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 18:06:48 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	ebiederm@...ssion.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security@...ntu.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

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?
-- 
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