Archive for February, 2007

Compiling Linux Kernel Sources

Sunday, February 4th, 2007

1. Downloading the kernel source code

In order to compile a new kernel we have to download the source code of the Linux kernel. We can download the source from www.kernel.org. Here we can find all versions of the Linux kernel source code. Let’s take an example. Suppose we want to compile the 2.6.9 version of the linux kernel. We have to download the 2.6.9 source code from:

http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.9.tar.bz2

It’s better to download the bzipped version, as that will be more compressed than its gzipped counterpart; hence will take less time to download. A wget from the command line will look like:

wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.9.tar.bz2

Once we download the required kernel version source, we need to bunzip and untar it. We can do the following:

tar xvjf linux-2.6.9.tar.bz2

2. Configuring the kernel

We have to configure the kernel before we start compiling it. During the configuration phase, we will select the components which we want to be part of the kernel. For example: suppose we are using the ext3 filesystem. Then we need to select the ext3 filesystem support while configuring the kernel. Typically we need to run following command.

make menuconfig

This will bring up the ncurses interface for configuring the kernel. There are other options such as ‘make xconfig’ and ‘make config’. The former will bring up the configuration menu in graphical mode and the latter in text mode. Once we select the different components we want for our kernel, we can exit the configuration interface. We should select the option to save the configuration from the configuration menu, before exiting.

After we have configured the kernel as mentioned above, we can find a file named ‘.config‘ in the top level directory of the source. This file is the configuration file. It contains various options and their states (whether they are selected or not). For example, if we choose to have the PCI support in our kernel we can find an entry of the form:

CONFIG_PCI=y

in the .config file. Similarly, options which are selected as not required will appear as not set. Suppose we have not selected the XFS filesystem support in our kernel we will find the following in the .config

# CONFIG_XFS_FS is not set

A great feature of 2.6 kernels is that if we are running make menuconfig (or xconfig or config) for the first time, then the configuration menu we are presented with is based on our current kernel configuration. In my case, I have a Fedora Core 1 system. The kernel which I run is ‘2.4.22-1.2115.nptl’. Hence when I run a ‘make menuconfig’ for the first time on the source then the configuration menu presented will contain the options as given in ‘/boot/config-2.4.22-1.2115.nptl’.

3. Building Dependencies

This step is required in kernels prior to 2.6 series (here I am only referring to the stable series kernels). For example if we are using a 2.4 kernel then we have to build the dependencies explicitly. We have to run the following:

make dep

This will build the dependencies. But for a 2.6 kernel we can skip this step. The dependencies are automatically created when making the final image with a 2.6 kernel.

4. Creating the final image

We can build various types of kernel binary images. We can build a plain kernel image, or a compressed version of it; the usual choice is compressed, or the ‘bzImage’. We can create the bzImage by running

make bzImage

In 2.6 kernels this step will also resolve the dependencies and proceed to create a bzImage image.

After the compilation is over we can find the kernel image at the path arch/i386/boot/bzImage in case of an image for a 386 based processor (Pentium, AMD etc.).

5. Compiling and Installing the modules

In the configuring section if we have selected some components to be built as kernel modules then we need to compile those modules. To compile the modules we should run the command:

make modules

This command will compile the components (which are selected for module compilation) to modules. In a 2.4 kernel the result will be .o files of the corresponding components. But in a 2.6 kernel the output file will be a .ko module. For example if we have given the option for the Network driver of Realtek cards to be built as modules then after giving a ‘make modules’ we can find in ‘driver/net/‘ a file named 8139too.o in the case of a 2.4 kernel and 8139too.koin the case of a 2.6 kernel.

After we have compiled the modules, it’s time now to install the modules. To install the modules run:

make modules_install

as root. This will install the modules and other necessary files into the /lib/modules/2.6.9directory.

6. Booting from the new kernel

Once we are done with the installation of modules, we can go for an automatic installation procedure for the kernel binary. We just have to run

make install

This will update the kernel image on to the /boot area, update the configuration file of the bootloader (lilo.conf or grub.conf) and then do the necessary actions to make the new kernel bootable.

After this we need to reboot the machine. When the machine boots next time the boot menu will present us with the option to boot from the new kernel we built.