[lxc-devel] device namespaces

[riya khanna]

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 at xmission.com>
wrote:

> riya khanna <riyakhanna1983 at 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.
>
> Eric
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