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

[Glauber Costa]

On 08/31/2012 02:15 AM, 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.
> 
> 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.
> 
> For hacks and controversial ideas FUSE is good because it makes it
> someone else's problem and it means it isn't something we have to
> support in the kernel for the indefinite future.  At the same time in
> general a FUSE solution does not really solve anything it just sort of
> papers over a problem.
> 

Two main points here:

I would love to see this solved transparently by the kernel. In all
previous attempts, this seemed to fall under "controversial idea" as you
described above.

It is hard to call it "consensus", indeed, but this idea gets quite a
lot of traction (I am far from the only one), specifically because it
allows us to present userspace with something in the time being.
Besides, the interface is just the standard /proc interface. If the
kernel eventually does something here, we just have to replace it.

About mechanism for communicating with applications, this is completely
orthogonal. Applications will keep requiring the standard interface for
a while. We can, however, do a lot better with a library for those,
using some other non-standard mechanism that helps containers and
non-containers alike. But this is in addition to, at least for the time
being.