8.3. Linux-2.6.8.1

Building the kernel involves a few steps—configuration, compilation, and installation. Read the README file in the kernel source tree for alternate methods to the way this book configures the kernel.

Prepare for compilation by running the following command:

make mrproper

This ensures that the kernel tree is absolutely clean. The kernel team recommends that this command be issued prior to each kernel compilation. Do not rely on the source tree being clean after un-tarring.

Also, ensure that the kernel does not attempt to pass hotplugging events to userspace until userspace specifies that it is ready:

sed -i 's@/sbin/hotplug@/bin/true@' kernel/kmod.c

If, in Section 7.6, “Configuring the Linux Console,” it was decided to compile the keymap into the kernel, issue the command below:

loadkeys -m /usr/share/kbd/keymaps/[path to  keymap] > \
    drivers/char/defkeymap.c

For example, if using a Dutch keyboard, use /usr/share/kbd/keymaps/i386/qwerty/nl.map.gz.

Configure the kernel via a menu-driven interface:

make menuconfig

Alternatively, make oldconfig may be more appropriate in some situations. See the README file for more information.

If desired, skip kernel configuration by copying the kernel config file, .config, from the host system (assuming it is available) to the unpacked linux-2.6.8.1 directory. However, we do not recommend this option. It is often better to explore all the configuration menus and create the kernel configuration from scratch.

For POSIX-shared memory support, ensure that the kernel config option “Virtual memory file system support” is enabled. It resides within the “File systems” menu and is normally enabled by default.

LFS bootscripts make the assumption that either both “Support for Host-side USB” and “USB device filesystem” have been compiled directly into the kernel, or that neither is compiled at all. Bootscripts will not work properly if it is a module (usbcore.ko).

Compile the kernel image and modules:

make

If using kernel modules, an /etc/modprobe.conf file may be needed. Information pertaining to modules and kernel configuration is located in the kernel documentation in the linux-2.6.8.1/Documentation directory. The modprobe.conf man page may also be of interest.

Be very careful when reading other documentation because it usually applies to 2.4.x kernels only. As far as we know, kernel configuration issues specific to Hotplug and Udev are not documented. The problem is that Udev will create a device node only if Hotplug or a user-written script inserts the corresponding module into the kernel, and not all modules are detectable by Hotplug. Note that statements like the one below in the /etc/modprobe.conf file do not work with Udev:

alias char-major-XXX some-module

Because of the complications with Hotplug, Udev, and modules, we strongly recommend starting with a completely non-modular kernel configuration, especially if this is the first time using Udev.

Install the modules, if the kernel configuration uses them:

make modules_install

If there are many modules and very little space, consider stripping and compressing the modules. For most users, such compression is not worth the time, but if the system is pressed for space, see http://www.linux-mips.org/archives/linux-mips/2002-04/msg00031.html.

After kernel compilation is complete, additional steps are required to complete the installation. Some files need to be copied to the /boot directory.

The path to the kernel image may vary depending on the platform being used. Issue the following command to install the kernel:

cp arch/i386/boot/bzImage /boot/lfskernel-2.6.8.1

System.map is a symbol file for the kernel. It maps the function entry points of every function in the kernel API, as well as the addresses of the kernel data structures for the running kernel. Issue the following command to install the map file:

cp System.map /boot/System.map-2.6.8.1

The kernel configuration file .config produced by the make menuconfig step above contains all the configuration selections for the kernel that was just compiled. It is a good idea to keep this file for future reference:

cp .config /boot/config-2.6.8.1

It is important to note that the files in the kernel source directory are not owned by root. Whenever a package is unpacked as user root (like we did inside chroot), the files have the user and group IDs of whatever they were on the packager's computer. This is usually not a problem for any other package to be installed because the source tree is removed after the installation. However, the Linux source tree is often retained for a long time. Because of this, there is a chance that whatever user ID the packager used will be assigned to somebody on the machine. That person would then have write access to the kernel source.

If the kernel source tree is going to retained, run chown -R 0:0 on the linux-2.6.8.1 directory to ensure all files are owned by user root.