linux-kernel - Re: [lxc-devel] device namespaces

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Date:	Wed, 24 Sep 2014 16:37:40 +0000
From:	Serge Hallyn <serge.hallyn@...ntu.com>
To:	riya khanna <riyakhanna1983@...il.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	LXC development mailing-list 
	<lxc-devel@...ts.linuxcontainers.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Tejun Heo <tj@...nel.org>,
	Seth Forshee <seth.forshee@...onical.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [lxc-devel] device namespaces

Isolation is provided by the devices cgroup.  You want something more
than isolation.

Quoting riya khanna (riyakhanna1983@...il.com):
> My use case for having device namespaces is device isolation. Isn't what
> namespaces are there for (as I understand)? Not everything should be
> accessible (or even visible) from a container all the time (we have seen
> people come up with different use cases for this). However, bind-mounting
> takes away this flexibility. I agree that assigning fixed device numbers is
> clearly not a long-term solution. Emulation for safe and flexible
> multiplexing, like you suggested either using CUSE/FUSE or something like
> devpts, is what I'm exploring.
> 
> On Wed, Sep 24, 2014 at 12:04 AM, Eric W. Biederman <ebiederm@...ssion.com>
> wrote:
> 
> > riya khanna <riyakhanna1983@...il.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.
> >
> > Eric
> > --
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