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Last Friday morning I had the joy of Jackhammers [1] not to long after waking up. This Friday I had the joy of not being able to log into X on my desktop machine.

I fairly quickly narrowed this down to some fairly bogus permissions on /dev/null, /dev/zero and likely many others. And I fairly quickly started to suspect udev. To cut a long story short, after much fussing about I narrowed the problem down to the presence of /etc/udev/rules.d/65_dmsetup.rules by comparing things with a working system.

After that, I found that this is Debian Bug #491107in dmsetup 2:1.02.27-2. The bug report describes the problem as well as I could, I experienced both the permissions and the no booting variants in the course of my journey :-)

From: Modestas Vainius
To: Debian Bug Tracking System
Subject: something in dmsetup.rules renders udev useless
Date: Wed, 16 Jul 2008 21:06:02 +0300

I'm reporting this as critical because it rendered my systems unbootable. When /etc/udev/rules.d/65_dmsetup.rules is present, udev stops functioning properly (various anomalies can be seen like wrong permissions on /dev/null etc.). If, unfortunately, initramfs image is regenerated and that rules gets included, system becomes unbootable due to "Waiting for root file system". Actually, even if udev daemon is started in this case, it simply sits there and won't coldplug a single device. Moving this file away resolves the problem. I reproduced this problem on both my computers with 2.6.26-trunk-amd64, 2.6.25-2-amd64 and a few other kernels (which had initramfs regenerated).

Lessons learnt:

1. If you run Debian unstable and update often then sometimes things break. No surprises there, thats part of its purpose. And I'm happy to say this has been the first nasty one in a while.
2. udev is more fragile than I would like for something so critical.

Fri, 18 Jul 2008 10:45:45 +1000| Permalink