Kernel testing, or the lack thereof, is considered to be a significant part
of the kernel quality problem. Recent kernels, while quite good in many
regards, contain more bugs than they should because people have not gotten
around to testing them before the final release. Many regressions are in
device drivers, which present special testing problems: drivers can only be
tested by people who have the relevant hardware. Core kernel code,
however, is hardware independent and should be easier to test. But bugs
can slip through in that code as well.
Consider, for example, the realtime rlimits feature, which can be used to
enable otherwise unprivileged users to run processes with elevated
priority. Andreas Steinmetz recently noticed that this feature does not work in the
2.6.13-rc3 kernel. This would seem to be just the sort of feedback the
process needs: a user, testing a feature in a -rc kernel, found a bug and
provided a patch to fix it. As a result, that particular bug will not be
present in 2.6.13.
The only problem is that, as confirmed by
Ingo Molnar, the bug is a little older than that. In fact, the realtime
resource limit feature does not work at all in the stable 2.6.12 kernel, and nobody
noticed until now. This is a feature which can be tested by just about
anybody, but that work clearly had not been done. Given that nobody
appears to be using this feature, Ingo is not
confident that the fix can go into a 2.6.12 stable release; this one
will have to wait for 2.6.13.
It should be said that testing realtime resource limits is not an entirely
straightforward operation; setting that limit requires changes to the PAM
library, C library, and the shells as well. Very few distributions - and
no major ones - are shipping those changes at this time. Even so,
unprivileged realtime scheduling is a feature that a number of people had
been asking for. It is a little surprising that none of those people
noticed that it failed to work in a major kernel release. Getting
comprehensive testing coverage for the kernel is clearly still a problem -
even before drivers are taken into account.
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