net: introduce skb_transport_header_was_set()

We have skb_mac_header_was_set() helper to tell if mac_header
was set on a skb. We would like the same for transport_header.

__netif_receive_skb() doesn't reset the transport header if already
set by GRO layer.

Note that network stacks usually reset the transport header anyway,
after pulling the network header, so this change only allows
a followup patch to have more precise qdisc pkt_len computation
for GSO packets at ingress side.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1492,6 +1492,11 @@ static inline void skb_set_inner_network_header(struct sk_buff *skb,
 	skb->inner_network_header += offset;
 }
 
+static inline bool skb_transport_header_was_set(const struct sk_buff *skb)
+{
+	return skb->transport_header != ~0U;
+}
+
 static inline unsigned char *skb_transport_header(const struct sk_buff *skb)
 {
 	return skb->head + skb->transport_header;
@@ -1580,6 +1585,11 @@ static inline void skb_set_inner_network_header(struct sk_buff *skb,
 	skb->inner_network_header = skb->data + offset;
 }
 
+static inline bool skb_transport_header_was_set(const struct sk_buff *skb)
+{
+	return skb->transport_header != NULL;
+}
+
 static inline unsigned char *skb_transport_header(const struct sk_buff *skb)
 {
 	return skb->transport_header;

net/core/dev.c

@@ -3352,7 +3352,8 @@ static int __netif_receive_skb(struct sk_buff *skb)
 	orig_dev = skb->dev;
 
 	skb_reset_network_header(skb);
-	skb_reset_transport_header(skb);
+	if (!skb_transport_header_was_set(skb))
+		skb_reset_transport_header(skb);
 	skb_reset_mac_len(skb);
 
 	pt_prev = NULL;

net/core/skbuff.c

@@ -260,6 +260,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 	skb->end = skb->tail + size;
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
 	skb->mac_header = ~0U;
+	skb->transport_header = ~0U;
 #endif
 
 	/* make sure we initialize shinfo sequentially */
@@ -328,6 +329,7 @@ struct sk_buff *build_skb(void *data, unsigned int frag_size)
 	skb->end = skb->tail + size;
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
 	skb->mac_header = ~0U;
+	skb->transport_header = ~0U;
 #endif
 
 	/* make sure we initialize shinfo sequentially */