Kernel building 101 - By Randy Loux.

This is a BASIC view of rebuilding the kernel. It is by no means the gospel. Please read the appropriate documentation before attempting to modify your system parameters. I take no responsibility whatsoever for your kernel. If it breaks in half, you get to keep both halves.

Two ways to rebuild your kernel, install a whole new kernel. (about 6MB download), or patch up. I prefer to install a new one, and thus won't cover patching up here.

What does the kernel do?

Where can I get one?

Your kernel is located in /usr/src directory, and should be called 'linux'. Most times the kernel is named with the extension of it's version, 'linux-2.0.30' for example.

Whole kernels and patches are available from many ftp sites around the world. The best bet is to find one that is up to date, and is close to you, but gets little traffic. Populare sites are:

• sunsite.unc.edu
• tsx-11.mit.edu
• ftp.cdrom.com
• ftp.kernel.org

Why does the kernel need rebuilding? Why would you want to do it?

• ... add support for new hardware
• ... remove support for an obsolete piece of hardware, or software.

What are the basics? What do I do?

1. Research
   • RTFM! READ THE (expletive) MANUAL! (http://sunsite.unc.edu/pub/Linux/docs/howto/Kernel-HOWTO)
   • Read the FAQ's
   • FULLY research your problem. See if you know someone with the same needs, and can offer some advice.

2. Make a boot disk.
   • If you start playing with your kernel, and you screw it up, you're severely inconvenienced if you don't have a boot disk.
   • Put a blank floppy in the disk drive and su to root.
   • cd /usr/src/linux/arch/i386/boot
   • cat zImage > /dev/fd0
   • Take the disk out, label it, and put it in a safe place.

I got my boot disk, now what?

• Download the new kernel and put it in /usr/src directory.
• Untar the file with: tar xvfz linux-2.0.X
• Build the config file (in one of the following three ways)

  Method ONE ...

  This is the text way. It's the way I learned. You cannot stop if you make a mistake, and you cannot back up. Does not recognize dependancies. You can make mistakes, and it'll let you.

  • Switch users to root. (su - root)
  • Type: cd /usr/src/linux
  • Type: make config

  Mothed TWO ...

  This is similar to the original installation procedure, so newbies will find it familiar looking. You can back up if you make a mistake, it remembers dependancies, so if you forget something, it'll help you out.

  • Switch users to root. (su - root)
  • Type: cd /usr/src/linux
  • Type: make menuconfig

  Method THREE ...

  This option is for users who are easily distracted by shiny objects. :-) Has very pretty buttons, and looks really official. Remembers dependancies, and lets you go back if you make a mistake. It runs under X, so it can be slow on older systems. It requires some special libraries, so it may not work on all systems.

  • log in and start X.
  • Switch users to root. Type: su - root
  • Set your display variable. Type: DISPLAY=:0.0; export DISPLAY
  • Type: cd /usr/src/linux
  • Type: make xconfig

The questions. Oh my GOD, the questions!

• At my last count, there were well over 100 parameters to be set as Y, N, or M to build the Linux kernel. I cannot cover them all here.
• If you're smart enough to run Linux, you're smart enough to press the <HELP> option when you don't know how you should answer a particular question.
• Most of the questions come with a general explanation of the option, and a suggestion on how you should answer.
• Once you have a good config file made, copy it to a safe place so you can see how you answered the next time you need to rebuild.
• Check the Kernel-HOWTO on sunsite for help with some of the more common problems.

Config file is made, now what?

• Check your /etc/lilo.conf file. Make sure the line "image = /boot/vmlinuz" says 'vmlinuz' and not vmlinuz-2.0.30, or whatever kernel you're currently on. If it does, change it.
• Check /boot to make sure 'vmlinuz' is a symlink to vmlinuz-2.0.30, and System.map is a symlink to System.map-2.0.30.
• Once you're sure of this, rm the symlinks. This'll make sure you have everything set up correctly for the new kernel once it's done.

Consider modules

Monolithic

Generally for systems that do one specific task, a dedicated web server, or ftp server. Systems that do one task, all the time.

