10. Making Changes to Kernel Code

We will have to make changes in the kernel in order to add features to it and get the features working. There are two steps to it. Making the changes and then compiling them.

The steps are quite simple

1. Make the changes
2. Compile the kernel.

Compiling the whole kernel is one way to make the changes you did into the kernel. If your changes are in a small module of the code you need not compile the whole code and you can just make that module of the code.

In this section we will make two changes to the kernel.

1. Effecting only a module.
2. Effecting the whole kernel.

• Make the changes in the required file, re-check it.
• make SUBDIRS="path" - to compile the particular module.
• make modules_install - to install the module, you can manually copy the module to the /lib/ directory.
• make install - update the whole system if required, not required generally.
• Check the working of the code.

10.1. Hands-On - Making changes to a small module

• Make changes to the ext2 code. In the ext2_fsync function, add a printk and compile the code. Copy the modules to the /lib/XX directory and test by mounting a ext2 file system. Check dmesg for the printed message. Open/Write a file using vim to see the effects.
• The diff is based of linux-3.4.6 Kernel - use the same kernel version to avoid unneccessary problems at this stage.

10.2. Hands-On - Making changes to a code which effects the whole kernel