On Tue, Feb 5, 2019 at 11:36 AM Michael Chan [off-list ref] wrote:

On Wed, Jan 30, 2019 at 5:00 PM Mahesh Bandewar (महेश बंडेवार)
[off-list ref] wrote:

quoted

On Wed, Jan 30, 2019 at 1:07 AM Michael Chan [off-list ref] wrote:

quoted

On Tue, Jan 22, 2019 at 10:29 AM Mahesh Bandewar (महेश बंडेवार)
[off-list ref] wrote:

quoted

The idea behind the fix is very simple and it is to create a dst-only
(unregistered) device with a very low MTU and use it instead of 'lo'
while invalidating the dst. This would make it *not* forward packets
to driver which might need fragmentation.

We tested the 2 patches many times and including an overnight test.  I
can confirm that the oversize UDP packets are no longer seen with the
patches applied.  However, I don't see the blackhole xmit function
getting called to free the SKBs though.

Thanks for the confirmation Michael. The blackhole device mtu is
really small, so I would assume the fragmentation code dropped those
packets before calling the xmit function (in ip_fragment), you could
verify that with icmp counters.

I've looked at this a little more.  The blackhole_dev is not IFF_UP |
IFF_RUNNING, right?  May be that's why the packets are never getting
to the xmit function?

Yes, so I added those two flags and ended up writing a test-module for
the device (which I will include while posting the patch-series).
However, adding those flags is also not sufficient since the qdisc is
initialized to noop_qdisc so qdisc enqueue will drop packets before
hitting the ndo_start_xmit().