net: more accurate network taps in transmit path

dev_queue_xmit_nit() should be called right before ndo_start_xmit()
calls or we might give wrong packet contents to taps users :

Packet checksum can be changed, or packet can be linearized or
segmented, and segments partially sent for the later case.

Also a memory allocation can fail and packet never really hit the
driver entry point.
Reported-by: NJamie Gloudon <jamie.gloudon@gmail.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2213,9 +2213,6 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 		if (dev->priv_flags & IFF_XMIT_DST_RELEASE)
 			skb_dst_drop(skb);
 
-		if (!list_empty(&ptype_all))
-			dev_queue_xmit_nit(skb, dev);
-
 		features = netif_skb_features(skb);
 
 		if (vlan_tx_tag_present(skb) &&
@@ -2250,6 +2247,9 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 			}
 		}
 
+		if (!list_empty(&ptype_all))
+			dev_queue_xmit_nit(skb, dev);
+
 		skb_len = skb->len;
 		rc = ops->ndo_start_xmit(skb, dev);
 		trace_net_dev_xmit(skb, rc, dev, skb_len);
@@ -2272,6 +2272,9 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 		if (dev->priv_flags & IFF_XMIT_DST_RELEASE)
 			skb_dst_drop(nskb);
 
+		if (!list_empty(&ptype_all))
+			dev_queue_xmit_nit(nskb, dev);
+
 		skb_len = nskb->len;
 		rc = ops->ndo_start_xmit(nskb, dev);
 		trace_net_dev_xmit(nskb, rc, dev, skb_len);