netdev - Re: veth regression with "don’t modify ip_summed; doing so treats packets with bad checksums as good."

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Date:	Thu, 24 Mar 2016 16:56:03 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	netdev <netdev@...r.kernel.org>, Evan Jones <ej@...njones.ca>,
	Vijay P <vijayp@...ayp.ca>, Cong Wang <cwang@...pensource.com>
Subject: Re: veth regression with "don’t modify ip_summed; doing so treats packets with bad checksums as good."

On Thu, Mar 24, 2016 at 3:01 PM, Ben Greear <greearb@...delatech.com> wrote:
> I have an application that creates two pairs of veth devices.
>
> a <-> b       c <-> d
>
> b and c have a raw packet socket opened on them and I 'bridge' frames
> between b and c to provide network emulation (ie, configurable delay).
>

IIUC, you create two raw sockets in order to bridge these two veth pairs?
That is, to receive packets on one socket and deliver packets on the other?


>
> I put IP 1.1.1.1/24 on a, 1.1.1.2/24 on d, and then create a UDP connection
> (using policy based routing to ensure frames are sent on the appropriate
> interfaces).
>
> This is user-space only app, and kernel in this case is completely
> unmodified.
>
> The commit below breaks this feature:  UDP frames are sniffed on both a and
> d ports
> (in both directions), but the UDP socket does not receive frames.
>
> Using normal ethernet ports, this network emulation feature works fine, so
> it is
> specific to VETH.
>
> A similar test with just sending UDP between a single veth pair:  e <-> f
> works fine.  Maybe it has something to do with raw packets?
>

Yeah, I have the same feeling. Could you trace kfree_skb() to see
where these packets are dropped? At UDP layer?

Thanks.