building a RHEL4 kernel w/ kdb support

Sometimes I have a need to reproduce bugs on other platforms. When that platform uses a heavily patched kernel, it makes it hard to debug. Below are some notes I took while building a RHEL4 kernel with kdb support.

1. get the kernel

It's not easy to find the kernel sources for RHEL4 if you don't have the CDs. I was working off-site so I didn't.

I finally found a security article on LinuxSecurity.com that talked about a security fix and referenced the FTP source for kernel-2.6.9-22.EL.src.rpm. I grabbed that.

2. install the rpm

This step is easy, just run rpm -i kernel-2.6.9-22.EL.src.rpm. The sources are unpacked into /usr/src/redhat directories.

3. prepare the kernel

This step unpacks the tarball and applies the millions of patches packages with the redhat kernel:

    $ rpmbuild --target=i686 -bp /usr/src/redhat/SPECS/kernel-2.6.spec

The kernel is now in /usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9

4. getting the KDB patches

If you need help, read the README.

You need to get two KDB patches. One common, and one for your architecture. In my case that's i386.
(I hope that is your case too, if not email me and maybe I can help you with the conversion of the patches)

The original patches (see below) require some manual fixing.

    $ cd /usr/src/redhat/SOURCES/
    $ wget http://www.jukie.net/~bart/kernel/2.6.9-22+kdb/kdb-v4.4-2.6.9-22.EL-common-2.patch.bz2
    $ bunzip2 kdb-v4.4-2.6.9-22.EL-common-2.patch.bz2

    $ wget http://www.jukie.net/~bart/kernel/2.6.9-22+kdb/kdb-v4.4-2.6.9-22.EL-i386-1.patch.bz2
    $ bunzip2 kdb-v4.4-2.6.9-22.EL-i386-1.patch.bz2

here are the links: kdb-v4.4-2.6.9-22.EL-common-2.patch.bz2, kdb-v4.4-2.6.9-22.EL-i386-1.patch.bz2. Note that you don't apply these patches yourself, the build system will do that for you.

For reference, these are the original patches I started with:

    $ wget ftp://oss.sgi.com/projects/kdb/download/v4.4/kdb-v4.4-2.6.9-common-2.bz2
    $ wget ftp://oss.sgi.com/projects/kdb/download/v4.4/kdb-v4.4-2.6.9-i386-1.bz2

5. patching the .spec file

To build an RPM you need a spec file. The spec file shipped with the .src.rpm contains a list of all the patches that will be applied, so we just need to add the two kdb patches. Here is the diff you will have to make:

add the patches into the list of patches at the end:

after Patch10001 add:

Patch10002: kdb-v4.4-2.6.9-22.EL-common-2.patch
Patch10003: kdb-v4.4-2.6.9-22.EL-i386-1.patch

add the options to pass to patch when applying our kdb patches:

after %patch10001 add:

# kdb-v4.4-2.6.9-22.EL-common-2.patch
%patch10002 -p1
# kdb-v4.4-2.6.9-22.EL-i386-1.patch
%patch10003 -p1

You can just get my spec patch:

    $ cd /usr/src/redhat/SPECS/
    $ wget http://www.jukie.net/~bart/kernel/2.6.9-22+kdb/kernel-2.6.spec.patch

here is the link: kernel-2.6.spec.patch

Then apply it by running

    $ cd /usr/src/redhat/SPECS/
    $ patch -p1 < kernel-2.6.spec.patch

8. patching the configs

I added 4 lines to each of the i686 config files in /usr/src/redhat/SOURCES:

    +CONFIG_KDB=y
    +# CONFIG_KDB_MODULES is not set
    +# CONFIG_KDB_OFF is not set
    +CONFIG_KDB_CONTINUE_CATASTROPHIC=0

You can just get my config patch:

    $ wget http://www.jukie.net/~bart/kernel/2.6.9-22+kdb/kernel-2.6.9-i686-configs.patch

here is the link: kernel-2.6.9-i686-configs.patch

Then apply it by running

    $ cd /usr/src/redhat/SOURCES/
    $ patch -p1 < kernel-2.6.9-i686-configs.patch

7. building the rpm packages

To build both source and binary packages run:

    $ rpmbuild --target=i686 -ba /usr/src/redhat/SPECS/kernel-2.6.spec

The final products will be stored in /usr/src/redhat/RPMS/i686/ and /usr/src/redhat/SRPMS/. Should something fail you can review the tree in /usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9 to see if you can fix it.

Note that this will build a 2.6.9-22.EL.root kernel. There is probably a good reason for including the user name of who built it. Eh, whatever.

This is what I get in the end:

    /usr/src/redhat/RPMS/i686/:
    total 40M
    9.9M kernel-2.6.9-22.EL.root.i686.rpm
    3.7M kernel-devel-2.6.9-22.EL.root.i686.rpm
    9.5M kernel-hugemem-2.6.9-22.EL.root.i686.rpm
    3.8M kernel-hugemem-devel-2.6.9-22.EL.root.i686.rpm
    9.6M kernel-smp-2.6.9-22.EL.root.i686.rpm
    3.7M kernel-smp-devel-2.6.9-22.EL.root.i686.rpm

    /usr/src/redhat/SRPMS:
    total 40M
    40M kernel-2.6.9-22.EL.root.src.rpm

8. installing the new rpms

To install the new package simply run

    $ cd /usr/src/redhat/RPMS/i686/
    $ rpm -i kernel-2.6.9-22.EL.root.i686.rpm kernel-devel-2.6.9-22.EL.root.i686.rpm

(I install the devel packages because I am building 3rd party modules against it)

I am not sure if this updates /etc/grub.conf. Check it. Make sure you have something like this:

    title Red Hat Enterprise Linux AS (2.6.9-22.EL.root)
            root (hd0,0)
            kernel /boot/vmlinuz-2.6.9-22.EL.root ro root=LABEL=/1 rhgb quiet
            initrd /boot/initrd-2.6.9-22.EL.root.img

Then cross your fingers and reboot.

9. using kdb

Read the Inside the Linux kernel debugger article from IBM.

Bart's Blog

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