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

[Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

On 09/04/2012 09:18 PM, Serge E. Hallyn wrote:
> Quoting Glauber Costa (glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org):
>> On 09/04/2012 07:25 PM, Serge Hallyn wrote:
>>> Quoting Glauber Costa (glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org):
>>>> On 09/04/2012 06:44 PM, Serge Hallyn wrote:
>>>>> 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?
>>>>>
>>>> Why shouldn't it provided by the kernel?, is the real question
>>>
>>> Because it's not the right place.  The origin of this thread proves that
>>> people want a per-init, not per-kernel, value.
>>>
>>
>> Not all files provided by the kernel are "per-kernel". /proc/self is
>> full of per-namespace stuff.
>>
>>>> The way I see it, every file we need to setup from the outside is a
>>>> hassle. Among many other things, it is just asking for duplication of
>>>> efforts among multiple userspaces.
>>>>
>>>> netns does this for its proc files. The only reason we don't do it for
>>>> cgroups-driven file, is that the semantics is very ill-defined. For this
>>>> file, it doesn't seem to be the case.
>>>
>>> But it is the case.  How do you intend to have the kernel decide what
>>> value to put in there for a process in a container, or in a chroot?
>>>
>>
>> one value per pidns.
> 
> ok.  (So should it be called /proc/pidns_uuid?  Well, whatever.  No
> objection from me - thanks.)
> 
> -serge
> 
For completeness, I believe it should live in the same place it lives
today, and become a symlink from /proc/self/boot_id.

Consistent with what we have today for other values like this.