netdev-linux: Favour inner packet for multi-encapsulated TSO.

Previously if an OVS configuration nested multiple layers of UDP tunnels
like VXLAN or GENEVE on top of each other through netdev-linux
interfaces, the vnet header would be incorrectly set to the outermost
UDP tunnel layer instead of the intermediary tunnel layer.

This resulted in the middle UDP tunnel not checksum offloading properly.

Fixes: 3337e6d ("userspace: Enable L4 checksum offloading by default.")
Reported-by: David Marchand <[email protected]>
Signed-off-by: Mike Pattrick <[email protected]>

lib/netdev-linux.c

@@ -7247,14 +7247,23 @@ netdev_linux_prepend_vnet_hdr(struct dp_packet *b, int mtu)
             vnet->csum_offset = (OVS_FORCE __virtio16) __builtin_offsetof(
                                     struct tcp_header, tcp_csum);
         } else if (dp_packet_hwol_l4_is_udp(b)) {
-            struct udp_header *udp_hdr = dp_packet_l4(b);
+            /* Favour the inner packet when indicating checksum offsets. */
+            void *l3_off = dp_packet_inner_l3(b);
+            void *l4_off = dp_packet_inner_l4(b);
+
+            if (!l3_off || !l4_off) {
+                l3_off = dp_packet_l3(b);
+                l4_off = dp_packet_l4(b);
+            }
+            struct udp_header *udp_hdr = l4_off;
+
             ovs_be16 csum = 0;
 
             if (dp_packet_hwol_is_ipv4(b)) {
-                const struct ip_header *ip_hdr = dp_packet_l3(b);
+                const struct ip_header *ip_hdr = l3_off;
                 csum = ~csum_finish(packet_csum_pseudoheader(ip_hdr));
             } else if (dp_packet_hwol_tx_ipv6(b)) {
-                const struct ovs_16aligned_ip6_hdr *ip6_hdr = dp_packet_l3(b);
+                const struct ovs_16aligned_ip6_hdr *ip6_hdr = l4_off;
                 csum = ~csum_finish(packet_csum_pseudoheader6(ip6_hdr));
             }