WORKING WITH THE SUSE 2.6.x and 3.x KERNEL SOURCES Andreas Gruenbacher <>, SUSE Labs, 2003, 2004, 2005, 2006 Michal Marek <>, SUSE Labs, 2010 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 HOWTO that describes how to build driver update disks (among other things) is available at: 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, on top of which a number of patches are applied. The resulting kernel source tree is configured and built, resulting in a binary kernel. The add-on patches and configuration files are maintained in a GIT repository at A script (scripts/ packs up the files in the 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-devel A subset of the kernel-source package that is needed to build external modules. * kernel-$FLAVOR A number of binary kernels (for example, kernel-default for uniprocessor machines, kernel-smp for smp machines, etc.). These packages are all generated from the same kernel sources, and differ in the kernel configurations used. * kernel-$FLAVOR-base A subset of kernel-$FLAVOR, for use in paravirtualized quests that only require a few device and filesystem drivers. * kernel-$FLAVOR-devel The files used for generating kernel module packages for use with kernel-$FLAVOR. * kernel-syms A meta package that pulls in the relevant kernel-$FLAVOR-devel packages for a given architecture. The repository contains the configuration files (.config) for all SUSE kernel flavors. All configuration files are included in the dependencies of the kernel-syms 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/share/doc/packages/kernel-source/ This document. The kernel-$FLAVOR-devel packages install the following files: * /usr/src/linux-$VERSION-$RELEASE-obj/$ARCH/$FLAVOR/ Kernel build object files for one kernel flavor. These files are used for compiling additional kernel modules. * /usr/src/linux-obj/$ARCH/$FLAVOR A symbolic link to /usr/src/linux-$VERSION-$RELEASE-obj/$ARCH/$FLAVOR. COMPILING YOUR OWN KERNEL The kernel sources are found in the kernel-source package. The recommended way to produce a binary kernel is: (1) Install kernel-source. Change to the /usr/src/linux directory. (2) Create a build directory for use in configuring and building the kernel. Using /usr/src/linux directly requires root priviledges and will cause problems if you need to build kernel modules for other installed kernels. (3) Configure the kernel (for example, ``make -C /usr/src/linux O=$(pwd) oldconfig'', see HOW TO CONFIGURE THE KERNEL SOURCES). (4) Build the kernel and all its modules (``make''). (5) Make sure that /etc/modprobe.d/unsupported-modules contains allow_unsupported_modules 1 otherwise modprobe will refuse to load any modules. (6) 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''). (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-$FLAVOR packages using RPM. BUILDING ADDITIONAL (EXTERNAL) MODULES A single binary kernel module generally only works for a specific version of the kernel source tree, for a specific architecture and configuration. This means that for each binary kernel that SUSE ships, a custom module must be built. This requirement is to some extent relaxed by the modversion mechanism: modversions attach a checksum to each symbol (function or variable) exported to modules by the kernel. This allows to use kernel modules that have been built for a kernel with a different version or release number in many cases, as long as none of the symbols the module uses have changed between the two kernel versions. When releasing maintenance or security update kernels for a specific product, we carefully try to keep the kernel ABI stable. Despite this, we sometimes have no choice but to break binary compatibility. In this case, those kernel modules must be rebuilt. Additional kernel modules for one of the SUSE kernel flavors can be built in three different ways: (1) by configuring the kernel sources in a separate build directory (see HOW TO CONFIGURE THE KERNEL SOURCES), or (2) by using one of the standard configurations in /usr/src/linux-obj/$ARCH/$FLAVOR, or (3) by creating a Kernel Module Package (KMP) as described in the Kernel Module Packages Manual, The first method involves the following steps: (1) Install the kernel-source package. (2) Configure the kernel, see HOW TO CONFIGURE THE KERNEL SOURCES. (3) Create files required for compiling external modules: ``make prepare'' and ``make scripts''. (4) Compile the module(s) by changing into the module source directory and typing ``make -C $(your_build_dir) M=$(pwd)''. (5) Install the module(s) by typing ``make -C $(your_build_dir) M=$(pwd) modules_install''. The second method involves the following steps: (1) Install the kernel-devel package. (2) Install the kernel-$FLAVOR-devel package. This is necessary for symbol version information (CONFIG_MODVERSIONS). (3) Compile the module(s) by changing into the module source directory and typing ``make -C /usr/src/linux-obj/$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). (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. 