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

[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.