linux-kernel - Re: [RFC][PATCH 0/7] Clone PTS namespace

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Date:	Wed, 9 Apr 2008 20:59:55 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Containers <containers@...ts.osdl.org>, clg@...ibm.com,
	linux-kernel@...r.kernel.org, Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [RFC][PATCH 0/7] Clone PTS namespace

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> On Tue, 2008-04-08 at 17:53 -0700, H. Peter Anvin wrote:
> > sukadev@...ibm.com wrote:
> > > Devpts namespace patchset
> > > 
> > > In continuation of the implementation of containers in mainline, we need to
> > > support multiple PTY namespaces so that the PTY index (ie the tty names) in
> > > one container is independent of the PTY indices of other containers.  For
> > > instance this would allow each container to have a '/dev/pts/0' PTY and
> > > refer to different terminals.
> > > 
> > 
> > Why do we "need" this?  There isn't a fundamental need for this to be a 
> > dense numberspace (in fact, there are substantial reasons why it's a bad 
> > idea; the only reason the namespace is dense at the moment is because of 
> > the hideously bad handing of utmp in glibc.)  Other than indicies, this 
> > seems to be a more special case of device isolation across namespaces, 
> > would that be a more useful problem to solve across the board?
> 
> In short application migration.  When you move a running applicaiton
> from one machine to another you want to be able to keep the same pseudo
> devices.
> 
> The isolation that you have noticed is also an important application and
> like the rest of the namespaces if we can solve the duplicate identifier
> problem needed to restore checkpoints we also largely solve the
> isolation problem.
> 
> This problem is much larger then ptys.  ptys are the really in your face
> aspect of it.  There are a more pseudo devices in the kernel and it is
> the device number to device mapping that we are abstracting.  So this
> really should be done as a device namespace not a pty namespace.
> 
> I would be happy if the first version of the device namespace could not
> map anything but pty's (assuming an incremental implementation path).  I
> really don't think we should do a special case for each kind of device.

Sounds like we're all agreed on this and just doing
s/CLONE_NEWPTS/CLONE_NEWDEV/ on the current patchset suffices for now.
But,

> Oh and just skimming the patch summary I'm pretty certain this
> implementation breaks /sys/class/tty/ptyXX/uevent.  Which is another
> reason why it would be good to have a single device namespace.  So we
> only to capture one more namespace and figure out how to deal with it
> when mounting sysfs. 

Feh, so of course sysfs would have the most interactions for a device
namespace, but now we have pty, network, and user namespace all needing
some sort of sysfs solution.  For a quickfix for
CONFIG_USER_SCHED+CONFIG_USER_NS, I just moved /sys/kernel/uids/<uid>
to /sys/kernel/uids/<userns_address>/<uid>.  But what would be a *good*
general solution?

ln -s /sys /proc/self/sys?

-serge
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