Virtualizing /proc/sys/kernel/random/boot_id per container ?

Daniel P. Berrange berrange at redhat.com
Thu Aug 30 23:22:39 UTC 2012


On Thu, Aug 30, 2012 at 03:15:17PM -0700, Eric W. Biederman wrote:
> "Daniel P. Berrange" <berrange at redhat.com> writes:
> 
> > One of the features that SystemD folks have asked us to fix in LXC, is
> > to make sure that /proc/sys/kernel/random/boot_id changes each time a
> > container is started.
> 
> There may be a good reason for this.  Most of the time what I have seen
> of kernel requests from the direction of SystemD is that while there may
> be a real problem but usually their imagined solution is not a
> particularly good solution.  So a description of the problem is needed.
> 
> Justifying something with just SystemD wants this is a good way to get
> a nack.
> 
> > The current semantics are that this file produces a new random UUID each
> > time the host OS is booted. Obviously each time we start a container now,
> > they just see the host's random boot_id, so from a container's POV this
> > does not change each time it starts.
> 
> That is correct.  As I recall the contract with boot_id is to provide
> a unique per boot value to assist in dealing with boots etc.  I seem
> to recall emacs uses the combination of hostname+boot_id to help
> generate unique lock files names.
> 
> I would definitely need a refresher on how boot_id is used in practice
> by applications other than SystemD before I could suggest a good design.

This post seems to describe what emacs wants boot_id for:

  http://marc.info/?l=linux-kernel&m=93613053109494&w=2

With this info, I think emacs inside a container would expect the boot_id
to change each time the container is started, so they can detect stale
locks from an emacs instance in a previous boot of the container.

Daniel
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