Compiling (or recompiling) your Linux kernel

Before I get into the compilation of my kernel I'm going to make a boot floppy just to be on the safe side. To do this, slap a floppy in your drive, su to root, and run the command:

dd if=/boot/bzImage of=/dev/fd0 bs=8192

dd if=/boot/vmlinuz-2.2.16-22 of=/dev/fd0 bs=8192

Back to compiling the kernel

The first step is to locate the source code for your particular distribution. Typically, these files are located in /usr/source/linux. (This holds true for most Linux distributions).

Once you've located the source, you are ready to actually start the compilation process.

Make xconfig

Since I am going to take the easy route, let's stick with the simplest (and sanest) kernel configuration tool. The xconfig tool is a graphical tool offering simple buttons that, when clicked, present the many kernel configuration choices. The main xconfig window looks like the one shown in Figure A.

Figure A

Above, you see the main xconfig window.

Each time you click on a button within the xconfig window, you will be offered a list of choices. In Figure B, you can see where I've chosen the Character Devices button. Each choice will have three selections: yes (y), no (n), and modular (m). The first two choices are obvious: yes means add the module; no means do not add the module. The third choice isn't so obvious. When modularly adding a kernel module, you are, in effect, saying you want the modules available and will load or unload them as you see fit.

Go ahead and run through the other buttons to see what's there. It's quite a hefty assortment of modules (giving you an idea of how complete the Linux kernel really is). Once you've run the gamut, you can click Save And Exit and move on to the next step of the compilation!

Figure B

Above you see a small sample of possible options to choose while compiling your kernel. .

Make and its options

While in the /usr/src/linux directory, you are going to run the following commands (as root):

make mrproper
make dep
make clean
make bzImage
make modules
make modules_install

Each of the above commands performs a very different and very critical role in the compiling of the Linux kernel. Once you've run the above set of commands without error, your kernel is compiled!

Postcompilation process

Now, you are ready for the final steps of your kernel compilation. These steps are very important, and it's crucial that you follow them closely.

BzImage to boot image

First, open up a second console window and cd to the /boot directory. Once you're in this directory, run the ls command to see what's there. On my Red Hat 7.0 system, I see the following ls output:

What you're actually looking for is your current kernel image and the System.map file. In my case, the image is vmlinuz-2.2.16-22, and the System.map is a symlink to System.map-2.2.16-22.

Sticking with the above example, rename the file vmlinuz-2.2.16-22 to vmlinuz-2.2.16-22-old, and the System.map-2.2.16-22 to System.map-2.2.16-22-old. To do this, run these commands:

cd /boot
mv vmlinuz-2.2.16-22 vmlinuz-2.2.16-22-old
mv System.map-2.2.16-22 System.map-2.2.16-22-old

With these files renamed, you're ready to move the new files into /boot.

Now, change to the directory holding the new kernel image with:

cd /usr/src/linux/arch/i386/boot/bzImage /boot/

and then copy the bzImage file into /boot with the command:

cp bzImage /boot/vmlinuz-2.2.16-22

Then, it's time to do the same with the System.map file. Change to the /usr/src/linux directory and run the command:

cp System.map /boot/System.map-2.2.16-22

You are nearly done!

LILO, reboot and special circumstances

LILO

It's time to edit your /etc/lilo.conf file. In reality, you are going to simply add a new entry into this file to reflect the new kernel image. The section you want to edit looks like:

image=/boot/vmlinuz-2.2.16-22
label=linux
read-only
root=/dev/hde

and will finally (in our example) look like:

image=/boot/vmlinuz-2.2.16-22
label=test
read-only
root=/dev/hde
image=/boot/vmlinuz-2.2.16-22-old
label=linux
read-only
root=/dev/hde

Save this file and rerun lilo with the command:

/sbin/lilo

You should receive a return with something like:

Added linux *
Added test

and you're ready to rock and roll!

Reboot

With your boot floppy in hand (just in case), it's time to reboot. At the lilo: prompt, you can hit [Tab] and see both the linux and the test entry.

Special circumstances

Of course, not every situation is going to be the same. You may be compiling a new kernel. The biggest difference here will be the location of the source files. If you have downloaded a new kernel, and plan to compile it, you will want to move the source of your current kernel so as not to overwrite the source of the working kernel.

Before you unpack the new kernel, check to see if the /usr/src/linux directory is not merely a symbolic link to /usr/src/linux-2.*.* (where 2.*.* is the name of your current, running kernel). To check this, run the command:

ls -l /usr/src/linux

If you receive something like:

lrwxrwxrwx 1 root root 12 Nov 4 12:08 linux -> linux-2.2.16

you know it's a symbolic link. With this in mind, you will want to remove the symbolic link with the command:

rm /usr/src/linux

Now, you can unpack your new kernel, which will create a new directory. Let's say you are planning on compiling a 2.2.17 kernel. When you unpack the source, you will have a new /usr/src/linux directory. You'll want to rename that directory with the command: mv /usr/src/linux /usr/src/linux-2.2.17

and then create a symbolic link to /usr/src/linux with the command:

ln -s /usr/src/linux-2.2.17 /usr/src/linux

You can now move into the /usr/src/linux directory and start the compilation process!

Editorial disclaimer: The authors and editors have taken care in preparation of the content contained herein but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.

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