On Thu, Apr 18, 2019 at 12:53:00AM +0100, Al Viro wrote:

Why would you want to duplicate large chunks of anything?
The above is not even compile-tested, but...  I can put
together a patch if you wish.  Or am I missing something
here?

Actually, there's another broken part - pppox.  And that one
is nastier - you've just lost everything outside of protocol
family ioctls on those sockets.  Try e.g.  SIOCGIFMTU in 32bit
process on those.

We really can't reuse native ->ioctl() there - proto family
->compat_ioctl() *must* return -ENOIOCTLCMD on everything it
doesn't handle.