13.3.1. Installation of the kernel
Building the kernel involves a few steps—configuration,
compilation, and installation. Read the README
file in the kernel source tree for alternative 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.
Configure the kernel via a menu-driven interface.
Please note that the udev bootscript requires "rtc" and "tmpfs" to be
enabled and built into the kernel, not as modules. CBLFS has
some information regarding particular kernel configuration requirements of
packages outside of CLFS at http://cblfs.cross-lfs.org/:
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 root directory of the unpacked kernel
sources. However, we do not recommend this option. It is often better
to explore all the configuration menus and create the kernel configuration
from scratch.
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 Documentation
directory of the kernel
sources tree. Also, modprobe.conf(5)
may
be of interest.
Be very careful when reading other documentation relating to
kernel modules 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 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
After kernel compilation is complete, additional steps are
required to complete the installation. Some files need to be copied to
the /boot
directory.
Issue the following command to install the kernel:
cp -v vmlinux /boot/clfskernel-2.6.39
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 -v System.map /boot/System.map-2.6.39
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 -v .config /boot/config-2.6.39
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 do inside the final-system
build environment), 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-2.6.39
directory to
ensure all files are owned by user root
.
Warning
Some kernel documentation recommends creating a symlink from
/usr/src/linux
pointing to the
kernel source directory. This is specific to kernels prior to the
2.6 series and must not be created on a CLFS
system as it can cause problems for packages you may wish to build
once your base CLFS system is complete.
Also, the headers in the system's include
directory should
always be the ones against which Glibc was
compiled and should never be replaced by headers
from a different kernel version.