Appendix A. Building a Custom Kernel

Many people new to Linux often ask, "Why should I build my own kernel?" Given the advances that have been made in the use of kernel modules, the most accurate response to that question is, "Unless you already know why you need to build your own kernel, you probably do not need to."

In the past, you had to recompile the kernel if you added new hardware on your system. In other words, the kernel was static. Improvements in the Linux 2.0.x kernels allowed for many hardware drivers to be modularized into components that are loaded on demand. However, major problems existed when users had multiple kernels that had been compiled for different configuration options on their system; for example, SMP versus UP kernels. Further Linux 2.4.x kernel modularization advancements allow for multiple kernels to co-exist more easily, but they can not share modules.

For information on handling kernel modules see Chapter 24. Unless you are recompiling a customized kernel for your system, you will not see many changes in how kernel modules are handled.

Building a Modularized Kernel

The instructions in this section apply to building a modularized kernel. If you are interested in building a monolithic kernel instead, see the section called Building a Monolithic Kernel for an explanation of the different aspects of building and installing a monolithic kernel.

The following steps will guide you through building a custom kernel for the x86 architecture:

	Note
	This example uses 2.4.7-3 as the kernel version. Your kernel version might differ. To determine your kernel version, type the command `uname -r`. Replace 2.4.7-3 with your kernel version.

The most important step is to make sure that you have a working emergency boot disk in case you make a mistake. If you didn't make a boot disk during the installation, use the mkbootdisk command to make one now. The standard command is similar to mkbootdisk --device /dev/fd0 2.4.x, where 2.4.x is the full version of your kernel (such as 2.4.7-3). Once done, test the boot disk to make sure that it will boot the system.
You must have both the kernel-headers and kernel-source packages installed. Issue the commands rpm -q kernel-headers and rpm -q kernel-source to determine their versions, if they are installed. If they are not installed, install them from the Red Hat Linux CD 1 or the Red Hat FTP site available at ftp://ftp.redhat.com (a list of mirrors is available at http://www.redhat.com/mirrors.html). Refer to Chapter 25 for information on installing RPM packages.
Open a shell prompt and change to the directory /usr/src/linux-2.4. All commands from this point forward must be issued from this directory.
It is important that you begin a kernel build with the source tree in a known condition. Therefore, it is recommended that you begin with the command make mrproper. This will remove any configuration files along with the remains of any previous builds that may be scattered around the source tree. If you already have a working configuration file (/usr/src/linux-2.4/.config) that you want to use, back it up to a different directory before running this command and copy it back after running the command. If you use an existing configuration file, skip the next step.
Now you must create a configuration file that will determine which components to include in your new kernel.
If you are running the X Window System, the recommended method is to use the command make xconfig. Components are listed in different levels of menus and are selected using a mouse. You can select Y (yes), N (no), or M (module). After choosing your components, click the Save and Exit button to create the configuration file /usr/src/linux-2.4/.config and exit the Linux Kernel Configuration program.
Other available methods for kernel configuration are listed below:
- make config — An interactive text program. Components are presented in a linear format and you answer them one at a time. This method does not require the X Window System and does not allow you to change your answers to previous questions.
- make menuconfig — A text-mode, menu driven program. Components are presented in a menu of categories; you select the desired components in the same manner used in the text-mode Red Hat Linux installation program. Toggle the tag corresponding to the item you want included: [*] (built-in), [ ] (exclude), <M> (module), or < > (module capable). This method does not require the X Window System.
- make oldconfig — This is a non-interactive script that will set up your configuration file to contain the default settings. If you're using the default Red Hat kernel, it will create a configuration file for the kernel that shipped with Red Hat Linux for your architecture. This is useful for setting up your kernel to known working defaults and then turning off features that you don't want.
Note

To use kmod (see Chapter 24 for details) and kernel modules you must answer Yes to kmod support and module version (CONFIG_MODVERSIONS) support during the configuration.
After creating a /usr/src/linux-2.4/.config file, use the command make dep to set up all the dependencies correctly.
Use the command make clean to prepare the source tree for the build.
The next step in making a modularized kernel is to edit /usr/src/linux-2.4/Makefile so that you do not overwrite your existing kernel. The method described here is the easiest to recover from in the event of a mishap. If you are interested in other possibilities, details can be found at http://www.redhat.com/mirrors/LDP/HOWTO/Kernel-HOWTO.html or in the Makefile in /usr/src/linux-2.4 on your Linux system.
Edit /usr/src/linux-2.4/Makefile and modify the line beginning with EXTRAVERSION = to match a "unique" name by appending the date to the end of the string. For example, if you are compiling kernel version 2.4.7-3 you can append the flag to look similar to EXTRAVERSION = -0.1.21-jul2001). This will allow you to have the old working kernel and the new kernel, version 2.4.7-3-jul2001, on your system at the same time.
Build the kernel with make bzImage.
Build any modules you configured with make modules.
Install the kernel modules (even if you didn't build any) with make modules_install. Make sure that you type the underscore (_). This will install the kernel modules into the directory path /lib/modules/KERNELVERSION/kernel/drivers, where KERNELVERSION is the version specified in the Makefile. Our example would be /lib/modules/2.4.7-3-jul2001/kernel/drivers/.
If you have a SCSI adapter and you made your SCSI driver modular, build a new initrd image (see the section called Making an initrd Image; note that there are few practical reasons to make the SCSI driver modular in a custom kernel). Unless you have a specific reason to create an initrd image, do not create one and do not add it to lilo.conf.
Use make install to copy your new kernel and its associated files to the proper directories.
The kernel is built and installed now. The next step is configuring the boot loader to boot the new kernel. Refer to the section called Configuring the Boot Loader for more information.