[Courses][Linux comands] customizing a 2.4 kernel

[Carla Schroder]

I've been in a kernel building frenzy, so this week's course will be a 
kernel-building howto. I'm on a mission, 'cause there's a lot of 
misinformation out there masquerading as howtos. Thanks to the gang on 
Techtalk for helping me over the bumpy parts.

You probably want to use kernel sources supplied by your distribution, 
especially on distros like Red Hat, SuSE, and Mandrake, that ship heavily 
modified kernels. Pristine kernels can be downloaded from kernel.org. Every 
distribution has its own particular kernel building tools, and you can also 
get nice pre-fab kernels to play with. As we are True Geeks, and scorn wussy 
pre-fab kernels, we will customize our kernels all to heck.

This lesson focuses on customizing your stock kernel, and removing all the 
unnecessary baggage. You can easily add things as well. Here's the steps:

Make hard copies of the outputs of dmesg, lscpi, /proc/cpuinfo, and lsusb. 
This contains all of your hardware information.

Download and unpack new kernel sources into a folder in your home directory, 
such as ~/src. CD to this directory.

Edit the new kernel makefile (~/src/linux-2.4.25/Makefile), giving a custom 
value to EXTRAVERSION, such as EXTRAVERSION = -new-kernel. This gives your 
new kernel a unique name (2.4.25-new-kernel), so you can easily identify it. 

Then, download and unpack your shiny new sources into your home directory. I 
like to use ~/src. No, do not use /usr/src/linux. That is bad and wrong. See 
the README in your toplevel sources directory. Also read 
Documentation/Changes, this tells gcc and other program versions you'll need. 
There are tons of docs in the source tree, read them.

CD to your top-level sources directory, for example ~/src/linux-2.4.25. 
Everything will be done from here.

$ make mrproper
$ make menuconfig
$ make dep
$ make bzImage
$ make modules
$ su
# make modules_install
# cp ~/src/arch/i386/boot/bzImage  /boot/bzImage-2.4.25-new-kernel
# cp ~/src/System.map /boot/System.map-2.4.25-new-kernel
# ln -s /boot/System.map-2.4.25-new-kernel /boot/System.map
$ make clean

README says to use gcc 2.95.3. You can have multiple versions of gcc 
installed. Select the one you want to use with

$ make bzImage CC=gcc-2.95.3

'make mrproper' cleans the build tree, removing configuration files, 
dependency information, and object files. It is good to do this even with 
freshly downloaded sources. Mr. Proper, according to popular lore, is the 
European version of Mr. Clean, for those occasions when you need to make 
something cleaner than clean. In this case, mrproper cleans the build tree 
more thoroughly than make clean, which removes object files, but does not 
touch configuration or dependency files.

'make menuconfig' is the most time-consuming, and most important part. 
menuconfig contains abundant help for every item, read the help. You can 
eliminate a lot of useless driver modules, ISDN support, PCMCIA/Cardbus, 
which are unnecessary on a desktop machine, and fine-tune what you want 
compiled statically, and what you want as loadable modules. 

'make bzImage' compiles the new kernel. This can take up to an hour, depending 
on the speed of your PC, and how complex your new kernel is.

'make modules' compiles all of the necessary modules.

'make modules_install' is the first operation that requires superuser 
privileges. This installs your new modules in /lib/modules/2.4.25.

'cp ~/src/arch/i386/boot/bzImage  /boot/bzImage-2.4.25-new-kernel' copies your 
nice new kernel image to the /boot directory, and renames it. It is 
important, when installing multiple kernels, to make sure each one has a 
unique name. And to use the exact kernel name in your bootloader.

'cp ~/src/System.map System.map-2.4.25-new-kernel' copies the new System.map 
to /boot.

'ln -s /boot/System.map-2.4.25-new-kernel /boot/System.map' creates a soft 
link. It may be necessary to use ln -fs, to force overwriting of the existing 
link. Be very careful of your typing here! A typo may render your system 
unbootable.

'make clean' tidies up the build tree, sweeping away around 10 megabytes of 
binary object files you don't need anymore.

Edit your bootloaders so they know about the new kernel, using the appropriate 
values for your system:

GRUB sample config:
title     Kernel 2.4.25, new kernel
root      (hd0,5)
kernel      /boot/bzImage-2.4.25-newkernel root=/dev/hda6 ro
savedefault
boot

LILO looks like this:

image=/boot/bzImage-2.4.25-newkernel
    label=Kernel 2.4.25 new kernel
    root=/dev/hda6
    read-only

And remember to re-run LILO:

$ /sbin/lilo 

-- 
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Carla Schroder
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