From: ebiederm@xmission.com (Eric W. Biederman)
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Greg KH <greg@kroah.com>, Jiri Slaby <jslaby@suse.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Aurelien Jarno <aurelien@aurel32.net>,
        Andy Lutomirski <luto@amacapital.net>,
        Florian Weimer <fw@deneb.enyo.de>, Al Viro <viro@zeniv.linux.org.uk>,
        Serge Hallyn <serge.hallyn@ubuntu.com>, Jann Horn <jann@thejh.net>,
        "security\@kernel.org" <security@kernel.org>,
        "security\@ubuntu.com \>\> security" <security@ubuntu.com>,
        security@debian.org, Willy Tarreau <w@1wt.eu>,
        linux-kernel@vger.kernel.org
References: <43AD2BA7-B594-4299-95F3-D86FD38AF21B@zytor.com>
	<87egexpf4o.fsf@x220.int.ebiederm.org>
	<CA+55aFw9Bg+Zh_T4zP487n3ieaxoMHgZ_nNJVdpSR4kQK9gQ9w@mail.gmail.com>
	<1CB621EF-1647-463B-A144-D611DB150E15@zytor.com>
	<20151208223135.GA8352@kroah.com>
	<87oae0h2bo.fsf@x220.int.ebiederm.org> <56677DE3.5040705@zytor.com>
	<20151209012311.GA11794@kroah.com>
	<84B136DF-55E4-476A-9CB2-062B15677EE5@zytor.com>
	<20151209013859.GA12442@kroah.com> <20151209083225.GA30452@1wt.eu>
	<87wpskyds7.fsf_-_@x220.int.ebiederm.org>
	<566F1CD7.20502@hurleysoftware.com>
	<87y4cq6t1j.fsf@x220.int.ebiederm.org>
Date: Sat, 19 Dec 2015 22:11:50 -0600
In-Reply-To: <87y4cq6t1j.fsf@x220.int.ebiederm.org> (Eric W. Biederman's
	message of "Sat, 19 Dec 2015 15:13:28 -0600")
Message-ID: <87a8p54v3t.fsf@x220.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance
Sender: linux-kernel-owner@vger.kernel.org
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ebiederm@xmission.com (Eric W. Biederman) writes:

>>> In that system ptys simply did not work after boot when I tested
>>> associating /dev/ptmx with the first mount of the devpts filesystem.
>>
>> Assuming userspace isn't broken by that patch, is a fixed association
>> with first mount otherwise an acceptable solution for magic /dev/ptmx
>> (where /dev/ptmx is not a symlink to /dev/pts/ptmx)?
>
> I do not believe a fixed association with the first mount is an
> acceptable solution for implementing  /dev/ptmx in association with
> a change to cause mount of devpts to be an independent filesystem.
> Such an association fails to be backwards compatible with existing
> userspace, and it is extremely fragile.

Ugh.  After reviewing the userspace code that mounts devpts we have
to do use a magic /dev/ptmx to solve the issue we are trying to solve.

The fragility of detecting the primary system devpts seems solvable.

CentOS5 and openwrt-15.05 mount devpts, unmount devpts,
then mount devpts again.  So a rule of mouting the internal devpts if it
isn't mounted would work for those.

CentOS6 uses switch_root and moves it's early mount of devpts onto the
primary root, and then because devpts is also in /etc/fstab tries and
fails to mount devpts once more at the same location.  Implying
newinstance will make that mounting devpts twice.  That sounds solvable
but I don't see a clean way of detecting that case yet.

Ugh.

I am going to pound my head up against what is needed to find the
primary system mount of devpts for a bit more and see if I can solve
that.  Otherwise this exercise is pointless.

Eric

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