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

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Date:	Tue, 12 Apr 2016 02:47:59 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 08:23:00PM -0500, Eric W. Biederman wrote:

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

Correct.  For a very good reason - the same superblock can bloody well end up
in many places; having it tied to _the_ mountpoint is just plain wrong.

You know how it would look done right?
	a) mount --after support
	b) devpts containing both /ptmx and /pts/1,...
	c) mount --type devpts --after none /dev

That's it.  And this would be way, way more useful that overlay-style unions;
it would *NOT* recurse into subdirectories.  Just a search list done right...

Not an option, unfortunately, since it obviously breaks userland setups -
even if we go ahead and implement that kind of non-recursive unions.  But
if we had a chance to design it from scratch, that would've been an interesting
option.