tcp: Make SACK code to split only at mss boundaries

Sadly enough, this adds possible divide though we try to avoid
it by checking one mss as common case.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1248,20 +1248,39 @@ static int tcp_match_skb_to_sack(struct sock *sk, struct sk_buff *skb,
 {
 	int in_sack, err;
 	unsigned int pkt_len;
+	unsigned int mss;
 
 	in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
 		  !before(end_seq, TCP_SKB_CB(skb)->end_seq);
 
 	if (tcp_skb_pcount(skb) > 1 && !in_sack &&
 	    after(TCP_SKB_CB(skb)->end_seq, start_seq)) {
-
+		mss = tcp_skb_mss(skb);
 		in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);
 
-		if (!in_sack)
+		if (!in_sack) {
 			pkt_len = start_seq - TCP_SKB_CB(skb)->seq;
-		else
+			if (pkt_len < mss)
+				pkt_len = mss;
+		} else {
 			pkt_len = end_seq - TCP_SKB_CB(skb)->seq;
-		err = tcp_fragment(sk, skb, pkt_len, skb_shinfo(skb)->gso_size);
+			if (pkt_len < mss)
+				return -EINVAL;
+		}
+
+		/* Round if necessary so that SACKs cover only full MSSes
+		 * and/or the remaining small portion (if present)
+		 */
+		if (pkt_len > mss) {
+			unsigned int new_len = (pkt_len / mss) * mss;
+			if (!in_sack && new_len < pkt_len) {
+				new_len += mss;
+				if (new_len > skb->len)
+					return 0;
+			}
+			pkt_len = new_len;
+		}
+		err = tcp_fragment(sk, skb, pkt_len, mss);
 		if (err < 0)
 			return err;
 	}