Recompiling your Linux Kernel

LinuxSA - July 1997 Meeting
Geoffrey D. Bennett
(geoffrey@linuxsa.org.au)

Introduction

• What is the job of a kernel?

• Why recompile your kernel?

• Kernel Versions

• Requirements

• Choosing to build modules

• Configuring the kernel

• Building the kernel

• Installing the kernel

• LILO Configuration

What is the job of a kernel?

• Controls execution of processes (starting, stopping, scheduling)

• Arranges memory management

• Provides a file system

• Provides for a portable interface for programs to communicate with hardware through device drivers

Why recompile your kernel?

• Want a newer version
  • New features
  • Bug fixes
  • Testing

• Need to apply patches for non-standard features

• Need support for more hardware

• Reduce size of kernel (perhaps use modules too)

• Learning experience :)

Kernel Versions

• Version numbers are in the format x.y.z
  • x indicates major version number
  • y indicates minor version number
  • z indicates patchlevel

• Examples:
  • 1.0.9
  • 1.1.7
  • 1.2.13
  • 1.3.36
  • 2.0.0
  • 2.1.23

• Two current development streams, indicated by the minor version number:
  • Even numbers: stable (1.2.13, 2.0.30)
  • Odd numbers: development (1.1.7, 2.1.23)

Requirements

• gcc 2.6.3 or newer (preferably 2.7.2 or newer)

• Kernel source unpacked in /usr/src/linux
  • From one of many FTP sites, including:
    • ftp.cc.monash.edu.au
      /pub/linux/kernel
    • sunsite.anu.edu.au
      /pub/linux/kernel
    • ftp.kernel.org
      /pub/linux/kernel
  • From Red Hat's kernel-source and kernel-headers packages

• Disk space:
  • ~6MB compressed kernel source
  • ~24MB when uncompressed
  • ~6MB extra to compile

Choosing to build modules

• Modules
  • Code (eg. device drivers) inserted into the kernel while it is running
  • Eg. floppy disk driver or printer port driver only needed when accessing those devices
  • Saves on memory (kernel code is not swappable)

• Nearly all device drivers can be built as modules

• Not usually a good idea to build boot-time devices such as a hard disk device driver as a module (however it can be done with initrd)

• Sound-card support is better built compiled-in, anything else that can be modularised should be

Configuring the kernel

• cd /usr/src/linux

• One of:
  • make config (text script)
  • make menuconfig (menu driven)
  • make xconfig (X menu driven)

• Choosing what to turn on or off:
  • Configuration programs include help
  • Kernel-HOWTO

Building the kernel

• make dep (build dependencies)

• make clean (clear out anything from previous builds)

• make boot
  • make zImage (alternative, is Intel-specific)
  • make bzImage (for big kernels)

• make modules (if any modules were selected)

Installing the kernel

• cp arch/i386/boot/zImage /boot/pick-a-name
  • The name is arbitrary, but commonly begins with zImage or vmlinuz
  • Also common to append version number/build number to keep track of multiple kernels (eg. vmlinuz-2.0.30-1)

• Installation of modules:
  • Move any old modules in /lib/modules/kernel-version out of the way, eg:
    cd /lib/modules
    mv 2.0.30 2.0.30-old
  • cd /usr/src/linux
    make modules_install

LILO Configuration

• Sample /etc/lilo.conf:

  boot=/dev/hda
  map=/boot/map
  install=/boot/boot.b
  prompt
  timeout=50
  other=/dev/hda1
          label=dos
          table=/dev/hda
  image=/boot/vmlinuz
          label=linux
          root=/dev/hda2
          read-only
  

• "image=" section can be repeated as required to boot different kernels

• You must rerun /sbin/lilo when any changes to this file or any kernels are made

References

• /usr/src/linux/README

• Red Hat User's Guide - "Building a Custom Kernel"

• Kernel-HOWTO

Questions