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

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Message-ID: <20160412013436.GN25498@ZenIV.linux.org.uk>
Date:	Tue, 12 Apr 2016 02:34:36 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	"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 Mon, Apr 11, 2016 at 07:10:47PM -0500, Eric W. Biederman wrote:
> 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).

_What_ dentry?  Which filesystem would that be done to?  Whatever we have
on /dev?  Or we suddenly get the fucking dentry operations change when
dentry is attached to magical cdev inode?