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

Serge Hallyn serge.hallyn at canonical.com
Fri Aug 31 13:25:36 UTC 2012


Quoting Eric W. Biederman (ebiederm at xmission.com):
> "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.
> 
> There is also a question of uptime.
> 
> > There seems to be general agreement that, aside from the PID directories,
> > changes to data in  proc should be done by a FUSE filesystem overlay of
> > some kind.
> 
> No.  I have yet to see a justification for using FUSE in containers on
> top of proc files.
> 
> I have seen a lot of bad ideas suggested like hacking /proc/cpuinfo
> instead of providing a proper mechanism to tell applications how
> parallel they can/should be.

Should we be talking about a new set of library functions to gather info
like available memory and cpus, etc?  The library functions could internally
take into account the per-host procfiles, cgroup info, and, as the need
arises, new sources of information.

-serge


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