Creating Custom Kernel RPMs

Creating custom CentOS kernel RPMs

Source

I need to build a custom kernel

• Are you sure? CentOS is designed to function as a complete environment. If you replace a critical component, it may very well affect how the rest of the system acts.
• ARE YOU ABSOLUTELY SURE? Seriously, 99.9999% of users no longer need to rebuild their own kernel. You may simply just need to build a module, in which case see I need the kernel source.
• Okay, last warning.. if you break the kernel or your machine, you get to keep it, and as a bonus, you get to keep all the associated crying about how your kernel doesn't build.
• Is the functionality you need in the plus kernel located in Repositories/CentOSPlus
• Is the functionality you need available as a separate module to the current kernel?

If you want to build a more recent kernel, see http://howtoforge.com/kernel_compilation_centos. This site is not endorsed as to its build method, however, as it builds the SRPM as root, which is unsafe, and basically flawed in approach. See, more correctly, building as non-root

This tutorial will cover rebuilding the centos source rpm with your own options or modifications.

These actions are for your own personal use only. Custom kernels are not supported, as the development team has no control over the build environment, options chosen etc. If you chose to build your own kernel, you will be responsible for continuing to maintain it for security updates, new releases, the second coming of $DEITY, and any other possible scenario which may warrant an update.

Build Prep

You're going to need a few packages to build a kernel properly.

• yum groupinstall "Development Tools" #This is overkill, but will make sure you have all the needed packages to build.
• yum install kernel-devel # or kernel-smp-devel depending on what you're running.
• from the KBS-Extras repository at http://centos.karan.org/, install fedora-rpmdevtools
• yum install qt-devel #only necessary if you want to use make xconfig instead of make menuconfig

rpm -i http://mirror.centos.org/centos/4/updates/SRPMS/kernel-2.6.9-42.0.3.EL.src.rpm

[user@host]$ cd ~/rpmbuild/SPECS
[user@host SPECS]$ rpmbuild -bp --target=`uname -m` kernel-2.6.spec

Now that we've got the buildroot set up, it's time to start modifying the kernel to suit your needs. First we need to put the current config into place. To do this copy the appropriate config from the buildroot's SOURCES directory to the build directory. See the example below.

[user@host]$ cd ~/rpmbuild/BUILD/kernel-2.6.9/linux-2.6.9
[user@host]$ cp ~/rpmbuild/SOURCES/kernel-2.6.9-i686.config .config

From here, you're ready to run make menuconfig or make xconfig and any other changes you see fit (if you copied down config files from the SOURCES directory, and not the configs/ directory to .config, you will need to run make oldconfig prior to make menuconfig or make xconfig). If you patch the kernel source here, you need to make sure to generate a patch, or keep the patch you're using because we're not going to build the kernel here. We're generating a new source package and configs to generate a new kernel rpm. To those of you saying 'but I can just run make rpm!' you certainly can, but it will not generate the initrd, so you may have issues booting, and you'll have to modify grub to handle your image. Trust me, this way is a little more work, but it's the proper way.

Note: For CentOS-5 kernels, you need to add a line to the top of the .config file before you copy it back to the SOURCES directory that contains the Hardware Platform (uname -i) that you are building the kernel for. That is i386 for i[3,4,5,6]86 arches, x86_64 for the x86_64 arch. It needs to be remarked out with a # and be the first line of the .config file.

For CentOS 5 only, add this as the first line of the .config file

# i386

# x86_64

[user@host]$ cp .config ~/rpmbuild/SOURCES/kernel2.6.9-i686.config

Now it's time to modify the kernel spec file to accept the changes that we've made.

[user@host]$ cd ~/rpmbuild/SPECS
[user@host]$ vim kernel-2.6.spec

%define release 42.0.3.EL

%define release 42.0.3.1.custom

Patch40000: my-custom-kernel.patch

%patch40000 -p1

Remove or remark out this code to build CentOS-5 modified kernels:

#if a rhel kernel, apply the rhel config options
%if 0%{?rhel}
  for i in %{all_arch_configs}
  do 
    mv $i $i.tmp
    $RPM_SOURCE_DIR/merge.pl $RPM_SOURCE_DIR/config-rhel-generic $i.tmp > $i
    rm $i.tmp
  done
  for i in $RPM_SOURCE_DIR/kernel-%{kversion}-{i586,i686,i686-PAE,x86_64}*.config
  do
    echo i is this file  $i
    mv $i $i.tmp
    $RPM_SOURCE_DIR/merge.pl $RPM_SOURCE_DIR/config-rhel-x86-generic $i.tmp > $i
    rm $i.tmp
  done
  for i in $RPM_SOURCE_DIR/kernel-%{kversion}-ppc64.config
  do
    echo i is this file  $i
    mv $i $i.tmp
    $RPM_SOURCE_DIR/merge.pl $RPM_SOURCE_DIR/config-rhel-ppc64-generic $i.tmp > $i
    rm $i.tmp
  done
%endif
#if a olpc kernel, apply the olpc config options
%if 0%{?olpc}
  for i in %{all_arch_configs}
  do
    mv $i $i.tmp
    $RPM_SOURCE_DIR/merge.pl $RPM_SOURCE_DIR/config-olpc-generic $i.tmp > $i
    rm $i.tmp
  done
%endif

Building your new kernel

We're just about done now, and you're only a couple short steps from your own custom kernel. Now we need to put the build into motion.

[user@host SPECS]$ rpmbuild -ba --target=`uname -m` kernel-2.6.spec

Make sure that you install these rpms using yum localinstall, or rpm -ivh kernel-*.rpm.

UNDER NO CIRCUMSTANCES install your kernel rpms with rpm -Uvh, as this will update and overwrite the current kernel install, so if you have a problem with your version, you will not be able to roll back.