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

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Message-ID: <20120124201600.GB20039@us.ibm.com>
Date:	Tue, 24 Jan 2012 12:16:00 -0800
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	containers@...ts.linux-foundation.org,
	Dave Hansen <haveblue@...ibm.com>,
	linux-kernel@...r.kernel.org, Andy Whitcroft <apw@...onical.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC] fix devpts mount behavior

Serge Hallyn [serge@...lyn.com] wrote:
| 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?

With DEVPTS_MULTIPLE_INSTANCES=n, there is only _one_ (global) instance
right ? Why would we need a 'pts/ptmx' node ? To keep the symlink (i.e
user space scripts) valid for both single and multiple instance cases ?

Sukadev

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