2.2.2.1. Your first Linux kernel hack
Let’s hack up the Linux kernel entry point, which is an easy place to start.
Open the file:
vim submodules/linux/init/main.c
and find the start_kernel function, then add there a:
pr_info("I'VE HACKED THE LINUX KERNEL!!!");
Then rebuild the Linux kernel, quit QEMU and reboot the modified kernel:
./build-linux ./run
and, surely enough, your message has appeared at the beginning of the boot:
<6>[ 0.000000] I'VE HACKED THE LINUX KERNEL!!!
So you are now officially a Linux kernel hacker, way to go!
We could have used just build to rebuild the kernel as in the initial build instead of build-linux, but building just the required individual components is preferred during development:
-
saves a few seconds from parsing Make scripts and reading timestamps
-
makes it easier to understand what is being done in more detail
-
allows passing more specific options to customize the build
The build script is just a lightweight wrapper that calls the smaller build scripts, and you can see what ./build does with:
./build --dry-run
see also: Dry run to get commands for your project.
When you reach difficulties, QEMU makes it possible to easily GDB step debug the Linux kernel source code, see: Section 3, “GDB step debug”.