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

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Date:	Sat, 9 Apr 2016 17:01:27 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Peter Hurley <peter@...leysoftware.com>,
	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.com>,
	Aurelien Jarno <aurelien@...el32.net>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated
 devpts via path lookup

On Sat, Apr 9, 2016 at 3:37 PM, H. Peter Anvin <hpa@...or.com> wrote:
>
> On the flipside, if we were to allow ourselves to break userspace, at this point I would suggest making /dev/pts/ptmx have a different device number and make the legacy /dev/ptmx print a warning message, after which it can at least eventually be deleted.

You don't need a different device number.

The /dev/pts/ptmx file may look like it's the same node as /dev/ptmx,
but it is trivial to recognize as the pts one:

     if (dentry->d_sb->s_magic == DEVPTS_SUPER_MAGIC)

and you're done.

But nobody actually uses /dev/pts/ptmx, because it has never had sane
permissions.

So the fact is, /dev/ptmx is what people use, and we're not breaking userspace.

But when we fix bad semantics (and always just looking up the initial
pts mount really is crazy semantics) that doesn't mean that we have to
bend over backwards to not make the changed semantics visible. We
don't _break_ user space, but we also don't care about some random
test-program that checks for particular semantics.

And I can pretty much _guarantee_ that nobody has ever done the "let's
bind-mount a 'ptmx' node in a /dev directory, and then expect that to
bind to some _other_ pts thing than the one in /dev/pts/".

Except as a test-program, or possibly as a "why the f*ck doesn't this
work? Oh, I need to use the single-instance thing because the
multi-instance pts thing is broken. Damn shitty implementation".

                Linus