netdev - Re: Stack sends oversize UDP packet to the driver

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Message-ID: <CAF2d9jiMMez62n7U+k02V7sRp9Q-usFC1X0-h8aqq-BtjmtVmA@mail.gmail.com>
Date:   Wed, 30 Jan 2019 17:00:19 -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 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.

> Thanks.