linux-kernel - Re: [RFC] fix devpts mount behavior

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Message-ID: <20120124010758.GJ23916@ZenIV.linux.org.uk>
Date:	Tue, 24 Jan 2012 01:07:58 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Serge Hallyn <serge.hallyn@...onical.com>,
	Dave Hansen <haveblue@...ibm.com>, sukadev@...ux.vnet.ibm.com,
	Andy Whitcroft <apw@...onical.com>,
	Matt Helsley <matthltc@...ibm.com>,
	linux-kernel@...r.kernel.org, containers@...ts.linux-foundation.org
Subject: Re: [RFC] fix devpts mount behavior

On Mon, Jan 23, 2012 at 04:41:25PM -0800, Linus Torvalds wrote:

> Right. I think the opportunity for problems should be pretty small.
> 
> And it's not like the pty itself wouldn't continue to work - it's just
> that programs like /usr/bin/tty wouldn't be able to *find* it.
> 
> Although who knows - maybe there is some other subtle interaction.

FWIW, the subtle and nasty part in all that is  that you can mknod /dev/ptmx
and it *will* work, refering to the "initial" instance.  That's what
concerns me about the chroot scenarios -
	mknod /jail/dev/ptmx c 5 2
	mkdir /jail/dev/pts
	mount -t devpts /jail/dev/pts
	chroot /jail
works fine right now, but with that change behaviour will be all wrong -
opening /dev/ptmx inside of jail will grab you a pts, all right, but
it will *not* show up in (jail) /dev/pts/* as it does with the current
kernel.

Note that if you replace that mknod with symlink pts/ptmx /jail/dev/ptmx
the things will keep working.  However, that will _only_ work for kernels
with DEVPTS_MULTIPLE_INSTANCES - without it you won't get ptmx inside
devpts (which is arguably wrong, BTW)
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