Duncan Mac-Vicar P.

I really enjoy learning new things, and I spend most of my computer time digging into stuff. My productive times always coincide with the quality of my development environment.

If you can’t keep up with a easy and own system to checkout, develop, compile and try code, it is very likely you will spend more time doing meta-development.

I have been always interested into learning more about the kernel. I would conform myself with having a sandbox where easily modify and try things, so if one day I feel like doing something, I could just sit and start.

We are going to use QEMU, a PC emulator, which is quite nice because:

• It can launch kernels directly. Uses some magic to load the kernel in the emulator and boot it, so it is not needed to setup a tftp server with a kernel.
• It doesn’t require kernel modules, except if you need more speed, then you can use kqemu.
• It is free

The fact QEMU can launch a kernel directly, means we can launch the image we just compiled.

To get the Linux kernel source do:

git-clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git linux

To avoid having to create a image of the system to use as a root disk all the time, we can just mount a directory on our own machine via NFS.

 make xconfig

Be sure to enable nfsroot option, which at the same time requires kernel ip level autoconfiguration. Also be sure to put your network card in the kernel, not as a module, otherwise you will need to create a initrd image too.

 make bzimage
 make modules

We have everything ready to boot the kernel. But we need a root filesystem too. We will use the Kiwi tool to generate a custom root filesystem. With kiwi you can generate a root filesystem and then turn it into a LiveDVD, a USB stick distro or a Xen image. For us the first stage is enough. See the website for info how to get SUSE packages for Kiwi

Edit /usr/share/kiwi/image/netboot/suse-10.2/config.xml and now create the image:

 kiwi --root /space/nfsroot/work --prepare /usr/share/kiwi/image/netboot/suse-10.2

When trying this, I got a strange error. Looking at work.log, the problem was when kiwi called smart to install the packages, at commit time, the rpm database did not exist.

 Committing transaction...^M
 error: cannot open Packages index using db3 - No such file or directory (2)^M
 error: cannot open Packages database in work/var/lib/rpm

So I edited /usr/share/kiwi/modules/KIWIManager.pm and near line 630 I added these two lines:

 print $fd "mkdir -p $root/var/lib/rpm\n";
 print $fd "rpm --root $root --initdb\n";

I am not sure if it the right fix. I will ask on the mailing list later. But the workaround worked for me. The final section of the code looks now like:

 # Add package manager to package list
 #------------------------------------------
 push (@packs,$manager);
 #==========================================
 # Create screen call file
 #------------------------------------------
 print $fd "smart update @channelList\n";
 print $fd "mkdir -p $root/var/lib/rpm\n";
 print $fd "rpm --root $root --initdb\n";
 print $fd "test \$? = 0 && smart install @packs @installOpts\n";
 print $fd "echo \$? > $screenCall.exit\n";
 print $fd "rm -f $root/etc/smart/channels/*\n";

Edit /etc/exports to share that directory via NFS:

 /space/nfsroot/work   (rw,root_squash,sync)

Now, go to your kernel source directory and launch it:

qemu -kernel arch/i386/boot/bzImage
 -append
  "nfsroot=192.168.0.101:/space/nfsroot/work,rw
   ip=::::diskless:eth0:dhcp root=/dev/nfs"
 -hda /dev/zero -net nic -net user

Final result:

I still can’t login on the machine, but that is a minor issue. Some error messages about read-only filesystem too.

Things I haven’t figured yet:

• How to install modules to the root filesystem (something like make modules_install DESTDIR=/space/image)