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

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Message-ID: <4F80B9AC.2010103@zytor.com>
Date:	Sat, 07 Apr 2012 15:03:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"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>,
	Kay Sievers <kay.sievers@...y.org>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Subject: Re: RFC: deprecating/removing the legacy mode of devpts

On 04/07/2012 02:27 PM, Ted Ts'o wrote:
> On Sat, Apr 07, 2012 at 01:02:33PM -0700, H. Peter Anvin wrote:
>> The problem is that the lack of migration at the root causes real
>> problems for the people who need the extra funkiness, and unlike BSD
>> ttys it's not an application-level change at all.
> 
> Could you explain what the issues are?  I haven't been following
> devpts all that carefully.
> 

The issue is that to make the "ptys are private to an instance of the
devpts filesystem", /dev/ptmx has to live inside the /dev/pts
filesystem.  This was traditionally not the case.  To avoid breaking
everything all at once, we have a legacy mode which supports the "all
devpts instances are the same" (effectively bind mounts) and which
support a /dev/ptmx outside /dev/pts.

The problem is that anyone who wants to take advantage of the new
functionality has to make sure *all* instances of devpts work with the
new protocol.  This means modifying your distro to:

1. Add "newinstance" and "ptmxmode" to /etc/fstab [easy]
2. Make /dev/ptmx a symlink to /dev/pts/ptmx.

2 would be easy if it wasn't for udev, which makes it very hard.  It is
not reasonable for udev to support both modes, so the *only* mode that
is reasonable for it to support is the new mode.  However, it is
increasingly obvious that if we don't force it, this will never happen.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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