WORKING WITH THE SUSE 2.6.x KERNEL SOURCES

Andreas Gruenbacher <agruen@suse.de>, SUSE Labs, 2003, 2004, 2005, 2006


This document gives an overview of how SUSE Linux kernels are
created, and describes tasks like building individual kernels
and creating external kernel modules.

A companion Update Media HOWTO that describes how to build driver update
disks (among other things) is available at:

    ftp://ftp.suse.com/pub/people/hvogel/Update-Media-HOWTO.


TABLE OF CONTENTS

  Overview
  Compiling your own kernel
  Building additional (external) modules
  Supported vs. unsupported modules
  Patch selection mechanism
  Where to find configuration files
  How to configure the kernel sources
  Module load paths


OVERVIEW

The kernels for SUSE are generated from the vanilla Linux kernel sources
found at http://ftp.kernel.org, on top of which a number of patches are
applied. The resulting kernel source tree is configured and built,
resulting in a binary kernel.

Internally, the add-on patches and configuration files are maintained in
a CVS repository. A script (scripts/tar-up.sh) packs up the files in the
CVS repository in a form suitable for rpmbuild. When building the RPM
packages, the following binary packages get created:

  *  kernel-source

     The kernel source tree, generated by unpacking the vanilla kernel
     sources and applying the patches. The kernel sources are used by
     a number of other packages. They can also be used for compiling
     additional kernel modules.

  *  kernel-$FLAVOR

     A number of binary kernels (for example, kernel-default for
     uniprocessor machines, kernel-smp for smp machines, etc.). These
     packages are all generated from the same kernel sources, and
     differ in the kernel configurations used.

  *  kernel-syms

     Kernel symbol version information for compiling external modules:
     Functions and data structures that the kernel exports have version
     information attached. When loading kernel modules, this version
     information is used to make sure that the modules match the running
     kernel.


The CVS repository contains the configuration files (.config) for all
SUSE kernel flavors. All configuration files are included in the
kernel-source package (see WHERE TO FIND CONFIGURATION FILES below).


In the installed system, the kernel-source package installs files in the
following directories:


  *  /usr/src/linux-$VERSION-$RELEASE/

     The kernel sources.
     
  *  /usr/src/linux

     A symbolic link to /usr/src/linux-$VERSION-$RELEASE.

  *  /usr/src/linux-$VERSION-$RELEASE-obj/$ARCH/$FLAVOR/

     Kernel build object files for one kernel flavor. These
     files are used for compiling additional kernel modules.

  *  /usr/src/linux-obj

     A symbolic link to /usr/src/linux-$VERSION-$RELEASE-obj/$ARCH/$FLAVOR.

  *  /usr/share/doc/packages/kernel-source/

     This document and an external kernel module example.

  *  /etc/init.d/running-kernel

     Init script that adapts the kernel sources in /usr/src/linux to
     the running kernel.


COMPILING YOUR OWN KERNEL

The kernel sources are found in the kernel-source.$ARCH.rpm package. The
recommended way to produce a binary kernel is:

  (1)  Install kernel-source.$ARCH.rpm. Change to the /usr/src/linux
       directory.

  (2)  Configure the kernel (for example, ``make oldconfig'' or ``make
       cloneconfig'', see HOW TO CONFIGURE THE KERNEL SOURCES).

  (3)  Build the kernel and all its modules (``make'').

  (5)  Install the kernel and the modules (``make modules_install'',
       followed by ``make install''). This will automatically create
       an initrd for the new kernel as well (see ``mkinitrd -h'').

  (6)  Make sure that /etc/modprobe.d/unsupported-modules contains

           allow_unsupported_modules 1

       otherwise modprobe will refuse to load any modules.

  (7)  Add the kernel to the boot manager. When using lilo, run ``lilo''
       to update the boot map.

Instead of building binary kernels by hand, you can also build
one of the kernel-$FLAVOR packages using RPM.


BUILDING ADDITIONAL (EXTERNAL) MODULES

A single binary kernel module generally only works for a specific
version of the kernel source tree, for a specific architecture and
configuration. This means that for each binary kernel that SUSE ships, a
custom module must be built. This requirement is to some extent relaxed
by the modversion mechanism: modversions attach a checksum to each
symbol (function or variable) exported to modules by the kernel. This
allows to use kernel modules that have been built for a kernel with a
different version or release number in many cases, as long as none of
the symbols the module uses have changed between the two kernel
versions.

When releasing maintenance or security update kernels for a specific
product, we carefully try to keep the kernel ABI stable.  Despite this,
we sometimes have no choice but to break binary compatibility. In this
case, those kernel modules must be rebuilt.

Additional kernel modules for one of the SUSE kernel flavors can be
built in three different ways:

  (1) by configuring the kernel sources in /usr/src/linux (or a copy,
      see HOW TO CONFIGURE THE KERNEL SOURCES), or

  (2) by using one of the standard configurations in
      /usr/src/linux-obj/$ARCH/$FLAVOR, or

  (3) by creating a Kernel Module Package (KMP) as described in the
      Kernel Module Packages Manual, http://www.suse.de/~agruen/KMPM/.


