In the last post, I found that SystemTap threw errors when I tried to add probe points into the operating system.
It appears that the solution is to install the debug version of the kernel, which contains a mapping from human-readable symbol names to machine-executable addresses.
There is an excellent post on the Ubuntu wiki which describes how to do this, including doing a dedicated kernel build.
This post contains simplified instructions specific to Ubuntu 14.04 LTS. Please note that I’m running the 32bit version in a VM, since my hypervisor doesn’t support 64bit guest operating systems without tweaking (Note: I subsequently figured out how to make this work).
Finding the Ubuntu release name:
All Ubuntu releases have a marketing code name along with the more traditional version number.
It turns out that the code name is important for this task, because it is used to index into Ubuntu’s online package repositories.
I can find the appropriate code name as follows – this applies to all Ubuntu releases, not just 14.04 LTS:
cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty
DISTRIB_DESCRIPTION="Ubuntu 14.04.1 LTS"
So, my code name is ‘trusty’.
Adding repository references:
Now that I have the code name, I need to modify my repository setup to access the repository that holds the debug symbols.
The repositories are configured by the following file:
/etc/apt/sources.list
In this case, here’s what I need to add to ‘sources.list’:
deb http://ddebs.ubuntu.com/ trusty main restricted universe multiverse
deb http://ddebs.ubuntu.com/ trusty-security main restricted universe multiverse
deb http://ddebs.ubuntu.com/ trusty-updates main restricted universe multiverse
deb http://ddebs.ubuntu.com/ trusty-proposed main restricted universe multiverse
Note: A descritpion of what all these ‘universe’ and ‘multiverse’ tags mean can be found
here.
The key thing here is that the lines contain ‘ddebs’, which means ‘debug-debian, rather than the normal repositories which contain stripped packages.
Once I’ve added these lines, I need to force the package management system to re-read the file:
sudo apt-get update
When this completed, I noticed that the package system wasn’t able to validate the package signatures:
W: GPG error: http://ddebs.ubuntu.com trusty-proposed Release:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ECDCAD72428D7C01
This is easily fixed, as follows:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys ECDCAD72428D7C01
Note that the value passed to ‘apt-key’ is the same value as reported in the warning message. This process is the same for any repository where you get this warning – assuming you trust the repository, of course!
Once I’ve gotten the keys straightened out, I need to get run the update process again:
sudo apt-get update
Installing the debug symbols:
The final stage should be easy. I need to download the kernel package with debug symbols that matches our current kernel version:
sudo apt-get install linux-image-$(uname -r)-dbgsym
It’s worth noting that the package is pretty big, at about 400Mbytes. Go get a coffee while it downloads!
