linux-kernel - Re: Per-instance devpts

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Date:	Sat, 2 Aug 2008 22:08:00 -0700
From:	sukadev@...ibm.com
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...l.org>, serue@...ibm.com,
	matthltc@...ibm.com, Pavel Emelyanov <xemul@...nvz.org>,
	Containers <containers@...ts.osdl.org>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg KH <greg@...ah.com>, sukadev@...ibm.com
Subject: Re: Per-instance devpts

H. Peter Anvin [hpa@...or.com] wrote:
> Since the issue of PTY namespaces came up (and was rejected) back in April, 
> I have thought a little bit about changing ptys to be tied directly into a 
> devpts instance.  devpts would then be a "normal" filesystem, which can be 
> mounted multiple times (or not at all).  pty's would then become private to 
> a devpts instance.

Sorry, I thought we were going with a complete device namespace - since that
would address other devices as well and would avoid the following user-space
issue. 

I guess this issue came up in OLS recently and have been looking into this
again.  I have some helper patches to explore multiple mounts of devpts
without namespace stuff and can send them out in a couple of days.

>
> This is what it would appear would have to change, and I'd like to get 
> people's feeing for the user-space impact:
>
> 1. /dev/ptmx would have to change to a symlink, ptmx -> pts/ptmx.

IIRC, /dev/tty also needs a similar symlink.

> 2. Permissions on /dev/ptmx would not be persistent, and would have to
>    be set via devpts mount options (unless they're 0666 root.tty, which
>    would presumably be the default.)
> 3. The /proc/sys/kernel/pty limit would be global; a per-filesystem
>    limit could be added on top or instead (presumably via a filesystem
>    mount options.)
>
> I worry #1 would have substantial user-space impact, but I don't see a way 
> around it, since there would be no obvious way to associate /dev/ptmx with 
> a filesystem.

Sukadev
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