The first method involves the following steps:

  (1)  Install kernel-source.$ARCH.rpm.
  
  (2)  Change to the /usr/src/linux directory.  Configure the kernel
       (for example, ``make oldconfig'' or ``make cloneconfig'', see
       HOW TO CONFIGURE THE KERNEL SOURCES).

  (3)  Create files required for compiling external modules:
       ``make scripts'' and ``make prepare''.

  (4)  Compile the module(s) by changing into the module source directory
       and typing ``make -C /usr/src/linux M=$(pwd)''.

  (5)  Install the module(s) by typing
       ``make -C /usr/src/linux M=$(pwd) modules_install''.


The second method involves the following steps:

  (1)  Install kernel-source.$ARCH.rpm.

  (2)  Install kernel-syms.$ARCH.rpm. This package is necessary for
       symbol version information (CONFIG_MODVERSIONS).

  (3)  Compile the module(s) by changing into the module source directory
       and typing ``make -C /usr/src/linux-obj/$ARCH/$FLAVOR M=$(pwd)''.
       Substitute $ARCH and $FLAVOR with the architecture and flavor
       for which to build the module(s).

       If the installed kernel sources match the running kernel, you
       can build modules for the running kernel by using the path
       /lib/modules/$(uname -r)/build as the -C option in the above
       command. (build is a symlink to /usr/src/linux-obj/$ARCH/$FLAVOR).

       Starting with SuSE Linux 9.2 / SLES9 Service Pack 1, the
       modversion information for the running kernel is also
       contained in the kernel-$FLAVOR packages, and so for building
       modules for the running kernel, the kernel-syms package is no
       longer required.

  (4)  Install the module(s) with
       ``make -C /usr/src/linux-obj/$ARCH/$FLAVOR M=$(pwd) modules_install''.


Whenever building modules, please use the kernel build infrastructure as
much as possible, and do not try to circumvent it. The
Documentation/kbuild directory in the kernel sources documents kbuild
makefiles.

Please take a look at the demo module installed under
/usr/share/doc/packages/kernel-source for a simple example of an Kernel
Module Package (KMP).


SUPPORTED VS. UNSUPPORTED MODULES

As an extension to the mainline kernel, modules can be tagged as
supported (directly by SUSE, or indirectly by a third party) or
unsupported. Modules which are known to be flakey or for which SUSE does
not have the necessary expertise are marked as unsupported.  Modules for
which SUSE has third-party support agreements are marked as externally
supported. Modules for which SUSE provides direct support are marked as
supported.

The support status of a module can be queried with the modinfo tool.
Modinfo will report one of the following:

  - direct support by SUSE: "supported: yes"
  - third-party support: "supported: external"
  - unsupported modules: no supported tag.

At runtime, the setting of the" unsupported" kernel command line
parameter and /proc/sys/kernel/unsupported determines whether
unsupported modules can be loaded or not, and whether or not loading an
unsupported module causes a warning in the system log:

  0 = only allow supported modules,
  1 = warn when loading unsupported modules,
  2 = don't warn.

Irrespective of this setting, loading an externally supported or unsupported
module both set a kernel taint flag. The taint flags are included in
Oopses. The taint status of the kernel can be inspected in
/proc/sys/kernel/tainted: Bits 0 to 4 have the following meanings:

  bit  0 = a module with a GPL-incompatible license was loaded (tainted & 1),
  bit  1 = module load was enforced (tainted & 2),
  bit  2 = an SMP-unsafe module was loaded (tainted & 4),
  bit  3 = (reserved),
  bit  4 = an unsupported module was loaded (tainted & 16),
  bit  5 = a module with third-party support was loaded (tainted & 32).
  bit 10 = a machine check exception has occurred (taint & 1024; x86_64 only
	   so far).

The corresponding codes for the taint flags in Oopses are (x = unknown):

  - "Pxxx" if bit 0 set or else
    "Gxxx" if bit 0 unset,
  
  - "xFxx" if bit 1 set or else
    "x xx" if bit 1 unset,
  
  - "xxSx" if set or else
    "xx x" if bit 2 unset,
  
  - "xxxU" if bit 4 set or else
    "xxxX" if bit 5 set or else
    "xxx ".

By default, external modules will not have the supported flag (that is,
they wil be marked as unsupported). For building externally supported
modules, please get in touch with Kurt Garloff <garloff@suse.de>.


