[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 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?

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

-serge
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