10. Debian and the kernel

10.1 What tools does Debian provide to build custom kernels?

Users who wish to (or must) build a custom kernel are encouraged to download the package kernel-package_VVV_all.deb (it is stored in section misc at the Debian FTP archives). This package contains the script to build the kernel package, and provides the capability to create a Debian kernel-image package just by running the command make-kpkg kernel_image in the top-level kernel source directory. Help is available by executing the command make-kpkg --help, and through the manual page for make-kpkg(8).

Users must separately download the source code for the most recent kernel (or the kernel of their choice) from their favorite Linux archive site.

To build a custom kernel, users must have these packages installed: gcc, libc5-dev, bin86, binutils, gawk, make, gzip, and grep.

Executing the command dpkg --install kernel-package_VVV_all.deb sets up the directory /usr/src/linux-VVV/, and sets up the link /usr/src/linux to point to the directory /usr/src/linux-VVV/ containing the kernel sources.

Detailed instructions for using the package are given in the file /usr/doc/kernel-package/README. Briefly, one should:

• Unpack the kernel sources, and cd to the newly created directory.
• Modify the kernel configuration using one of these commands:
  • make config (for a tty one-line-at-a-time-interface).
  • make menuconfig (for an ncurses-based menu driven interface). Note that to use this option, the ncurses3.0-dev package must be installed.
  • make xconfig (for an X11 interface). Using this option requires that relevant X packages be installed.
  Any of the above steps generates a new .config in the top-level kernel source directory.
• Execute the command: make-kpkg -r Custom.N kernel_image, where N is a revision number assigned by the user. The new Debian archive thus formed would have revision Custom.1, e.g., kernel-image-2.0.27-Custom.1.deb for the Linux kernel 2.0.27.

• Install the package created.
  • Run dpkg --install /usr/src/kernel-image_VVV-Custom.N.deb to install the kernel itself. The installation script will:
    • run the boot loader, LILO (if it is installed),
    • install the custom kernel in /boot/vmlinuz_VVV-Custom.N, and set up appropriate symbolic links to the most recent kernel version.
    • prompt the user to make a boot floppy. This boot floppy will contain the raw kernel only. See additional notes for making a custom boot floppy.
  • To employ a secondary boot loaders (e.g., loadlin), copy this image to other locations (e.g., an MS-DOS partition).

10.2 How can I make a custom boot floppy?

This task is greatly aided by the Debian package boot-floppies_VVV-RRR.deb, normally found in the binary/devel section of the Debian FTP archive. Shell scripts in this package produce boot floppies in the SYSLINUX format. These are MS-DOS formatted floppies whose master boot records have been altered so that they boot Linux directly (or whatever other operating system has been defined in the syslinux.cfg file on the floppy). Other shell scripts in this package produce emergency root disks and can even reproduce the base disks.

When the boot-floppies package is installed, it produces files which reside in /usr/src/boot-floppies/, including the three scripts, called bootdisk.sh, rootdisk.sh, and basedisk.sh. Users of these scripts are encouraged to read them and customize as appropriate. In particular, one should include the kernel of your choice when invoking the bootdisk.sh script. Be aware that this kernel cannot be used to load the root disk into a RAMDISK unless it was compiled with CONFIG_RAMDISK=y, and CONFIG_INITRD=y.

To write the kernel-image-2.0.27 package to the 1.44 MByte disk in drive A: run this command:

./bootdisk.sh /usr/src/kernel-image-2.0.27_1.00.deb /dev/fd0 1440

Be sure to include the editor and whatever other tools you prefer when building your custom root disk with ./rootdisk.sh.

Some people found that the boot and root disks make good emergency floppies.

10.3 What special provisions does Debian provide to deal with modules?

Debian's modconf package provides a shell script (/usr/sbin/modconf) which can be used to customize the configuration of modules. This script presents a menu-based interface, prompting the user for particulars on the loadable device drivers in his system. The responses are used to customize the file /etc/conf.modules (which lists aliases, and other arguments that must be used in conjunction with various modules), and /etc/modules (which lists the modules that must be loaded at boot time).

Like the (new) Configure.help files that are now available to support the construction of custom kernels, the modconf package comes with a series of help files (in /usr/lib/modules_help/) which provide detailed information on appropriate arguments for each of the modules.

10.4 Can I safely de-install an old kernel, and if so how?

Yes. The kernel-image-NNN.prerm script checks to see whether the kernel you are currently running is the same as the kernel you are trying to de-install. Therefore you can remove unwanted kernel image packages using this command:

dpkg --purge --force-remove-essential kernel-image-NNN