Re: Stack sends oversize UDP packet to the driver

["Mahesh Bandewar (महेश बंडेवार)" <maheshb@google.com>]

On Tue, Feb 5, 2019 at 11:36 AM Michael Chan <michael.chan@broadcom.com> wrote:
>
> On Wed, Jan 30, 2019 at 5:00 PM Mahesh Bandewar (महेश बंडेवार)
> <maheshb@google.com> wrote:
> >
> > On Wed, Jan 30, 2019 at 1:07 AM Michael Chan <michael.chan@broadcom.com> wrote:
> > >
> > > On Tue, Jan 22, 2019 at 10:29 AM Mahesh Bandewar (महेश बंडेवार)
> > > <maheshb@google.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().