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

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Message-ID: <m1k44gj1cu.fsf@fess.ebiederm.org>
Date:	Tue, 24 Jan 2012 12:24:01 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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

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.

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

Eric
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