net: use skb_is_gso_sctp() instead of open-coding

As well as the basic conversion, I noticed that a lot of the
SCTP code checks gso_type without first checking skb_is_gso()
so I have added that where appropriate.

Also, document the helper.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/segmentation-offloads.txt

@@ -155,7 +155,10 @@ Therefore, any code in the core networking stack must be aware of the
 possibility that gso_size will be GSO_BY_FRAGS and handle that case
 appropriately.
 
-There are a couple of helpers to make this easier:
+There are some helpers to make this easier:
+
+ - skb_is_gso(skb) && skb_is_gso_sctp(skb) is the best way to see if
+   an skb is an SCTP GSO skb.
 
  - For size checks, the skb_gso_validate_*_len family of helpers correctly
    considers GSO_BY_FRAGS.

net/core/skbuff.c

@@ -4904,7 +4904,7 @@ static unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
 			thlen += inner_tcp_hdrlen(skb);
 	} else if (likely(shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6))) {
 		thlen = tcp_hdrlen(skb);
-	} else if (unlikely(shinfo->gso_type & SKB_GSO_SCTP)) {
+	} else if (unlikely(skb_is_gso_sctp(skb))) {
 		thlen = sizeof(struct sctphdr);
 	}
 	/* UFO sets gso_size to the size of the fragmentation

net/sched/act_csum.c

@@ -350,7 +350,7 @@ static int tcf_csum_sctp(struct sk_buff *skb, unsigned int ihl,
 {
 	struct sctphdr *sctph;
 
-	if (skb_is_gso(skb) && skb_shinfo(skb)->gso_type & SKB_GSO_SCTP)
+	if (skb_is_gso(skb) && skb_is_gso_sctp(skb))
 		return 1;
 
 	sctph = tcf_csum_skb_nextlayer(skb, ihl, ipl, sizeof(*sctph));

net/sctp/input.c

@@ -106,6 +106,7 @@ int sctp_rcv(struct sk_buff *skb)
 	int family;
 	struct sctp_af *af;
 	struct net *net = dev_net(skb->dev);
+	bool is_gso = skb_is_gso(skb) && skb_is_gso_sctp(skb);
 
 	if (skb->pkt_type != PACKET_HOST)
 		goto discard_it;
@@ -123,8 +124,7 @@ int sctp_rcv(struct sk_buff *skb)
 	 * it's better to just linearize it otherwise crc computing
 	 * takes longer.
 	 */
-	if ((!(skb_shinfo(skb)->gso_type & SKB_GSO_SCTP) &&
-	     skb_linearize(skb)) ||
+	if ((!is_gso && skb_linearize(skb)) ||
 	    !pskb_may_pull(skb, sizeof(struct sctphdr)))
 		goto discard_it;
 
@@ -135,7 +135,7 @@ int sctp_rcv(struct sk_buff *skb)
 	if (skb_csum_unnecessary(skb))
 		__skb_decr_checksum_unnecessary(skb);
 	else if (!sctp_checksum_disable &&
-		 !(skb_shinfo(skb)->gso_type & SKB_GSO_SCTP) &&
+		 !is_gso &&
 		 sctp_rcv_checksum(net, skb) < 0)
 		goto discard_it;
 	skb->csum_valid = 1;
@@ -1218,7 +1218,7 @@ static struct sctp_association *__sctp_rcv_lookup_harder(struct net *net,
 	 * issue as packets hitting this are mostly INIT or INIT-ACK and
 	 * those cannot be on GSO-style anyway.
 	 */
-	if ((skb_shinfo(skb)->gso_type & SKB_GSO_SCTP) == SKB_GSO_SCTP)
+	if (skb_is_gso(skb) && skb_is_gso_sctp(skb))
 		return NULL;
 
 	ch = (struct sctp_chunkhdr *)skb->data;

net/sctp/inqueue.c

@@ -170,7 +170,7 @@ next_chunk:
 
 		chunk = list_entry(entry, struct sctp_chunk, list);
 
-		if ((skb_shinfo(chunk->skb)->gso_type & SKB_GSO_SCTP) == SKB_GSO_SCTP) {
+		if (skb_is_gso(chunk->skb) && skb_is_gso_sctp(chunk->skb)) {
 			/* GSO-marked skbs but without frags, handle
 			 * them normally
 			 */

net/sctp/offload.c

@@ -45,7 +45,7 @@ static struct sk_buff *sctp_gso_segment(struct sk_buff *skb,
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
 	struct sctphdr *sh;
 
-	if (!(skb_shinfo(skb)->gso_type & SKB_GSO_SCTP))
+	if (!skb_is_gso_sctp(skb))
 		goto out;
 
 	sh = sctp_hdr(skb);