Configuring and compiling the kernel

Purpose of this lab

The purpose of this lab is to enable an experimental feature of the Linux 2.4 kernel: the Device File System (devfs). This file system simplifies device driver development significantly. Because the devfs is built-in in the kernel core (i.e. it is not available as a separate program or device driver), it is necessary to reconfigure and recompile the kernel. In addition, we will need to download and install the devfs management daemon.

Before you start

The kernel is the heart of the operating system. Modifying the kernel or even changing its configuration may result in an unusable machine and in having to reinstall the whole system. Recompiling the kernel is also very time consuming. For these reasons you should double-check every step of the lab before modifying or recompiling the kernel.
IMPORTANT: always make backup copies of the files that you are going to modify. Put lots of comments in every file that you create or modify, indicating what you did and why.

Some of the tasks of the labs - such as installing a kernel - require special super-user privileges. You will be granted partial root privileges on the lab's machines. These privileges do not apply to the other computers on campus. As root, aka superuser, you have full control over the system, which means that you have the power to delete critical system files (or even the whole system!) and make the machine unusable. Once again, before you execute a command as root, think two times on what you are doing.

You can execute commands in super-user mode with the sudo (Super-User DO) command. This is how it works:

sudo <command>

where <command> is the command that you want to execute as super-user. For security reasons, sudo will ask for your password before running the command.

For example, try the following commands:

cat /etc/shadow

and

sudo cat /etc/shadow

What are the results? Can you exlpain what happens? Notice that shadow is a privileged file containing the users' login names and their respective encrypted passwords, along with other information.

The kernel source tree

The kernel source code is mostly written in C and assembly language. It is located under the

The Device File System Management Daemon

Configuring the kernel

Kernel configuration is possible via a user interface which allows you to select the features that you want include in the new kernel.

Change the current directory to the root of the linux source tree (/usr/src/linux-2.4/)

cd /usr/src/linux-2.4/

Then type the following:

make xconfig

After a few seconds, the following window should appear on the screen:

Each button corresponds to a different configuration domain of the kernel.

In our new kernel we want to include support for a special file system called Device Filesystem (DEVFS). Because this file system is still experimental (not fully tested), first we need to enable kernel support for experimental features. To do this, click on the Code maturity level options button.

Select "y" as shown above. Notice that each configuration option has a "Help" button that displays really helpful information about that option. Often "Help" will tell you which choice is better for your needs (another very important source of information is the Documentation directory under the Linux source tree). Click on the "Help" button and read carefully the information that it gives you. When you are done, click on "Main Menu" to go back to the main menu.

Now it's time to enable the Device Filesystem support. Click on the File Systems button. On the window that pops up, select "yes" for the /dev file system support (EXPERIMENTAL) option, the Automatically mount at boot option just below it, and "no" (default) for the Debug devfs option. This is illustrated by the picture below. Again, you are encouraged to read the "Help" of these three options. When you are done, click on "Main Menu".

Also, in the same configuration window, make sure that the Ext3 journaling file system support is enabled.

Finally, we want to enable one more special feature, which is located in the Kernel Hacking section. Select "yes" to Kernel Debugging and Magic SysRq key. Everything else should be "no", as shown below.

Congratulations! You are done configuring the kernel. The next step is to save the configuration and compile the kernel. To save the configuration click on the "Save and Exit" button on the main menu window. To compile the kernel, see the next section.

Compiling the kernel

Change your current directory to the linux source root and run make as following:

$ cd /usr/src/linux-2.4
$ make clean dep bzImage

clean will remove old compiled files, dep will determine dependencies between modules, and, finally, bzImage will create the kernel image. You will have to wait about 10-15 minutes for make to finish compiling the kernel. When make is done, the kernel image is found under

/usr/src/linux-2.4/arch/i386/boot/bzImage

Installing the kernel

In order to boot your computer with the new kernel, the kernel image needs to be copied to the /boot directory and renamed to vmlinuz-2.4.18-devfs-lab. This can be done in one step:

$ sudo cp bzImage /boot/vmlinuz-2.4.18-devfs-lab

Notice two things:

• We need to be root in order to copy the image to the /boot directory.
• By convention, we include "devfs" in the file name to indicate that the kernel has been built with DEVFS support, and "lab" to indicate that the kernel has been built during a lab session.

IMPORTANT: be careful not to overwrite other kernel images located under the /boot directory! Kernel images are named vmlinuz-*.

Finally, add the following lines to the end of /boot/grub/grub.conf:

title Linux 2.4.18 Lab
        root (hd0,0)
        kernel /vmlinuz-2.4.18-devfs-lab DEVFS=y

Try it !

Now it's time to try the new kernel. Reboot your computer and select "Linux 2.4.18 Lab" when prompted. You may see some error messages (FAILED) during the boot. For now we can ignore these error messages as long as the system does not crash.

Questions

1. What is the latest unstable version of the Linux kernel?
2. Suppose you have a Pentium 4 at home. How do you turn on support for that CPU in the kernel?
3. What is the "symmetric multi-processing support" (Processor Type and Features section)?

Resources