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<b>Linux</b></font><p>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<UL>
<LI><A HREF="#Heading1">- 56 -</A>
<UL>
<LI><A HREF="#Heading2">Working with the Kernel</A>
<UL>
<LI><A HREF="#Heading3">WARNING</A>
<LI><A HREF="#Heading4">Upgrading and Installing New Kernel Software</A>
<LI><A HREF="#Heading5">CAUTION</A>
<LI><A HREF="#Heading6">Compiling the Kernel from Source Code</A>
<LI><A HREF="#Heading7">Adding Drivers to the Kernel</A>
<LI><A HREF="#Heading8">Upgrading Libraries</A>
<LI><A HREF="#Heading9">An Introduction to the Linux Source Tree</A>
<LI><A HREF="#Heading10">Making Loadable Modules for Linux</A>
<LI><A HREF="#Heading11">The rc.modules File</A>
<LI><A HREF="#Heading12">Summary</A>
</UL>
</UL>
</UL>
<P>
<HR SIZE="4">
<H2 ALIGN="CENTER"><A NAME="Heading1<FONT COLOR="#000077">- 56 -</FONT></H2>
<H2 ALIGN="CENTER"><A NAME="Heading2<FONT COLOR="#000077">Working with the
Kernel</FONT></H2>
<P><I>by Kamran Husain and Tim Parker</I></P>
<P>IN THIS CHAPTER</P>
<UL>
<LI>Upgrading and Installing New Kernel Software
<P>
<LI>Compiling the Kernel from Source Code
<P>
<LI>Adding Drivers to the Kernel
<P>
<LI>Upgrading Libraries
<P>
<LI>An Introduction to the Linux Source Tree
<P>
<LI>Making Loadable Modules for Linux
<P>
<LI>The rc.modules File
</UL>
<P>This chapter will look at
<UL>
<LI>How to install and recompile the kernel of Linux
<P>
<LI>How to add new software to the kernel
<P>
<LI>How to install new versions of the shared libraries
<P>
<LI>The steps necessary to create a new Linux kernel
<P>
<LI>How to use the kernel source files
<P>
<LI>Using dynamic modules
<P>
<LI>An introduction to the ELF format
</UL>
<P>Usually, you will want to leave the kernel alone, except when you are performing
a major upgrade or installing a new device driver that has special kernel modifications.
The details of the process are usually supplied with the software. However, this
chapter gives you a good idea of the general process.</P>
<P>Few people will want to change the details in the kernel source code, because
they lack the knowledge to do so (or have enough knowledge to know that hacking the
kernel can severely damage the system). However, most users will want to install
new versions of Linux, add patches, or modify the kernel's behavior a little.
<DL>
<DT></DT>
</DL>
<DL>
<DD>
<HR>
<A NAME="Heading3<FONT COLOR="#000077"><B>WARNING: </B></FONT>Don't modify
the kernel unless you know what you are doing. If you damage the source code, your
kernel may be unusable, and in the worst cases, your file system may be affected.
Take care and follow instructions carefully. You need to know several things about
kernel manipulation, and we can only look at the basics in this chapter.
<HR>
</DL>
<P>Several versions of Linux are commonly used, with a few inconsistencies between
them. For that reason, the exact instructions given here may not work with your version
of Linux. However, the general approach is the same, and only the directory or utility
names may be different. Most versions of Linux supply documentation that lists the
recompilation process and the locations of the source code and compiled programs.</P>
<P>Before you do anything with the kernel or utilities, make sure you have a good
set of emergency boot disks, and preferably a complete backup on tape or diskette.
Although the process of modifying the kernel is not difficult, every now and then
it does cause problems that can leave you stranded without a working system. Boot
disks are the best way to recover, so make at least one extra set.
<H3 ALIGN="CENTER"><A NAME="Heading4<FONT COLOR="#000077">Upgrading and Installing
New Kernel Software</FONT></H3>
<P>Linux is a dynamic operating system. New releases of the kernel, or parts of the
operating system that can be linked into the kernel, are made available at regular
intervals to users. Whether or not you want to upgrade to the new releases is up
to you and usually depends on the features or bug fixes that the new release offers.
You will probably have to recompile and relink the kernel when new software is added,
unless it is loaded as a utility or device driver.
<DL>
<DT></DT>
</DL>
<DL>
<DD>
<HR>
<A NAME="Heading5<FONT COLOR="#000077"><B>CAUTION:</B> </FONT>You should avoid
upgrading your system with every new release for a couple of reasons. The most common
problem with constant upgrades is that you may be stuck with a new software package
that causes backward compatibility problems with your existing system or that has
a major problem with it that was not patched before the new software was released.
This can cause you no end of trouble. Most new software releases wipe out existing
configuration information, so you have to reconfigure the packages that are being
installed from scratch.
<HR>
</DL>
<P>Another problem with constant upgrades is that the frequency with which new releases
are made available is so high that you can probably spend more time simply loading
and recompiling kernels and utilities than actually using the system. This becomes
tiresome after a while. Because most major releases of the Linux operating system
are available, the number of changes to the system is usually quite small. Therefore,
you should read the release notes carefully to ensure that the release is worth the
installation time and trouble.</P>
<P>The best advice is to upgrade only once or twice a year, and only when there is
a new feature or enhancement to your system that will make a significant difference
in the way you use Linux. It's tempting to always have the latest and newest version
of the operating system, but there is a lot to be said for having a stable, functioning
operating system, too.</P>
<P>If you do upgrade to a new release, bear in mind that you don't have to upgrade
everything. The last few Linux releases have changed only about 5 percent of the
operating system with each new major package upgrade. Instead of replacing the entire
system, just install those parts that will have a definite effect, such as the kernel,
compilers and their libraries, and frequently used utilities. This saves time and
reconfiguration.
