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Date:   Tue, 12 Feb 2019 00:55:46 -0800
From:   Michael Chan <>
To:     Mahesh Bandewar (महेश बंडेवार) 
Cc:     Daniel Axtens <>, Netdev <>,
        David Miller <>,
        Eric Dumazet <>,
        Willem de Bruijn <>
Subject: Re: Stack sends oversize UDP packet to the driver

On Fri, Feb 8, 2019 at 12:26 PM Mahesh Bandewar (महेश बंडेवार)
<> wrote:
> On Wed, Feb 6, 2019 at 8:51 PM Mahesh Bandewar (महेश बंडेवार)
> <> wrote:
> >
> > On Tue, Feb 5, 2019 at 11:36 AM Michael Chan <> wrote:
> > > 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().
> I have another version of the fix (with help from Eric) and this
> should hit the .ndo_start_xmit() of the blackhole_dev. I'm adding
> these flags during the setup and then calling dev_activate() to change
> noop qdisc to null qdisc. Please give this patch set a try and let me
> know if the blackhole_dev xmit path gets exercised in your test
> scenario.

The new version still works in the sense that no oversize packets are
seen in the NIC driver's xmit function.  But I still don't see any
packets hitting the blackhole's xmit function.  I'm not 100% sure but
I think the blackhole dev has no IP address and so the UDP packets are
dropped in ip_finish_output2() because there is no neigh.  Something
like that.

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