vlan: more careful checksum features handling

When combining real_dev's features and vlan_features, simple
bitwise AND is used. This doesn't work well for checksum
offloading features as if one set has NETIF_F_HW_CSUM and the
other NETIF_F_IP_CSUM and/or NETIF_F_IPV6_CSUM, we end up with
no checksum offloading. However, from the logical point of view
(how can_checksum_protocol() works), NETIF_F_HW_CSUM contains
the functionality of NETIF_F_IP_CSUM and NETIF_F_IPV6_CSUM so
that the result should be IP/IPV6.

Add helper function netdev_intersect_features() implementing
this logic and use it in vlan_dev_fix_features().
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -3153,6 +3153,20 @@ const char *netdev_drivername(const struct net_device *dev);
 
 void linkwatch_run_queue(void);
 
+static inline netdev_features_t netdev_intersect_features(netdev_features_t f1,
+							  netdev_features_t f2)
+{
+	if (f1 & NETIF_F_GEN_CSUM)
+		f1 |= (NETIF_F_ALL_CSUM & ~NETIF_F_GEN_CSUM);
+	if (f2 & NETIF_F_GEN_CSUM)
+		f2 |= (NETIF_F_ALL_CSUM & ~NETIF_F_GEN_CSUM);
+	f1 &= f2;
+	if (f1 & NETIF_F_GEN_CSUM)
+		f1 &= ~(NETIF_F_ALL_CSUM & ~NETIF_F_GEN_CSUM);
+
+	return f1;
+}
+
 static inline netdev_features_t netdev_get_wanted_features(
 	struct net_device *dev)
 {

net/8021q/vlan_dev.c

@@ -678,9 +678,9 @@ static netdev_features_t vlan_dev_fix_features(struct net_device *dev,
 	struct net_device *real_dev = vlan_dev_priv(dev)->real_dev;
 	netdev_features_t old_features = features;
 
-	features &= real_dev->vlan_features;
+	features = netdev_intersect_features(features, real_dev->vlan_features);
 	features |= NETIF_F_RXCSUM;
-	features &= real_dev->features;
+	features = netdev_intersect_features(features, real_dev->features);
 
 	features |= old_features & NETIF_F_SOFT_FEATURES;
 	features |= NETIF_F_LLTX;