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

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Date:	Mon, 11 Apr 2016 20:23:00 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security\@kernel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security\@ubuntu.com \>\> security" <security@...ntu.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Willy Tarreau <w@....eu>,
	Aurelien Jarno <aurelien@...el32.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jann Horn <jann@...jh.net>, Greg KH <greg@...ah.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>, Florian Weimer <fw@...eb.enyo.de>
Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

"H. Peter Anvin" <hpa@...or.com> writes:

> On April 11, 2016 5:10:47 PM PDT, ebiederm@...ssion.com wrote:
>>Linus Torvalds <torvalds@...ux-foundation.org> writes:
>>
>>> On Mon, Apr 11, 2016 at 4:37 PM, Eric W. Biederman
>>> <ebiederm@...ssion.com> 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