V2: just check napi->dev->netdev_ops instead of getting clever with the
netdev registration state.
Original cover letter:
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 assumed 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 172.16.122.6/23 brd 172.16.123.255 scope global wlan0
$ sysctl -w net.core.busy_read=50
$ nc -l 172.16.122.6 5001
Then transmitted some data to this socket from a second host:
$ echo "foo" | nc 172.16.122.6 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 on how to handle bugs in features
that no longer exist in mainline.
Thanks,
Josh