<H3 ALIGN="CENTER"><A NAME="Heading6<FONT COLOR="#000077">Compiling the Kernel
from Source Code</FONT></H3>
<P>Upgrading, replacing, or adding new code to the kernel is usually a simple process:
You obtain the source for the kernel, make any configuration changes, compile it,
and then place it in the proper location on the file system to run the system properly.
The process is often automated for you by a shell script or installation program,
and some upgrades are completely automated--you don't need to do anything except
start the upgrade utility.</P>
<P>Kernel sources for new releases of Linux are available from CD-ROM distributions,
FTP sites (see Appendix A, "FTP Sites and Newsgroups"), user groups, and
many other locations. Most kernel versions are numbered with a version and a patch
level, so you will see kernel names such as 1.12.123, where 1 is the major release,
12 is the minor version release, and 123 is the patch number. Most sites of kernel
source code maintain several versions simultaneously, so check through the source
directories for the latest version of the kernel.</P>
<P>Patch releases are sometimes numbered differently and do not require the entire
source of the kernel to install. They require just the source of the patch. In most
cases, the patch overlays a section of existing source code and a simple recompilation
is all that's necessary to install the patch. Patches are released quite frequently.</P>
<P>Most kernel source programs are maintained as a gzipped <TT>tar</TT> file. Unpack
the files into a subdirectory called <TT>/usr/src</TT>, which is where most of the
source code is kept for Linux. Some versions of Linux keep other directories for
the kernel source, so you may want to check any documentation supplied with the system
or look for a <TT>README</TT> file in the <TT>/usr/src</TT> directory for more instructions.</P>
<P>Often, unpacking the gzipped <TT>tar</TT> file in <TT>/usr/src</TT> creates a
subdirectory called <TT>/usr/src/linux</TT>, which can overwrite your last version
of the kernel source. Before starting the unpacking process, rename or copy any existing
<TT>/usr/src/linux</TT> (or whatever name is used with the new kernel) so you have
a backup version in case of problems.</P>
<P>After the kernel source has been unpacked, you need to create two symbolic links
to the <TT>/usr/include</TT> directory, if they are not created already or set by
the installation procedure. Usually, the link commands required are<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">ln -sf /usr/src/linux/include/linux /usr/include/linux
ln -sf /usr/src/linux/include/asm /usr/include/asm
</FONT></PRE>
<P>If the directory names shown are different from your version of Linux, substitute
the new directory names for <TT>/usr/src/linux</TT>. Without these links, the upgrade
or installation of a new kernel cannot proceed.</P>
<P>After the source code has been ungzipped and untarred and the links have been
established, the compilation process can begin. You must have a version of <TT>gcc</TT>
or <TT>g++</TT> (the GNU C and C++ compilers) or some other compatible compiler available
for the compilation. You may have to check with the source code documentation to
make sure you have the correct versions of the compilers, because occasionally new
kernel features are added that are not supported by older versions of <TT>gcc</TT>
or <TT>g++</TT>.</P>
<P>Check the file <TT>/usr/src/linux/Makefile</TT> (or whatever path the <TT>Makefile</TT>
is in with your source distribution). There will be a line in the file that defines
the <TT>ROOT_DEV</TT>, which is the device that is used as the <TT>root</TT> file
system when Linux boots. Usually the line looks like this:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">ROOT_DEV = CURRENT
</FONT></PRE>
<P>If you have any other value, make sure it is correct for your file system configuration.
If the <TT>Makefile</TT> has no value, set it as shown in the preceding code line.</P>
<P>The compilation process begins when you change to the <TT>/usr/src/linux</TT>
directory and issue the following command:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">make config
</FONT></PRE>
<P>This command invokes the <TT>make</TT> utility for the C compiler. The process
may be slightly different for some versions of Linux, so you should check with any
release or installation notes supplied with the source code.</P>
<P>The <TT>config</TT> program issues a series of questions and prompts that you
need to answer to indicate any configuration issues that need to be completed before
the actual compilation begins. These may be about the type of disk drive you are
using, the CPU, any partitions, or other devices, such as CD-ROMs. Answer the questions
as well as you can. If you are unsure, choose the default values or the choice that
makes the most sense. The worst case is that you might have to redo the process if
the system doesn't run properly. (You do have an emergency boot disk ready, don't
you?)</P>
<P>Next, you have to set all the source dependencies. This is a step that is commonly
skipped, and it can cause several problems if it is not performed for each software
release. Issue the following command:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">make dep
</FONT></PRE>
<P>If the software you are installing does not have a <TT>dep</TT> file, check with
the release or installation notes to ensure that the dependencies are correctly handled
by the other steps.</P>
<P>After that, you can finally compile the new kernel. The command to start the process
is<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">make zImage
make zdisk
make zlilo
</FONT></PRE>
<P>This compiles the source code and leaves the new kernel image file in the current
directory (usually <TT>/usr/src/linux</TT>). If you want to create a compressed kernel
image, you can use the command<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">make zImage
</FONT></PRE>
<P>Not all releases or upgrades to the kernel support compressed image compilation.</P>
<P>The last step in the process is to copy the new kernel image file to the boot
device or a boot floppy. Use the following command to place the file on a floppy:<FONT
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