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Date: Wed, 24 Sep 2014 16:38:16 +0000
From: Serge Hallyn <serge.hallyn@ubuntu.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: LXC development mailing-list 
	<lxc-devel@lists.linuxcontainers.org>,
        linux-kernel@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>,
        fuse-devel <fuse-devel@lists.sourceforge.net>,
        Tejun Heo <tj@kernel.org>, Seth Forshee <seth.forshee@canonical.com>
Subject: Re: [lxc-devel] device namespaces
Message-ID: <20140924163816.GE3865@ubuntumail>
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Quoting Eric W. Biederman (ebiederm@xmission.com):
> riya khanna <riyakhanna1983@gmail.com> writes:
> 
> > (Please pardon multiple emails, artifact of merging all separate conversations)
> >
> > Thanks for your feedback! 
> >
> > Letting the kernel know about what devices a container could access (based on
> > device cgroups) and having devtmpfs in the kernel create device nodes for a
> > container that map to corresponding CUSE nodes is what I thought of. For
> > example, "echo 29:0 > /proc/<pid>/devices" would prepare a virtual framebuffer
> > (based on real fb0 SCREENINFO properties) for this process provided permissions
> > allow this operation. To view the framebuffer, the CUSE based virtual device
> > would talk to the actual hardware. Since namespaces would have different view of
> > the underlying devices, "sysfs" has to made aware of this as well. 
> >
> > Please let me know your inputs. Thanks again!
> 
> The solution hugely depends on what you are trying to do with it.
> 
> The situation today is that device nodes are slowly fading out.  In
> another 20 years linux may not have any device nodes at all.
> 
> Therefore the question becomes what are you trying to support.
> 
> If it is just filtering of existing device nodes.  We can do a pretty
> good approximation with bind mounts.
> 
> If you want to emulate a device you can use normal fuse (not cuse).
> As normal fuse file will support arbitrary ioctls.
> 
> There are a few cases where it is desirable to emulate what devpts
> does for allowing arbitrary users to creating virtual devices in the
> kernel.  Loop devices in particular.
> 
> Ultimately given the existence of device hotplug I don't see any call
> for being able to create device nodes with well known device numbers
> (fundamentally what a device namespace would be about).
> 
> The conversation last year was about people wanting to multiplex devices
> that don't have multiplexer support in the kernel.  If that is your
> desire I think it is entirely reasonable to device type by device type
> add support for multiplexing that device type to the kernel, or
> potentially just use fuse or cuse to implement your multiplexer in
> userspace but that has the potential to be unusably slow.

It would be helpful to have a list of devices that may want that
multiplexing.  Is it really just loop and graphics drivers?