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, out-of-tree modules will not have the supported flag (that is, they will be marked as unsupported). For building externally supported modules, please get in touch with Solid Driver Program team, led by Scott Bahling, at <>. PATCH SELECTION MECHANISM The SUSE kernels consist of the vanilla kernel sources on top of which a number of patches is applied. The file series.conf determines which patches are applied and which are excluded. 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. 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 -xjf "$f" || break; \ done $ tar -xjf linux-2.6.5.tar.bz2 # Apply the patches $ for p in $(./guards < series.conf); do patch -d linux-2.6.5 -p1 < $p || break done The configuration script config.conf which is similar to series.conf is used for configuration file selection. See the section WHERE TO FIND CONFIGURATION FILES. The file format of series.conf and config.conf should be obvious from the comments in series.conf, and from the guards(1) manual page. (The guards(1) manual page can be generated by running pod2man on the guards script.) WHERE TO FIND CONFIGURATION FILES Kernel configuration files are stored in the kernel GIT repository. When packing up the repository, they end up in config.tar.bz2. The kernel-$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-$VER_STR (for example, /boot/config-2.6.5-99-default). The config is also packaged in the kernel-$FLAVOR-devel package 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 silentoldconfig''. 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-syms package installs 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 your build directory. - Run the following commands in sequence to apply the configuration, generate version information files, etc.: make -C /usr/src/linux O=$PWD clean make -C /usr/src/linux O=$PWD 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. Once this step is completed, a Makefile will have been created that eliminates the need to specify the locations of the kernel source and the build directory. For configuring the kernel to match the running kernel, you can use the file /proc/config.gz, unpack it into into .config, and then run ``make oldconfig''. HOW TO ADD CUSTOM PATCHES Typically patches are added to the appropriate patches.* directory (e.g. patches.fixes) and to series.conf. When the kernel-source package is exported from the git repository, the patch will be automatically added to the appropriate patch tarball. If your goal is to create a kernel with only a few additional patches and you don't want to be bothered with using the git repository, there is an easier way. The kernel-source SRPM ships with two empty archives that can be be filled and automatically expanded when building the kernel. You can use these to add your own patches and config options without disturbing the rest of the kernel package. This is useful if you are using the openSUSE Build Service and link to the main kernel-source project instead of creating your own branch. The advantage to this is that your project will automatically receive all the changes that go into the main project without any further effort. To add a patch using this mechanism, just add it to the patches.addon.tar.bz2 archive and add an entry to the series.conf file. The archive will be expanded automatically with the other kernel patches when the source tree is constructed. Some patches may add new Kconfig options. The config.addon.tar.bz2 archive contains the same hierarchy as config.tar.bz2, but is under config.addon. You can add your new config options to files named after their config/ counterparts. For example, the file used to configure the i386 default kernel is named config/i386/default. To add config options to that kernel, you would create a new file called config.addon/i386/default with the options as formatted in a normal Linux kernel .config file. This is important because the kernel build is non-interactive and will fail if it encounters new config options without entries in the config file. 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, Linux Weekly News, Rusty's Remarkably Unreliable Guides (Kernel Hacking and Kernel Locking guides), Kernel newbies, Loadable Kernel Modules Peter Jay Salzman and Ori Pomerantz: Linux Kernel Module Programming Guide, Version 2.4, April 2003, Kernel Module Packages Andreas Gruenbacher: Kernel Module Packages Manual. Versions for CODE9 (SLES9, SUSE LINUX 10.0) and CODE10 (SUSE Linux 10.1, SLES10),