Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

[H. Peter Anvin]

On April 11, 2016 5:10:47 PM PDT, ebiederm@xxxxxxxxxxxx wrote:
>Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
>
>> On Mon, Apr 11, 2016 at 4:37 PM, Eric W. Biederman
>> <ebiederm@xxxxxxxxxxxx> wrote:
>>>
>>> My practical concern if we worked through the implementation details
>>> would be how would it interact with people who bind mount
>/dev/pts/ptmx
>>> on top of /dev/ptmx.  We might get some strange new errors.
>>
>> Yes, please don't let's play "clever" games. The semantics should be
>> fairly straightforward.
>
>Actually for me this is about keeping the semantics simpler, and coming
>up with a higher performance implementation.
>
>A dentry that does an automount is already well defined.
>
>Making the rule that accessing /dev/ptmx causes an automount of
>/dev/pts/ptmx on top of the device node at /dev/ptmx is really simple,
>with no special games.  It also makes it more obvious to userspace what
>is going on.  AKA allows userspace to know which superblock does an
>open
>ptmx master tty belongs to (and it happens in a backwards and forwards
>compatible way).
>
>My only concern is with this very minor change in semantics will
>anything care.  I need to implement and test to find out.
>
>I think I see an implementation that Al won't grumble too loudly about.
>
>Anyway I am going to try this and see what I can see.
>
>Eric

Why bother with an automount?  You can look up ../ptmx from the devpts get_super method and just do the bind mount once.  No fuss, no muss.  What's wrong with that?
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