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

[Serge Hallyn <serge.hallyn-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>]

Quoting Eric W. Biederman (ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org):
> Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org> writes:
> 
> > On 08/31/2012 04:13 AM, Eric W. Biederman wrote:
> >> "Daniel P. Berrange" <berrange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> writes:
> >> 
> >>> On Thu, Aug 30, 2012 at 03:15:17PM -0700, Eric W. Biederman wrote:
> >>>> "Daniel P. Berrange" <berrange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 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.
> >>>
> >>> SystemD records log messages for all system services in their journal.
> >>> They can show you all log messages for the current service execution,
> >>> all log messages for a service since system boot, or all log messsages
> >>> ever. The boot_id value is used as a unique tag to allow grouping of
> >>> the log messages per system boot. When we run systemd inside a container
> >>> we want to get that grouping of log messages generated by services inside
> >>> the container, to take account of the container boot, not the host boot.
> >>> Hence the desire to have the boot_id value reflect when a container is
> >>> booted.
> >> 
> >> Since SystemD post-dates containers and since the logging feature is not
> >> currently in wide use that use case is completely non-persuasive.
> >> 
> >> So far this just sounds like a plain SystemD bug and something that can
> >> be easily changed at this point in time.
> >> 
> >> It has been a long time but my fuzzy memory says that the originial
> >> boot_id justification was based on use cases that could not be solved
> >> any other way.
> >> 
> >> My memory says it was this thread https://lkml.org/lkml/1999/5/31/233
> >> that inspired the implementation of boot_id.  However reading the
> >> current emacs source code it appears emacs gave up before boot_id
> >> was implemented and stats /var/run/random-seed (which we seem to
> >> have removed) or looks in wtmp or utmp for the latest boot record.
> >> 
> >> I did a quick grep through the binaries on my system and I could not
> >> find anything using /proc/sys/random/boot_id.
> >> 
> >> That suggests to me that the proper solution is to actually just remove
> >> boot_id.
> >> 
> >> Hmm.  And then there is other interesting detail.  What should boot_id
> >> return after the processes have migrated from one system to another.
> >> 
> >
> > Since this would be a per-boot id, this clearly has to be carried over
> > with migration, along with all the tons of data we already carry.
> 
> The twist of course is what does a boot mean.  If we are really after
> machine boots than the current behavior is correct.
> 
> Looking back in the archives the desired behavior appears to be a value
> that can be used to see if a pid value must be stale.
> 
> As a stale pid detector boot_id is pretty lousy.  Pids can still be
> reused.
> 
> Still a role as a stale pid detector makes it clear which namespace
> boot_id should be in and how we should treat boot_id upon migration.
> 
> You can only serve as a stale pid detector if you are in the pid
> namespace.
> 
> So at this point patches are welcome.  Hopefully with a summary
> of the discussion.

I don't understand why this should be provided by the kernel.  Especially
given that we've proven that everyone really wants this to be per-container
as well.

So why not just have init, on startup, create a /run/boot_id file, perhaps
by sha1summing the time at which it started perhaps plus some nonce?