Hi Dave,

I stumbled across a reproducible kernel panic while playing around with
busy_poll on a Linux 4.9.86 kernel. There's an unfortunate interaction
between init_dummy_netdev, which doesn't bother to fill in netdev_ops, and
sk_busy_loop, which assumes netdev_ops is a valid pointer.

To reproduce on the device under test (DUT), I did:

  $ ip addr show dev wlan0
  8: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq [...]
      inet brd scope global wlan0
  $ sysctl -w net.core.busy_read=50
  $ nc -l 5001

Then transmitted some data to this socket from a second host:

  $ echo "foo" | nc 5001

The DUT immediately hits a kernel panic.

I've attached a patch that applies cleanly to the 4.9.87 stable release.
This fix isn't necessary for net/net-next (ndo_busy_poll was removed in
linux-4.11), but a further backport of this commit is likely required for
any stable releases older than linux-4.5.

I hope this is the right way to raise something like this. I couldn't find
a clear answer from the -stable and netdev howtos for bugs against features
that no longer exist in mainline.


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