linux-kernel - Re: RFC: deprecating/removing the legacy mode of devpts

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Date:	Sun, 08 Apr 2012 11:00:52 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kay Sievers <kay@...y.org>
CC:	"Ted Ts'o" <tytso@....edu>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Subject: Re: RFC: deprecating/removing the legacy mode of devpts

On 04/08/2012 12:30 AM, Kay Sievers wrote:
> 
> I don't think it has much to do with udev, it follows 100%
> instructions from the kernel, and it does not create the device node
> that is in the way here.
> 
> Udev does not name any device in the system since a long time. Since
> quite a while it has not even the code to do mknod() and requires
> devtmpfs. The device node part of udev these day is limited to manage
> device node permissions and creating additional symlinks. All device
> node creation happens inside the kernel itself - where it belongs -
> and not in userspace.
> 
> If the default behaviour of /dev/pts/* should be changed, the kernel
> should be changed to support the multi-instance mode right away
> without involving userspace. We better do not require userspace to
> gain any knowledge about such stuff. I'm confident, that we should not
> add more, or require to support multiple alternative ways of handling
> kernel internals in userspace.
> 
> So, I think we either remove the '/sys/class/tty/ptmx' device from the
> system, and let the devpts code create the symlink in the 'devtmpfs'
> filesystem, or alternatively the '/sys/class/tty/ptmx' device supports
> the multi-instance mode itself, instead of requiring a symlink. Such
> stuff belongs entirely into the kernel these day. Anything else seems
> to just ask for trouble.
> 

OK, this seems very reasonable, and something that could (and probably
*has to*) be done as an atomic change... maybe.

There is no way for /dev/ptmx to support the multiinstance mode since it
is located outside any devpts filesystem; it *has* to be inside the
devpts filesystem in order to function... if that wasn't the case this
whole thing would have been trivial from the get-go.

Greg, do you have any insights in what would have to be necessary
mechanics to make this.

	-hpa

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