netdev - [PATCH net] net: change skb->mac_header when Generic XDP calls adjust

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Message-ID: <394385d4-796a-9ada-8f01-53a52d73a418@solarflare.com>
Date:   Tue, 19 Sep 2017 18:45:56 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     David Miller <davem@...emloft.net>
CC:     netdev <netdev@...r.kernel.org>
Subject: [PATCH net] net: change skb->mac_header when Generic XDP calls
 adjust_head

Since XDP's view of the packet includes the MAC header, moving the start-
 of-packet with bpf_xdp_adjust_head needs to also update the offset of the
 MAC header (which is relative to skb->head, not to the skb->data that was
 changed).
Without this, tcpdump sees packets starting from the old MAC header rather
 than the new one, at least in my tests on the loopback device.

Fixes: b5cdae3291f7 ("net: Generic XDP")
Signed-off-by: Edward Cree <ecree@...arflare.com>
---
 net/core/dev.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index fb766d9..9a2254f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3892,6 +3892,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 		__skb_pull(skb, off);
 	else if (off < 0)
 		__skb_push(skb, -off);
+	skb->mac_header += off;

 	switch (act) {
 	case XDP_REDIRECT: