net: Fix some corner cases in dev_can_checksum()

dev_can_checksum() incorrectly returns true in these cases:

1. The skb has both out-of-band and in-band VLAN tags and the device
   supports checksum offload for the encapsulated protocol but only with
   one layer of encapsulation.
2. The skb has a VLAN tag and the device supports generic checksumming
   but not in conjunction with VLAN encapsulation.

Rearrange the VLAN tag checks to avoid these.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 10 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index e8a8dc19365b..3c5fbfbb3181 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1696,22 +1696,18 @@ static bool can_checksum_protocol(unsigned long features, __be16 protocol)
 
 static bool dev_can_checksum(struct net_device *dev, struct sk_buff *skb)
 {
+        __be16 protocol = skb->protocol;
         int features = dev->features;
 
-        if (vlan_tx_tag_present(skb))
+        if (vlan_tx_tag_present(skb)) {
                 features &= dev->vlan_features;
-
-        if (can_checksum_protocol(features, skb->protocol))
-                return true;
-
-        if (skb->protocol == htons(ETH_P_8021Q)) {
+        } else if (protocol == htons(ETH_P_8021Q)) {
                 struct vlan_ethhdr *veh = (struct vlan_ethhdr *)skb->data;
-                if (can_checksum_protocol(dev->features & dev->vlan_features,
-                                          veh->h_vlan_encapsulated_proto))
-                        return true;
+                protocol = veh->h_vlan_encapsulated_proto;
+                features &= dev->vlan_features;
         }
 
-        return false;
+        return can_checksum_protocol(features, protocol);
 }
 
 /**