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

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Message-ID: <CACKFLimWHQA_52D_6FZDOeJftXt-YqFOdADMDFTFuUkK__Ay2Q@mail.gmail.com>
Date:   Tue, 5 Feb 2019 11:35:56 -0800
From:   Michael Chan <michael.chan@...adcom.com>
To:     Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.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 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?