net: Make skb_segment not to compute checksum if network controller supports checksumming

Problem:
TCP checksum in the output path is not being offloaded during GSO
in the following case:
The network driver does not support scatter-gather but supports
checksum offload with NETIF_F_HW_CSUM.

Cause:
skb_segment calls skb_copy_and_csum_bits if the network driver
does not announce NETIF_F_SG. It does not check if the driver
supports NETIF_F_HW_CSUM.
So for devices which might want to offload checksum but do not support SG
there is currently no way to do so if GSO is enabled.

Solution:
In skb_segment check if the network controller does checksum and if so
call skb_copy_bits instead of skb_copy_and_csum_bits.

Testing:
Without the patch, ran iperf TCP traffic with NETIF_F_HW_CSUM enabled
in the network driver. Observed the TCP checksum offload is not happening
since the skbs received by the driver in the output path have
skb->ip_summed set to CHECKSUM_NONE.

With the patch ran iperf TCP traffic and observed that TCP checksum
is being offloaded with skb->ip_summed set to CHECKSUM_PARTIAL.
Also tested with the patch by disabling NETIF_F_HW_CSUM in the driver
to cover the newly introduced if-else code path in skb_segment.

Link: https://lore.kernel.org/netdev/CA+FuTSeYGYr3Umij+Mezk9CUcaxYwqEe5sPSuXF8jPE2yMFJAw@mail.gmail.comSigned-off-by: Yadu Kishore <kyk.segfault@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -3926,14 +3926,21 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 			goto perform_csum_check;
 
 		if (!sg) {
-			if (!nskb->remcsum_offload)
-				nskb->ip_summed = CHECKSUM_NONE;
-			SKB_GSO_CB(nskb)->csum =
-				skb_copy_and_csum_bits(head_skb, offset,
-						       skb_put(nskb, len),
-						       len, 0);
-			SKB_GSO_CB(nskb)->csum_start =
-				skb_headroom(nskb) + doffset;
+			if (!csum) {
+				if (!nskb->remcsum_offload)
+					nskb->ip_summed = CHECKSUM_NONE;
+				SKB_GSO_CB(nskb)->csum =
+					skb_copy_and_csum_bits(head_skb, offset,
+							       skb_put(nskb,
+								       len),
+							       len, 0);
+				SKB_GSO_CB(nskb)->csum_start =
+					skb_headroom(nskb) + doffset;
+			} else {
+				skb_copy_bits(head_skb, offset,
+					      skb_put(nskb, len),
+					      len);
+			}
 			continue;
 		}