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 12:07:45 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>
Cc:	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>,
	"Eric W. Biederman" <ebiederm@...ssion.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 04/12/16 11:12, Linus Torvalds wrote:
>
> So what I want to happen is to "just make /dev/ptmx work".  Get rid of
> the broken "single instance" crap. The only reason it exists is
> exactly because /dev/ptmx does not work.
>
> I think the current situation is completely and utterly broken. We
> should never have done what we did. I want to *fix* the kernel, not
> add random new magic crap.
>

Agreed.  As far as I'm concerned, there seem to be two realistic 
variants, talking semantically as opposed to implementation-wise:

1. Change the default mode of /dev/pts/ptmx to default to 0666, and make 
/dev/ptmx have the effective semantics of the symlink which userspace 
and userdev/devramfs should have provided all along.

2. Make /dev/ptmx simply look up the pts superblock from its path and 
then act like /dev/pts/ptmx.  In that case we can probably remove the 
ptmx device node unless the ptmxmode mount option is given (in which 
case user space probably enabled the symlink.)

	-hpa