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

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Date:	Tue, 24 Jan 2012 18:21:16 +0000
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	containers@...ts.linux-foundation.org,
	Dave Hansen <haveblue@...ibm.com>,
	linux-kernel@...r.kernel.org, Andy Whitcroft <apw@...onical.com>,
	sukadev@...ux.vnet.ibm.com
Subject: Re: [RFC] fix devpts mount behavior

Quoting Al Viro (viro@...IV.linux.org.uk):
> 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)

Should /dev/pts/ptmx be created for DEVPTS_MULTIPLE_INSTANCES=n?

-serge
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