[RFC][PATCH 7/8]: Auto-create ptmx node when mounting devpts

[Eric W. Biederman]

sukadev at us.ibm.com writes:

> H. Peter Anvin [hpa at zytor.com] wrote:
>> Alan Cox wrote:
>>>> auto-created, than supporting mknod(2) inside the devpts filesystem. It's 
>>>> not a matter of "changing the user space"; it's a matter of what makes 
>>>> most sense inside the kernel.
>>> Having an extra node with different permissions suddenely appear without
>>> warning isn't I think good behaviour.
>>
>> Hm.  Given that the single-instance mode is the backwards compatibility 
>> mode (and it's accessible from outside the filesystem), it probably makes 
>> sense to suppress creating this device node when *not* applying the "newns" 
>> option, or whatever we want to call it.
>
> I had the new ptmx node only in 'multi-mount' mode initially. But if users
> want the multi-mount semantics, /dev/ptmx must be a symlink. If its a symlink,
> we break in the single-mount case (which does not have the ptmx node and
> we don't support mknod in pts).

Then have user space make it a file bind mount instead of symlink.
That should address all of the backwards compatibility concerns, and
allow us to only create it when open.

>>> I'm open to being convinced and the
>>> other problems with that code are more pressing.
>
> Yes, I will look at the latest in linux-next and the ->driver_data
> approach.
>
> But just to confirm, we do want try and keep single-mount semantics.

Definitely.

Eric