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Date: Wed, 6 Feb 2019 20:51:37 -0800 From: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.com> To: Michael Chan <michael.chan@...adcom.com> Cc: Daniel Axtens <dja@...ens.net>, Netdev <netdev@...r.kernel.org>, David Miller <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Willem de Bruijn <willemb@...gle.com> Subject: Re: Stack sends oversize UDP packet to the driver On Tue, Feb 5, 2019 at 11:36 AM Michael Chan <michael.chan@...adcom.com> wrote: > > On Wed, Jan 30, 2019 at 5:00 PM Mahesh Bandewar (महेश बंडेवार) > <maheshb@...gle.com> wrote: > > > > On Wed, Jan 30, 2019 at 1:07 AM Michael Chan <michael.chan@...adcom.com> wrote: > > > > > > On Tue, Jan 22, 2019 at 10:29 AM Mahesh Bandewar (महेश बंडेवार) > > > <maheshb@...gle.com> wrote: > > > > > > > > > > > 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().
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