Kernel debugging essentials workshop

Kernel debugging essentials workshop

Event start:

2014-02-19 20:00

Topics

File:Angry-tux.png

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: [1]

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
apt-get -y install kdump-tools crash git linux-headers linux-image-$(uname -r)-dbg linux-source ncurses-dbg	pkcon install kernel{-debug,}{-devel,} kexec-tools crash git gcc ncurses-devel debuginfo-install -y kernel{-debug,}

Get the kernel source tree

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

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
Edit /etc/default/kdump-tools Add crashkernel=256M to /etc/default/grub Run update-grub2 Reboot Check kdump-config status	system-config-kdump or Add crashkernel=256M to /etc/default/grub Run grub2-mkconfig >/etc/grub.conf systemctl enable kdump.service Reboot Check systemctl status kdump.service

Enable netconsole

Debian	Fedora
modprobe netconsole \ netconsole=@/,514@$(getent hosts odvarok.local | awk '{print $1}')/	Edit /etc/sysconfig/netconsole systemctl enable netconsole.service systemctl start netconsole.service

Running crash

The location of the raw vmlinux binary from the debugging packages differs in distros.

Debian	Fedora
crash /usr/lib/debug/boot/vmlinux-$(uname -r) /var/crash/201402090936/dump.201402090936	crash /usr/lib/debug/lib/modules/$(uname -r)/vmlinux /var/crash/127.0.0.1-2013.10.24-02:48:40/vmcore

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