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

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Message-ID: <20120124220247.GA26353@hallyn.com>
Date:	Tue, 24 Jan 2012 22:02:47 +0000
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.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>,
	sukadev@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC] fix devpts mount behavior

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> Al Viro <viro@...IV.linux.org.uk> writes:
> 
> > 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)
> 
> For testing I would recommend looking at the distro chroot build cases.

Do you have a specific example in mind?  I would expect build chroots
generally don't mount a devpts.

> It looks like relatively recent udev still creates /dev/ptmx and does

Boy, it does, and it's stubborn about it.  Removing the /lib/udev/rules.d
entry doesn't stop it.  (this is after I've had an init job replace the
devtmpfs-created ptmx entry with a symlink)

So current distros (well, Ubuntu and Fedora at least) would need to at least
(a) fix udev, (b) change the default devpts mount (done from initramfs) to
add ptmxmode=666, (c) (if not done in udev) create the /dev/ptmx symlink.

For safety I'd recommend creating /dev/pts/ptmx with
DEVPTS_MULTIPLE_INSTANCES=n (or dropping that support), and by default
setting ptmxmode to 666 as that's what udev does.

> not create the symlink.  So we might get into the awkward situation of
> /dev/ptmx not matching /dev/pts/ptmx with something as simple as
> initramfs mounting /dev/pts and then initscripts mounting /dev/pts.

That shouldn't matter with a symlink, though it is sloppy.

-serge
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