linux-kernel - Re: Multiple instances of devpts

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Date:	Sat, 12 Apr 2008 12:24:32 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Al Viro <viro@...IV.linux.org.uk>, sukadev@...ibm.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,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Multiple instances of devpts

Eric W. Biederman wrote:
>>
>> /dev/ptmx can be a symlink ptmx -> pts/ptmx, and we add a ptmx instance 
>> inside the devpts filesystem.  Each devpts filesystem is responsible for 
>> its own pool of ptys, with own numbering, etc.
>>
>> This does mean that entries in /dev/pts are more than just plain device 
>> nodes, which they are now (you can cp -a a device node from /dev/pts 
>> into another filesystem and it will still "just work"), but I doubt this 
>> actually matters to anyone.  If anyone cares, now I guess would be a 
>> good time to speak up.
> 
> Agreed.   That is another legitimate path.  And if all you care about is
> isolation and not dealing with the general class of problems with the
> global device number to device mapping that is sane.  I know we have
> several other virtual devices that we tend to care about but ptys are
> the real world pain point.
> 

Thinking about it further, allowing this restriction would also allow a 
whole lot of cleanups inside the pty setup, since it would eliminate the 
need to do a separate lookup to find the corresponding devpts entry in 
pty_open().  The benefit here comes from the closer coupling between the 
pty and the devpts filesystem and isn't at all related to namespaces, 
but it's a very nice side benefit.

	-hpa
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