PATCH SELECTION MECHANISM

The SUSE kernels consist of the vanilla kernel sources on top of which a
number of patches is applied. Almost all of these patches are applied on
all architectures; a few patches are only used on a subset of
architectures. The file series.conf determines which patches are applied
on which architectures. A script named "guards" converts series.conf
into a plain list of patch files to be applied. Guards decides which
patches to include and exclude based on a list of symbols. The symbols
used by default are computed by the helper script "arch-symbols". From
the kernel-source.src.rpm package, a fully patched kernel source tree
can be generated from vanilla sources + patches like this:

    # Install the package:

      $ rpm -i kernel-source.src.rpm

    # Unpack the patches and the kernel sources:
    
      $ cd /usr/src/packages/SOURCES
      $ for f in patches.*.tar.bz2; do		\
	  tar xfj $f || break;			\
	done
      $ tar xfj linux-2.6.5.tar.bz2

    # Apply the patches

      $ for p in $(./guards $(./arch-symbols) < series.conf); do
	  patch -d linux-2.6.5 -p1 < $p || break
	done

The configuration script config.conf which is similar to series.conf is
used for configuration file selection.  See the section WHERE TO FIND
CONFIGURATION FILES.

The file format of series.conf and config.conf should be obvious from
the comments in series.conf, and from the guards(1) manual page. (The
guards(1) manual page can be generated by running pod2man on the guards
script.)


WHERE TO FIND CONFIGURATION FILES

Kernel configuration files are stored in the kernel CVS repository. When
packing up the repository, they end up in config.tar.bz.  When
kernel-source.$ARCH.rpm is built, the config files are copied from
config/$ARCH/$FLAVOR to .config in the kernel source tree.

The kernel-$FLAVOR packages are based on config/$ARCH/$FLAVOR.
(kernel-default is based on config/$ARCH/default, for example).
The kernel-$FLAVOR packages install their configuration files as
/boot/config-$VER_STR (for example, /boot/config-2.6.5-99-default) as
well as /usr/src/linux-obj/$ARCH/$FLAVOR/.config.

In addition, the running kernel exposes a gzip compressed version of its
configuration file as /proc/config.gz. The kernel sources can be
configured based on /proc/config.gz with ``make cloneconfig''.


HOW TO CONFIGURE THE KERNEL SOURCES

Before a binary kernel is built or an additional loadable module
for an existing kernel is created, the kernel must be configured.

In order for a loadable module to work with an existing kernel, it must
be created with a configuration that is identical to the kernel's
configuration, or at least very close to that. Each configuration is
contained in a single file.  The kernel-source package contains
configurations for all standard SUSE kernel variants, so for building
only external kernel modules it is not necessary to configure the kernel
sources.

Configuring the kernel sources for a specific configuration is
straightfoward:

  - Locate the configuration file you want to use. (See WHERE TO FIND
    CONFIGURATION FILES above).

  - Copy the configuration to the file .config in the kernel source
    tree. The kernel-source package installs its source tree in
    /usr/src/linux.

  - Run the following commands in sequence to apply the configuration,
    generate version information files, etc.:

	make clean
	make oldconfig

    Alternatively to ``make oldconfig'', you can also use ``make
    menuconfig'' for a text menu oriented user interface. If the kernel
    sources do not match the configuration file exactly, ``make
    oldconfig'' will prompt for settings that are undefined.

    For configuring the kernel to match the running kernel, there is a
    shortcut ``make cloneconfig'' that expands the file /proc/config.gz
    into .config, and then runs ``make oldconfig''.


MODULE LOAD PATHS

Modules that belong to a specific kernel release are installed in
/lib/modules/2.6.5-99-smp and similar. Note that this path contains the
kernel package release number. Modules from KMPs must be installed
below /lib/modules/2.6.5-99-smp/updates/ and similar: modules below
updates/ have priority over other modules.

When KMPs contain modules that are compatible between multiple installed
kernels, symlinks are used to make those modules available to those
compatible kernels like this:

  /lib/modules/2.6.16-100-smp/weak-updates/foo.ko ->
      /lib/modules/2.6.16-99-smp/updates/foo.ko

Modules in the weak-updates directory have lower priority than modules
in /lib/modules/2.6.16-100-smp/updates/, and higher priority than other
modules in /lib/modules/2.6.16-100-smp.


REFERENCES

General

  Documentation in the kernel source tree.

  Linux Documentation Project, http://www.tldp.org/

  Linux Weekly News, http://lwn.net

  Rusty's Remarkably Unreliable Guides (Kernel Hacking
    and Kernel Locking guides),
    http://www.netfilter.org/unreliable-guides/

  Kernel newbies, http://www.kernelnewbies.org/


Loadable Kernel Modules

  Peter Jay Salzman and Ori Pomerantz: Linux Kernel Module
    Programming Guide, Version 2.4, April 2003,
    http://www.tldp.org/guides.html


Kernel Module Packages

    Andreas Gruenbacher: Kernel Module Packages Manual.
    Versions for CODE9 (SLES9, SUSE LINUX 10.0) and CODE10
    (SUSE Linux 10.1, SLES10),
    http://www.suse.de/~agruen/KMPM/