View source for Kernel debugging essentials workshop

{{Event|2014-02-19 20:00}}

== Topics ==

[[File:angry-tux.png|right]]

We'll be holding a workshop on basics of finding and debugging (and eventually fixing) common issues with Linux kernel.

Topics I'd initially cover would include leveraging in-kernel debugging facilities, techniques for capturing debug output from various crashes, understanding the captured data, and modifying, building and loading the altered kernel code with more debugging code or an actual fix.

Original mailing list reference: [http://lists.base48.cz/pipermail/public/2014-January/000235.html]

== Requirements ==

* Some elementary knowledge of C
* A laptop with Linux (Fedora 20 preferred, Debian 7 fine, we'll try to cover older versions and other distribution too) and root access, 2G + size of RAM free space
** If you don't have any available, please let me know in advance

== Language ==

I'm fine with holding the workshop in English language if there's anyone who doesn't speak Czech/Slovak.

== Setup instructions ==

Please prepare your laptop in advance so that we don't waste too much time at the workshop.

Extra hardware will be available for ones who won't be able to bring their own laptops.

=== Install required packages ===

{|
! Debian
! Fedora
|- style="vertical-align:top;"
|
# <code>apt-get -y install kdump-tools crash git linux-headers linux-image-$(uname -r)-dbg linux-source ncurses-dbg</code>
|
# <code>pkcon install kernel{-debug,}{-devel,} kexec-tools crash git gcc ncurses-devel
debuginfo-install -y kernel{-debug,}</code>
|}

=== Get the kernel source tree ===

<code>git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git</code>

== Cheatsheet ==

Here are some distro-dependent details. I'll be using Fedora, follow these to translate to Debian equivalents if you're running Debian.

These are here just for reference, no need to follow these prior to the workshop.

=== Enable kdump ===

{|
! Debian
! Fedora
|- style="vertical-align:top;"
|
# Edit <code>/etc/default/kdump-tools</code>
# Add <code>crashkernel=256M</code> to <code>/etc/default/grub</code>
# Run <code>update-grub2</code>
# Reboot
# Check <code>kdump-config status</code>
|
# <code>system-config-kdump</code>

or

# Add <code>crashkernel=256M</code> to <code>/etc/default/grub</code>
# Run <code>grub2-mkconfig >/etc/grub.conf</code>
# <code>systemctl enable kdump.service</code>
# Reboot
# <code>Check systemctl status kdump.service</code>
|}

=== Enable netconsole ===

{|
! Debian
! Fedora
|- style="vertical-align:top;"
|
# <code>modprobe netconsole \</code><br><code>    netconsole=@/,514@$(getent hosts odvarok.local |</code><br><code>    awk '{print $1}')/</code>
|
# Edit <code>/etc/sysconfig/netconsole</code>
# <code>systemctl enable netconsole.service</code>
# <code>systemctl start netconsole.service</code>
|}

=== Running crash ===

The location of the raw <code>vmlinux</code> binary from the debugging packages differs in distros.

{|
! Debian
! Fedora
|- style="vertical-align:top;"
|
# <code>crash /usr/lib/debug/boot/vmlinux-$(uname -r) /var/crash/201402090936/dump.201402090936</code>
|
# <code>crash /usr/lib/debug/lib/modules/$(uname -r)/vmlinux /var/crash/127.0.0.1-2013.10.24-02:48:40/vmcore</code>
|}

== Materials ==

Work-in-progress slides. Content will be extended and polished (as I learn beamer...), but outline will stay roughly the same: [[File:kernel-workshop.pdf.gz]]

* https://github.com/lkundrak/sad-ugly-and-broken This is a repository for variously broken kernel code, for demonstration
* https://github.com/lkundrak/kexec-tools Patched kexec-tools, for >= 3.11 kernels
* http://v3.sk/~lkundrak/firescope/ Firewire debugging tool