From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated
 devpts via path lookup
To: Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>
References: <878u0s3orx.fsf_-_@x220.int.ebiederm.org>
 <1459819769-30387-1-git-send-email-ebiederm@xmission.com>
 <CA+55aFwv7d8XfeN8KvYaVcSs7V9XoHtS-1cqtkG4gF4yJwB5nA@mail.gmail.com>
 <87twjcorwg.fsf@x220.int.ebiederm.org>
 <20160409140909.42315e6d@lxorguk.ukuu.org.uk>
 <83FE8CD2-C0A2-4ADB-AEBD-8DD89AD4F88A@zytor.com>
 <87bn5ij0x1.fsf@x220.int.ebiederm.org>
 <78205895-E11D-417F-91DC-4BCA0B61A122@zytor.com>
 <CA+55aFyB7y8TfQ=_EvMAFONS6oSPs=uY88f+S6n_QWh4MkKTHg@mail.gmail.com>
 <FC521A3F-4BEB-427E-AE8E-CA54C0128AA3@zytor.com>
 <CA+55aFw62q5bjNiK7Q9dEQQEDZs5fbUxNPxDq376OZJ6JKT=Rg@mail.gmail.com>
 <CALCETrUZ=QVZbTCr3gvhSd2-j_U4R02y5fCxbddG_HsQdnuBkw@mail.gmail.com>
 <CA+55aFzs00iDkYhvFCq=AZMVcNL0+oZT4SeimTeVurJq=5ZS3A@mail.gmail.com>
 <CALCETrUQ_Oad61dYvPpYFxDBT++jhZoAT3e4n4yiu22iTNcxNQ@mail.gmail.com>
 <CALCETrUJ_KuGJoVxuUjWGCNHBNs5Vk8E68MtsWoTdURJ+=iJnQ@mail.gmail.com>
 <CA+55aFzchGpOp0R0RjM6Bsk23GWp1t76fgFi8q6FZQndHq5EtA@mail.gmail.com>
Cc: security@debian.org, "security@kernel.org" <security@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "security@ubuntu.com >> security" <security@ubuntu.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Serge Hallyn <serge.hallyn@ubuntu.com>, Willy Tarreau <w@1wt.eu>,
        Aurelien Jarno <aurelien@aurel32.net>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Jann Horn <jann@thejh.net>, Greg KH <greg@kroah.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jiri Slaby <jslaby@suse.com>, Florian Weimer <fw@deneb.enyo.de>
Message-ID: <570D4781.3070600@zytor.com>
Date: Tue, 12 Apr 2016 12:07:45 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.7.1
MIME-Version: 1.0
In-Reply-To: <CA+55aFzchGpOp0R0RjM6Bsk23GWp1t76fgFi8q6FZQndHq5EtA@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 997
Lines: 25

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