[ 26/48] net: Correctly set segment mac_len in skb_segment().

From: Willy Tarreau
Date: Sun Nov 16 2014 - 17:05:00 EST

2.6.32-longterm review patch. If anyone has any objections, please let me know.


From: Vlad Yasevich <vyasevic@xxxxxxxxxx>

[ 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 <edumazet@xxxxxxxxxx>
Signed-off-by: Vlad Yasevich <vyasevic@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[wt: open-code skb_mac_len() as 2.6.32 doesn't have it]
Signed-off-by: Willy Tarreau <w@xxxxxx>
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 72ff527..b6707b8 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2573,7 +2573,6 @@ struct sk_buff *skb_segment(struct sk_buff *skb, int features)
tail = nskb;

__copy_skb_header(nskb, skb);
- nskb->mac_len = skb->mac_len;

/* nskb and skb might have different headroom */
if (nskb->ip_summed == CHECKSUM_PARTIAL)
@@ -2583,6 +2582,7 @@ struct sk_buff *skb_segment(struct sk_buff *skb, int features)
skb_set_network_header(nskb, skb->mac_len);
nskb->transport_header = (nskb->network_header +
+ nskb->mac_len = nskb->network_header - nskb->mac_header;
skb_copy_from_linear_data(skb, nskb->data, doffset);

if (fskb != skb_shinfo(skb)->frag_list)

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