[Devel] [RFC][PATCH 3/4]: Enable multiple mounts of /dev/pts

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at cs.columbia.edu):
>
>
> Serge E. Hallyn wrote:
>> Quoting Oren Laadan (orenl at cs.columbia.edu):
>>>
>>> Serge E. Hallyn wrote:
>>>> Quoting Pavel Emelyanov (xemul at openvz.org):
>>>>> Serge E. Hallyn wrote:
>>>>>> Quoting Pavel Emelyanov (xemul at openvz.org):
>>>>>>> sukadev at us.ibm.com wrote:
>>>>>>>> From: Sukadev Bhattiprolu <sukadev at us.ibm.com>
>>>>>>>> Subject: [RFC][PATCH 3/4]: Enable multiple mounts of /dev/pts
>>> [SNIP]
>>>
>>>>>>> That stuff becomes very very similar to that in proc :)
>>>>>>> Makes sense to consolidate. Maybe...
>>>>>> Yeah, and the mqns that Cedric sent too.  I think Cedric said he'd
>>>>>> started an a patch implementing a helper.  Cedric?
>>>>> Mmm. I wanted to send one small objection to Cedric's patches with 
>>>>> mqns,
>>>>> but the thread was abandoned by the time I decided to do-it-right-now.
>>>>>
>>>>> So I can put it here: forcing the CLONE_NEWNS is not very good, since
>>>>> this makes impossible to push a bind mount inside a new namespace, 
>>>>> which
>>>>> may operate in some chroot environment. But this ability is heavily
>>>> Which direction do you want to go?  I'm wondering whether mounts
>>>> propagation can address it.
>>>> Though really, I think you're right - we shouldn't break the kernel
>>>> doing CLONE_NEWMQ or CLONE_NEWPTS without CLONE_NEWNS, so we shouldn't
>>>> force the combination.
>>>>> exploited in OpenVZ, so if we can somehow avoid forcing the NEWNS flag
>>>>> that would be very very good :) See my next comment about this issue.
>>>>>
>>>>>> Pavel, not long ago you said you were starting to look at tty and pty
>>>>>> stuff - did you have any different ideas on devpts virtualization, or
>>>>>> are you ok with this minus your comments thus far?
>>>>> I have a similar idea of how to implement this, but I didn't thought
>>>>> about the details. As far as this issue is concerned, I see no reasons
>>>>> why we need a kern_mount-ed devtpsfs instance. If we don't make such,
>>>>> we may safely hold the ptsns from the superblock and be happy. The
>>>>> same seems applicable to the mqns, no?
>>>> But the current->nsproxy->devpts->mnt is used in several functions in
>>>> patch 3.
>>>>> The reason I have the kern_mount-ed instance of proc for pid namespaces
>>>>> is that I need a vfsmount to flush task entries from, but allowing
>>>>> it to be NULL (i.e. no kern_mount, but optional user mounts) means
>>>>> handing all the possible races, which is too heavy. But do we actually
>>>>> need the vfsmount for devpts and mqns if no user-space mounts exist?
>>>>>
>>>>> Besides, I planned to include legacy ptys virtualization and console
>>>>> virtualizatin in this namespace, but it seems, that it is not present
>>>>> in this particular one.
>>>> I had been thinking the consoles would have their own ns, since there's
>>>> really nothing linking them,  but there really is no good reason why
>>>> userspace should ever want them separate.  So I'm fine with combining
>>>> them.
>>> If you want to run something like an X server inside each container
>>> (eg each container holds a desktop session of a different user), then
>>> you need a separate virtual-console namespace for each container.
>> Ok, but whether the consoles and devpts are unshared with the same
>> cloneflag or not isn't an issue, right?
>
> true. (I misread your comment.)
> (
> modulo that we are additional-clone-flags-challenged ...)

Right, plus the fact that the number of clone flags involved becomes
almost obscene.  Let's see if Pavel and Suka have a preference, since
one of them seems likely to end up coding it  :)

>>> (yes, X per-se needs to provide remote display as opposed to use
>>> local hardware; see http://www.ncl.cs.columbia.edu/research/thinc/)

Nice, by the way :)

>> -serge

thanks,
-serge
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