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

[Eric W. Biederman]

"H. Peter Anvin" <hpa@xxxxxxxxx> writes:

> 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?

Perhaps I am reading the code wrong but as I read it that information is
simply not available in get_super.

Eric