tcp: fix for splice receive when used with software LRO

If an skb has nr_frags set to zero but its frag_list is not empty (as
it can happen if software LRO is enabled), and a previous
tcp_read_sock has consumed the linear part of the skb, then
__skb_splice_bits:

(a) incorrectly reports an error and

(b) forgets to update the offset to account for the linear part

Any of the two problems will cause the subsequent __skb_splice_bits
call (the one that handles the frag_list skbs) to either skip data,
or, if the unadjusted offset is greater then the size of the next skb
in the frag_list, make tcp_splice_read loop forever.
Signed-off-by: NOctavian Purdila <opurdila@ixiacom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -1292,12 +1292,14 @@ static int __skb_splice_bits(struct sk_buff *skb, unsigned int *offset,
 {
 	unsigned int nr_pages = spd->nr_pages;
 	unsigned int poff, plen, len, toff, tlen;
-	int headlen, seg;
+	int headlen, seg, error = 0;
 
 	toff = *offset;
 	tlen = *total_len;
-	if (!tlen)
+	if (!tlen) {
+		error = 1;
 		goto err;
+	}
 
 	/*
 	 * if the offset is greater than the linear part, go directly to
@@ -1339,7 +1341,8 @@ static int __skb_splice_bits(struct sk_buff *skb, unsigned int *offset,
 		 * just jump directly to update and return, no point
 		 * in going over fragments when the output is full.
 		 */
-		if (spd_fill_page(spd, virt_to_page(p), plen, poff, skb))
+		error = spd_fill_page(spd, virt_to_page(p), plen, poff, skb);
+		if (error)
 			goto done;
 
 		tlen -= plen;
@@ -1369,7 +1372,8 @@ static int __skb_splice_bits(struct sk_buff *skb, unsigned int *offset,
 		if (!plen)
 			break;
 
-		if (spd_fill_page(spd, f->page, plen, poff, skb))
+		error = spd_fill_page(spd, f->page, plen, poff, skb);
+		if (error)
 			break;
 
 		tlen -= plen;
@@ -1382,7 +1386,10 @@ static int __skb_splice_bits(struct sk_buff *skb, unsigned int *offset,
 		return 0;
 	}
 err:
-	return 1;
+	/* update the offset to reflect the linear part skip, if any */
+	if (!error)
+		*offset = toff;
+	return error;
 }
 
 /*