udplite: fast-path computation of checksum coverage

Commit 903ab86d of 1 March this year ("udp: Add
lockless transmit path") introduced a new fast TX path that broke the checksum
coverage computation of UDP-lite, which so far depended on up->len (only set
if the socket is locked and 0 in the fast path).

Fixed by providing both fast- and slow-path computation of checksum coverage.
The latter can be removed when UDP(-lite)v6 also uses a lockless transmit path.
Reported-by: Thomas Volkert <thomas@homer-conferencing.com>
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/udplite.h

@@ -66,21 +66,22 @@ static inline int udplite_checksum_init(struct sk_buff *skb, struct udphdr *uh)
 	return 0;
 }
 
-static inline int udplite_sender_cscov(struct udp_sock *up, struct udphdr *uh)
+/* Slow-path computation of checksum. Socket is locked. */
+static inline __wsum udplite_csum_outgoing(struct sock *sk, struct sk_buff *skb)
 {
+	const struct udp_sock *up = udp_sk(skb->sk);
 	int cscov = up->len;
+	__wsum csum = 0;
 
+	if (up->pcflag & UDPLITE_SEND_CC) {
 		/*
-	 * Sender has set `partial coverage' option on UDP-Lite socket
+		 * Sender has set `partial coverage' option on UDP-Lite socket.
+		 * The special case "up->pcslen == 0" signifies full coverage.
 		 */
-	if (up->pcflag & UDPLITE_SEND_CC)    {
 		if (up->pcslen < up->len) {
-		/* up->pcslen == 0 means that full coverage is required,
-		 * partial coverage only if  0 < up->pcslen < up->len */
-			if (0 < up->pcslen) {
+			if (0 < up->pcslen)
 				cscov = up->pcslen;
-			}
-			uh->len = htons(up->pcslen);
+			udp_hdr(skb)->len = htons(up->pcslen);
 		}
 		/*
 		 * NOTE: Causes for the error case  `up->pcslen > up->len':
@@ -93,13 +94,6 @@ static inline int udplite_sender_cscov(struct udp_sock *up, struct udphdr *uh)
 		 *       illegal, we fall back to the defaults here.
 		 */
 	}
-	return cscov;
-}
-
-static inline __wsum udplite_csum_outgoing(struct sock *sk, struct sk_buff *skb)
-{
-	int cscov = udplite_sender_cscov(udp_sk(sk), udp_hdr(skb));
-	__wsum csum = 0;
 
 	skb->ip_summed = CHECKSUM_NONE;     /* no HW support for checksumming */
 
@@ -115,16 +109,21 @@ static inline __wsum udplite_csum_outgoing(struct sock *sk, struct sk_buff *skb)
 	return csum;
 }
 
+/* Fast-path computation of checksum. Socket may not be locked. */
 static inline __wsum udplite_csum(struct sk_buff *skb)
 {
-	struct sock *sk = skb->sk;
-	int cscov = udplite_sender_cscov(udp_sk(sk), udp_hdr(skb));
+	const struct udp_sock *up = udp_sk(skb->sk);
 	const int off = skb_transport_offset(skb);
-	const int len = skb->len - off;
+	int len = skb->len - off;
 
+	if ((up->pcflag & UDPLITE_SEND_CC) && up->pcslen < len) {
+		if (0 < up->pcslen)
+			len = up->pcslen;
+		udp_hdr(skb)->len = htons(up->pcslen);
+	}
 	skb->ip_summed = CHECKSUM_NONE;     /* no HW support for checksumming */
 
-	return skb_checksum(skb, off, min(cscov, len), 0);
+	return skb_checksum(skb, off, len, 0);
 }
 
 extern void	udplite4_register(void);