[PATCH 11/11][v3]: Enable multiple instances of devpts

[sukadev at us.ibm.com]

H. Peter Anvin [hpa at zytor.com] wrote:
> Alan Cox wrote:
>> O> We can't, really, because it will open the global ptmx.  This is an 
>>> unfortunate side effect of the backwards-compatibility code.
>>>
>>> This is also why I don't like the bind mount; the symlink option has the 
>>> nice property that f*ckups are more obvious.
>> It's asking for trouble with existing systems and users that
>> upgrade. /dev/ptmx should remain a proper device file for the non
>> container case.
>
> I did say that as being the desired *eventual* goal.
>
>> Should /dev/ptmx give you a node in the 'master' pty namespace or a node
>> in your current containers pty namespace ?
>
> Well, since there is no "current containers pty namespace" per se, it will 
> give you a node in the default (initial) pty namespace unless the bind
> mount is set up.

But that node will not be accessible if there is a newinstance mount
without the bind mount ? IOW

	1. mount -t devpts -o newinstance lxcpts /dev/pts
	2. mount -o bind /dev/pts/ptmx /dev/ptmx

If both #1 and #2 or neither happen there is no problem.

If #1 is NOT followed by #2, ptys break in new namespace.

An open of /dev/ptmx in this case will allocate a pty in the
initial namespace, but since #1 is complete, we lookup the
pty (/dev/pts/7) in the new namespace and fail.