netdev - [PATCH 1/2] mactap: Fix checksum errors for non-gso packets in bridge mode

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Date:	Wed, 23 Apr 2014 12:51:40 -0400
From:	Vlad Yasevich <vyasevic@...hat.com>
To:	netdev@...r.kernel.org
Cc:	daniel.lezcano@...e.fr, nightnord@...il.com, kaber@...sh.net,
	eric.dumazet@...il.com, mst@...hat.com, jasowang@...hat.com,
	Vlad Yasevich <vyasevic@...hat.com>
Subject: [PATCH 1/2] mactap: Fix checksum errors for non-gso packets in bridge mode

The following is a problematic configuration:

 VM1: virtio-net device connected to macvtap0@...0
 VM2: e1000 device connect to macvtap1@...0

The problem is is that virtio-net supports checksum offloading
and thus sends the packets to the host with CHECKSUM_PARTIAL set.
On the other hand, e1000 does not support any acceleration.

For small TCP packets (and this includes the 3-way handshake),
e1000 ends up receiving packets that only have a partial checksum
set.  This causes TCP to fail checksum validation and to drop
packets.  As a result tcp connections can not be established.

Commit 3e4f8b787370978733ca6cae452720a4f0c296b8
	macvtap: Perform GSO on forwarding path.
fixes this issue for large packets wthat will end up undergoing GSO.
This commit adds a check for the non-GSO case and attempts to
compute the checksum for partially checksummed packets in the
non-GSO case.

CC: Daniel Lezcano <daniel.lezcano@...e.fr>
CC: Patrick McHardy <kaber@...sh.net>
CC: Andrian Nord <nightnord@...il.com>
CC: Eric Dumazet <eric.dumazet@...il.com>
CC: Michael S. Tsirkin <mst@...hat.com>
CC: Jason Wang <jasowang@...hat.com>
Signed-off-by: Vlad Yasevich <vyasevic@...hat.com>
---
 drivers/net/macvtap.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index ff111a8..ba91084 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -322,6 +322,13 @@ static rx_handler_result_t macvtap_handle_frame(struct sk_buff **pskb)
 			segs = nskb;
 		}
 	} else {
+		/* If we receive a partial checksum and the tap side
+		 * doesn't support checksum offload, compute the checksum.
+		 */
+		if (skb->ip_summed == CHECKSUM_PARTIAL &&
+		    !(features & NETIF_F_ALL_CSUM) &&
+		    skb_checksum_help(skb))
+			goto drop;
 		skb_queue_tail(&q->sk.sk_receive_queue, skb);
 	}
 
-- 
1.9.0

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