User-Agent: K-9 Mail for Android
In-Reply-To: <87oa9fbsag.fsf@x220.int.ebiederm.org>
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> <AC547146-B027-4AE2-9B26-111F0180C0B9@zytor.com> <877fg3emy7.fsf@x220.int.ebiederm.org> <CA+55aFzo=kJ57pWURnv9NxfQSaKuHP1yMCD9mPcMCu0M7Jf6ag@mail.gmail.com> <87oa9fbsag.fsf@x220.int.ebiederm.o!
 rg>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain;
 charset=UTF-8
Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Mon, 11 Apr 2016 18:06:48 -0700
To: ebiederm@xmission.com, Linus Torvalds <torvalds@linux-foundation.org>
CC: Andy Lutomirski <luto@amacapital.net>, 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>,
        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: <61F170A6-5A56-4BF5-BFB5-FC62AB6F3612@zytor.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1605
Lines: 39

On April 11, 2016 5:10:47 PM PDT, ebiederm@xmission.com wrote:
>Linus Torvalds <torvalds@linux-foundation.org> writes:
>
>> On Mon, Apr 11, 2016 at 4:37 PM, Eric W. Biederman
>> <ebiederm@xmission.com> wrote:
>>>
>>> My practical concern if we worked through the implementation details
>>> would be how would it interact with people who bind mount
>/dev/pts/ptmx
>>> on top of /dev/ptmx.  We might get some strange new errors.
>>
>> Yes, please don't let's play "clever" games. The semantics should be
>> fairly straightforward.
>
>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).
>
>My only concern is with this very minor change in semantics will
>anything care.  I need to implement and test to find out.
>
>I think I see an implementation that Al won't grumble too loudly about.
>
>Anyway I am going to try this and see what I can see.
>
>Eric

Why bother with an automount?  You can look up ../ptmx from the devpts get_super method and just do the bind mount once.  No fuss, no muss.  What's wrong with that?
-- 
Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.