update the modular dependency list: 
    # depmod -a
  4. Load the kernel module: 
    # modprobe -v test
     
    insmod /lib/modules/6.12.0-55.9.1.el10_0.x86_64/test.ko
  5. Verify that the kernel module was successfully loaded: 
    # lsmod | grep test
     
    test                   16384  0
  6. Read the latest messages from the kernel ring buffer: 
    # dmesg
     
    [74422.545004] Hello World
                This is a test