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Date:	Mon, 15 Sep 2014 15:06:59 -0700
From:	Kamal Mostafa <>
Cc:	Eric Dumazet <>,
	Vlad Yasevich <>,
	"David S. Miller" <>,
	Kamal Mostafa <>
Subject: [PATCH 3.13 009/187] net: Correctly set segment mac_len in skb_segment(). -stable review patch.  If anyone has any objections, please let me know.


From: Vlad Yasevich <>

[ Upstream commit fcdfe3a7fa4cb74391d42b6a26dc07c20dab1d82 ]

When performing segmentation, the mac_len value is copied right
out of the original skb.  However, this value is not always set correctly
(like when the packet is VLAN-tagged) and we'll end up copying a bad

One way to demonstrate this is to configure a VM which tags
packets internally and turn off VLAN acceleration on the forwarding
bridge port.  The packets show up corrupt like this:
16:18:24.985548 52:54:00:ab:be:25 > 52:54:00:26:ce:a3, ethertype 802.1Q
(0x8100), length 1518: vlan 100, p 0, ethertype 0x05e0,
        0x0000:  8cdb 1c7c 8cdb 0064 4006 b59d 0a00 6402 ...|...d@.....d.
        0x0010:  0a00 6401 9e0d b441 0a5e 64ec 0330 14fa ..d....A.^d..0..
        0x0020:  29e3 01c9 f871 0000 0101 080a 000a e833)....q.........3
        0x0030:  000f 8c75 6e65 7470 6572 6600 6e65 7470 ...unetperf.netp
        0x0040:  6572 6600 6e65 7470 6572 6600 6e65 7470 erf.netperf.netp
        0x0050:  6572 6600 6e65 7470 6572 6600 6e65 7470 erf.netperf.netp
        0x0060:  6572 6600 6e65 7470 6572 6600 6e65 7470 erf.netperf.netp

This also leads to awful throughput as GSO packets are dropped and
cause retransmissions.

The solution is to set the mac_len using the values already available
in then new skb.  We've already adjusted all of the header offset, so we
might as well correctly figure out the mac_len using skb_reset_mac_len().
After this change, packets are segmented correctly and performance
is restored.

CC: Eric Dumazet <>
Signed-off-by: Vlad Yasevich <>
Signed-off-by: David S. Miller <>
Signed-off-by: Kamal Mostafa <>
 net/core/skbuff.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f7eca05..808a270 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2872,9 +2872,9 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 		tail = nskb;
 		__copy_skb_header(nskb, head_skb);
-		nskb->mac_len = head_skb->mac_len;
 		skb_headers_offset_update(nskb, skb_headroom(nskb) - headroom);
+		skb_reset_mac_len(nskb);
 		skb_copy_from_linear_data_offset(head_skb, -tnl_hlen,
 						 nskb->data - tnl_hlen,

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