Re: [RFC] fix devpts mount behavior

[Al Viro <viro@ZenIV.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)