Modular

Pretty much everyone else. Production boxes, personal boxes, machines that require flexibility to function under varying conditions. Modular is faster (albeit arguably), smaller, and more flexible. It allows you to use programs or options that are basically incompatible. PPA and LP for example. PPA is a module that allows you to use your parallel port for misc. devices such as parallel scanners, or zip drives. LP is the module that allows you to use your parallel port for printers. They can't both be used at the same time, so you can take one out, and put the other in when you need to use it. (This is now a bad example since a patch has been made to solve this, but you get the point.)

Time to compile

• Log in as root.
• Type: cd /usr/src/linux
• Type: make dep
  • This makes sure that all the dependancies are where they're supposed to be.
• Type: make clean
  • This removes old object files and other errata that old versions of the kernel may leave behind.
• Type: make zlilo
  • This builds the new kernel, and places the compressed kernel image 'vmlinuz' in the / (root) directory, and installs the Lilo boot loader to the master boot record.
  • ** It is worth note that there are other options to 'make zlilo'. You can also use one of the following options, each of which have their own advantages.

    Type: make zImage

    This option makes the compressed kernel image and puts it in the directory: /usr/src/linux/arch/i386/boot. Use this option as one way to test your new kernel before actually installing it. Once it's done, you can cat it to a floppy, and boot from that floppy, or edit /etc/lilo.conf and change the boot image option to: 'image = /usr/src/linux/arch/i386/boot/vmlinuz'. If you do the latter, you'll need to re-run lilo before you reboot the system. (lilo -b /dev/hd?, where ? is the letter of your primary drive.)

    Type: make zdisk

    This makes the compressed kernel image, and puts it on a floppy. (Make sure you have the floppy in the drive when you start!) This way, you can boot on the new kernel from floppy before actually installing it.

What if something goes wrong?

There are many things that can go wrong with a kernel compile. But fear not! As long as you have followed these instructions (making a boot disk, and keeping your old kernel code, etc.), you won't harm your system, or your data in any way.

A good deal of the time, problems arise from having set a config parameter incorrectly, requiring that you start over with building the config file.

The best thing to do when a problem arises, is research the issue.

• Check the Kernel-HOWTO, make sure you followed it correctly.
• Read the FAQ's.
• Read the documentation included with the kernel you downloaded.
• Read the newsgroups. (comp.os.linux.installation, etc.)
• Ask someone! Ask your fellow CLUG members!

The kernel compiled successfully!

• Type: make modules
• Type: make modules install
• That will compile the modules, and leave them in /usr/src/linux/modules, where they can be accessed to insert and remove from the kernel at your convienience.
  • (for more information on modules, see the module-howto)
• If you did 'make zlilo' (which I'll assume you did), the kernel will be left in the / dir.
• Type: cd /boot
• If you haven't already removed the symlinks to your current kernel & map, do it now.
• Type: rm vmlinuz ; rm System.map
• Type: mv /vmlinuz /boot/vmlinuz-2.0.33 (for example)
• Type: mv /System.map /boot/System.map-2.0.33
• Type: ln -s /boot/System.map-2.0.33 /boot/System.map
• Type: ln -s /boot/vmlinuz /boot/vmlinuz-2.0.33
• Type: lilo -b /dev/hd? (where ? is the letter of your primary hard disk).
• The new kernel is now in place, restart your computer, and watch for errors when it starts coming back up. If you miss something, run 'dmesg|less' after you log in to see the boot messages.
• Type: sync ; sync ; shutdown -r now

Testing!

• Type: uname -a
  • This should return information about your system, and what kernel you're running, make sure it's the kernel version you just installed.
• Test your new kernel for no less than one week before removing your old one.
• Make sure you test all your favorite programs, and even those that you don't use frequently

A point of note. For Experienced users only!

After you successfully make a kernel, you can copy the .config file out to a safe place and use it the next time you need to recompile this kernel. (/root is generally a safe directory.)

All you'll need to do in the future, is edit the .config file with your editor of choice and change the options you need. (Once you open the file, it's pretty much self explanatory)

Once you're done editing the file to taste, put it in /usr/src/linux, and type: make oldconfig. That'll run through the config file, set up the dependancy information, and allow you to start off at 'make dep ; make clean'. This is a big timesaver when you only needto make minor changes.

Congratulations!

Randy Loux.
ra@choice.net
http://ww4.choice.net/~ra