[PATCH v1 net-next 11/14] udp: Optimise ulen tests in __udp[46]_lib_rcv().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

In __udp4_lib_rcv(), we need not call udp_hdr() unless we call
pskb_trim_rcsum().

In __udp6_lib_rcv(), we can save two unneeded conditions for every
jumbo payload that never be true.

	if (ulen > skb->len)
		goto short_packet;
	if (ulen == 0)
		ulen = skb->len;
	if (ulen < skb->len)

Note the number of tests for non-jumbo IPv6 payload is not changed.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
 net/ipv4/udp.c |  8 +++++---
 net/ipv6/udp.c | 30 +++++++++++++++++-------------
 2 files changed, 22 insertions(+), 16 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 23ebea2b84e4..eb968d20d5a8 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2325,10 +2325,12 @@ static int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable)
 	if (ulen > skb->len)
 		goto short_packet;
 
-	if (ulen < sizeof(*uh) || pskb_trim_rcsum(skb, ulen))
-		goto short_packet;
+	if (ulen < sizeof(*uh)) {
+		if (pskb_trim_rcsum(skb, ulen))
+			goto short_packet;
 
-	uh = udp_hdr(skb);
+		uh = udp_hdr(skb);
+	}
 
 	if (udp4_csum_init(skb, uh))
 		goto csum_error;
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index ee859679427a..6f5c29af4157 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -966,23 +966,27 @@ static int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable)
 	uh = udp_hdr(skb);
 
 	ulen = ntohs(uh->len);
-	if (ulen > skb->len)
-		goto short_packet;
+	if (ulen) {
+		if (ulen > skb->len)
+			goto short_packet;
 
-	/* Check for jumbo payload */
-	if (ulen == 0)
-		ulen = skb->len;
+		if (ulen < sizeof(*uh))
+			goto short_packet;
 
-	if (ulen < sizeof(*uh))
-		goto short_packet;
+		if (ulen < skb->len) {
+			if (pskb_trim_rcsum(skb, ulen))
+				goto short_packet;
 
-	if (ulen < skb->len) {
-		if (pskb_trim_rcsum(skb, ulen))
-			goto short_packet;
+			saddr = &ipv6_hdr(skb)->saddr;
+			daddr = &ipv6_hdr(skb)->daddr;
+			uh = udp_hdr(skb);
+		}
+	} else {
+		/* jumbo payload */
+		ulen = skb->len;
 
-		saddr = &ipv6_hdr(skb)->saddr;
-		daddr = &ipv6_hdr(skb)->daddr;
-		uh = udp_hdr(skb);
+		if (ulen < sizeof(*uh))
+			goto short_packet;
 	}
 
 	if (udp6_csum_init(skb, uh))
-- 
2.30.2