net: Make USO depend on CSUM offload

UDP segmentation offload inherently depends on checksum offload. It should
not be possible to disable checksum offload while leaving USO enabled.
Enforce this dependency in code.

There is a single tx-udp-segmentation feature flag to indicate support for
both IPv4/6, hence the devices wishing to support USO must offer checksum
offload for both IP versions.

Fixes: 10154dbd ("udp: Allow GSO transmit from devices with no checksum offload")
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240808-udp-gso-egress-from-tunnel-v4-1-f5c5b4149ab9@cloudflare.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -9912,6 +9912,15 @@ static void netdev_sync_lower_features(struct net_device *upper,
 	}
 }
 
+static bool netdev_has_ip_or_hw_csum(netdev_features_t features)
+{
+	netdev_features_t ip_csum_mask = NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM;
+	bool ip_csum = (features & ip_csum_mask) == ip_csum_mask;
+	bool hw_csum = features & NETIF_F_HW_CSUM;
+
+	return ip_csum || hw_csum;
+}
+
 static netdev_features_t netdev_fix_features(struct net_device *dev,
 	netdev_features_t features)
 {
@@ -9993,15 +10002,9 @@ static netdev_features_t netdev_fix_features(struct net_device *dev,
 		features &= ~NETIF_F_LRO;
 	}
 
-	if (features & NETIF_F_HW_TLS_TX) {
-		bool ip_csum = (features & (NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM)) ==
-			(NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM);
-		bool hw_csum = features & NETIF_F_HW_CSUM;
-
-		if (!ip_csum && !hw_csum) {
-			netdev_dbg(dev, "Dropping TLS TX HW offload feature since no CSUM feature.\n");
-			features &= ~NETIF_F_HW_TLS_TX;
-		}
+	if ((features & NETIF_F_HW_TLS_TX) && !netdev_has_ip_or_hw_csum(features)) {
+		netdev_dbg(dev, "Dropping TLS TX HW offload feature since no CSUM feature.\n");
+		features &= ~NETIF_F_HW_TLS_TX;
 	}
 
 	if ((features & NETIF_F_HW_TLS_RX) && !(features & NETIF_F_RXCSUM)) {
@@ -10009,6 +10012,11 @@ static netdev_features_t netdev_fix_features(struct net_device *dev,
 		features &= ~NETIF_F_HW_TLS_RX;
 	}
 
+	if ((features & NETIF_F_GSO_UDP_L4) && !netdev_has_ip_or_hw_csum(features)) {
+		netdev_dbg(dev, "Dropping USO feature since no CSUM feature.\n");
+		features &= ~NETIF_F_GSO_UDP_L4;
+	}
+
 	return features;
 }