block dangerous passthrough operation

[Sagi Grimberg]

>> It's not like just anyone can do this kind of craziness. It's like a
>> soft equivalent of pulling a cable. There are usually access
>> restrictions for untrusted people.
> 
> Yes, there are.  But that doesn't change us having to deal with the
> fallout.

Maybe we should do the inverse?
Whitelist what userspace is known to be allowed to do, and anything
outside of that, we don't prevent but rather log a big fat log message
that the driver behavior is unexpected?

That would prevent loss of flexibility, but still tell users that this
is not intended to be supported...