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This chapter is from the book

Building the Kernel

Building the kernel is easy. It is surprisingly easier than compiling and installing other system-level components, such as glibc. The 2.6 kernel series introduced a new configuration and build system, which made the job even easier and is a welcome improvement over earlier releases.

Configuring the Kernel

Because the Linux source code is available, it follows that you can configure and custom tailor it before compiling. Indeed, it is possible to compile support into your kernel for only the specific features and drivers you want. Configuring the kernel is a required step before building it. Because the kernel offers myriad features and supports a varied basket of hardware, there is a lot to configure. Kernel configuration is controlled by configuration options, which are prefixed by CONFIG in the form CONFIG_FEATURE. For example, symmetrical multiprocessing (SMP) is controlled by the configuration option CONFIG_SMP. If this option is set, SMP is enabled; if unset, SMP is disabled. The configure options are used both to decide which files to build and to manipulate code via preprocessor directives.

Configuration options that control the build process are either Booleans or tristates. A Boolean option is either yes or no. Kernel features, such as CONFIG_PREEMPT, are usually Booleans. A tristate option is one of yes, no, or module. The module setting represents a configuration option that is set but is to be compiled as a module (that is, a separate dynamically loadable object). In the case of tristates, a yes option explicitly means to compile the code into the main kernel image and not as a module. Drivers are usually represented by tristates.

Configuration options can also be strings or integers. These options do not control the build process but instead specify values that kernel source can access as a preprocessor macro. For example, a configuration option can specify the size of a statically allocated array.

Vendor kernels, such as those provided by Canonical for Ubuntu or Red Hat for Fedora, are precompiled as part of the distribution. Such kernels typically enable a good cross section of the needed kernel features and compile nearly all the drivers as modules. This provides for a great base kernel with support for a wide range of hardware as separate modules. For better or worse, as a kernel hacker, you need to compile your own kernels and learn what modules to include on your own.

Thankfully, the kernel provides multiple tools to facilitate configuration. The simplest tool is a text-based command-line utility:

$ make config

This utility goes through each option, one by one, and asks the user to interactively select yes, no, or (for tristates) module. Because this takes a long time, unless you are paid by the hour, you should use an ncurses-based graphical utility:

$ make menuconfig

Or a gtk+-based graphical utility:

$ make gconfig

These three utilities divide the various configuration options into categories, such as "Processor Type and Features." You can move through the categories, view the kernel options, and of course change their values.

This command creates a configuration based on the defaults for your architecture:

$ make defconfig

Although these defaults are somewhat arbitrary (on i386, they are rumored to be Linus's configuration!), they provide a good start if you have never configured the kernel. To get off and running quickly, run this command and then go back and ensure that configuration options for your hardware are enabled.

The configuration options are stored in the root of the kernel source tree in a file named .config. You may find it easier (as most of the kernel developers do) to just edit this file directly. It is quite easy to search for and change the value of the configuration options. After making changes to your configuration file, or when using an existing configuration file on a new kernel tree, you can validate and update the configuration:

$ make oldconfig

You should always run this before building a kernel.

The configuration option CONFIG_IKCONFIG_PROC places the complete kernel configuration file, compressed, at /proc/config.gz. This makes it easy to clone your current configuration when building a new kernel. If your current kernel has this option enabled, you can copy the configuration out of /proc and use it to build a new kernel:

$ zcat /proc/config.gz > .config
$ make oldconfig

After the kernel configuration is set—however you do it—you can build it with a single command:

$ make

Unlike kernels before 2.6, you no longer need to run make dep before building the kernel—the dependency tree is maintained automatically. You also do not need to specify a specific build type, such as bzImage, or build modules separately, as you did in old versions. The default Makefile rule will handle everything.

Minimizing Build Noise

A trick to minimize build noise, but still see warnings and errors, is to redirect the output from make:

$ make > ../detritus

If you need to see the build output, you can read the file. Because the warnings and errors are output to standard error, however, you normally do not need to. In fact, I just do

$ make > /dev/null

This redirects all the worthless output to that big, ominous sink of no return, /dev/null.

Spawning Multiple Build Jobs

The make program provides a feature to split the build process into a number of parallel jobs. Each of these jobs then runs separately and concurrently, significantly speeding up the build process on multiprocessing systems. It also improves processor utilization because the time to build a large source tree includes significant time in I/O wait (time in which the process is idle waiting for an I/O request to complete).

By default, make spawns only a single job because Makefiles all too often have incorrect dependency information. With incorrect dependencies, multiple jobs can step on each other's toes, resulting in errors in the build process. The kernel's Makefiles have correct dependency information, so spawning multiple jobs does not result in failures. To build the kernel with multiple make jobs, use

$ make -jn

Here, n is the number of jobs to spawn. Usual practice is to spawn one or two jobs per processor. For example, on a 16-core machine, you might do

$ make -j32 > /dev/null

Using utilities such as the excellent distcc or ccache can also dramatically improve kernel build time.

Installing the New Kernel

After the kernel is built, you need to install it. How it is installed is architecture- and boot loader-dependent—consult the directions for your boot loader on where to copy the kernel image and how to set it up to boot. Always keep a known-safe kernel or two around in case your new kernel has problems!

As an example, on an x86 system using grub, you would copy arch/i386/boot/bzImage to /boot, name it something like vmlinuz- version , and edit /boot/grub/grub.conf, adding a new entry for the new kernel. Systems using LILO to boot would instead edit /etc/lilo.conf and then rerun lilo.

Installing modules, thankfully, is automated and architecture-independent. As root, simply run

% make modules_install

This installs all the compiled modules to their correct home under /lib/modules.

The build process also creates the file System.map in the root of the kernel source tree. It contains a symbol lookup table, mapping kernel symbols to their start addresses. This is used during debugging to translate memory addresses to function and variable names.

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