[NET]: Clean up sk_buff walkers.

I noticed recently that, in skb_checksum(), "offset" and "start" are
essentially the same thing and have the same value throughout the
function, despite being computed differently. Using a single variable
allows some cleanups and makes the skb_checksum() function smaller,
more readable, and presumably marginally faster.

We appear to have many other "sk_buff walker" functions built on the
exact same model, so the cleanup applies to them, too. Here is a list
of the functions I found to be affected:

net/appletalk/ddp.c:atalk_sum_skb()
net/core/datagram.c:skb_copy_datagram_iovec()
net/core/datagram.c:skb_copy_and_csum_datagram()
net/core/skbuff.c:skb_copy_bits()
net/core/skbuff.c:skb_store_bits()
net/core/skbuff.c:skb_checksum()
net/core/skbuff.c:skb_copy_and_csum_bit()
net/core/user_dma.c:dma_skb_copy_datagram_iovec()
net/xfrm/xfrm_algo.c:skb_icv_walk()
net/xfrm/xfrm_algo.c:skb_to_sgvec()

OTOH, I admit I'm a bit surprised, the cleanup is rather obvious so I'm
really wondering if I am missing something. Can anyone please comment
on this?

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

5 files changed, 86 insertions, 158 deletions

diff --git a/net/appletalk/ddp.c b/net/appletalk/ddp.c
index f6a92a0b7aa6..16eda21fb38c 100644
--- a/net/appletalk/ddp.c
+++ b/net/appletalk/ddp.c
@@ -937,11 +937,11 @@ static unsigned long atalk_sum_partial(const unsigned char *data,
 static unsigned long atalk_sum_skb(const struct sk_buff *skb, int offset,
 				   int len, unsigned long sum)
 {
-	int start = skb_headlen(skb);
+	int end = skb_headlen(skb);
 	int i, copy;
 
 	/* checksum stuff in header space */
-	if ( (copy = start - offset) > 0) {
+	if ((copy = end - offset) > 0) {
 		if (copy > len)
 			copy = len;
 		sum = atalk_sum_partial(skb->data + offset, copy, sum);
@@ -953,11 +953,9 @@ static unsigned long atalk_sum_skb(const struct sk_buff *skb, int offset,
 
 	/* checksum stuff in frags */
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		int end;
+		BUG_TRAP(len >= 0);
 
-		BUG_TRAP(start <= offset + len);
-
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = offset + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			u8 *vaddr;
 			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
@@ -965,36 +963,31 @@ static unsigned long atalk_sum_skb(const struct sk_buff *skb, int offset,
 			if (copy > len)
 				copy = len;
 			vaddr = kmap_skb_frag(frag);
-			sum = atalk_sum_partial(vaddr + frag->page_offset +
-						  offset - start, copy, sum);
+			sum = atalk_sum_partial(vaddr + frag->page_offset,
+						copy, sum);
 			kunmap_skb_frag(vaddr);
 
 			if (!(len -= copy))
 				return sum;
 			offset += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
 		struct sk_buff *list = skb_shinfo(skb)->frag_list;
 
 		for (; list; list = list->next) {
-			int end;
-
-			BUG_TRAP(start <= offset + len);
+			BUG_TRAP(len >= 0);
 
-			end = start + list->len;
+			end = offset + list->len;
 			if ((copy = end - offset) > 0) {
 				if (copy > len)
 					copy = len;
-				sum = atalk_sum_skb(list, offset - start,
-						    copy, sum);
+				sum = atalk_sum_skb(list, 0, copy, sum);
 				if ((len -= copy) == 0)
 					return sum;
 				offset += copy;
 			}
-			start = end;
 		}
 	}
 
diff --git a/net/core/datagram.c b/net/core/datagram.c
index cb056f476126..e1afa7679445 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -247,8 +247,8 @@ EXPORT_SYMBOL(skb_kill_datagram);
 int skb_copy_datagram_iovec(const struct sk_buff *skb, int offset,
 			    struct iovec *to, int len)
 {
-	int start = skb_headlen(skb);
-	int i, copy = start - offset;
+	int end = skb_headlen(skb);
+	int i, copy = end - offset;
 
 	/* Copy header. */
 	if (copy > 0) {
@@ -263,11 +263,9 @@ int skb_copy_datagram_iovec(const struct sk_buff *skb, int offset,
 
 	/* Copy paged appendix. Hmm... why does this look so complicated? */
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		int end;
+		BUG_TRAP(len >= 0);
 
-		BUG_TRAP(start <= offset + len);
-
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = offset + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			int err;
 			u8  *vaddr;
@@ -277,8 +275,8 @@ int skb_copy_datagram_iovec(const struct sk_buff *skb, int offset,
 			if (copy > len)
 				copy = len;
 			vaddr = kmap(page);
-			err = memcpy_toiovec(to, vaddr + frag->page_offset +
-					     offset - start, copy);
+			err = memcpy_toiovec(to, vaddr + frag->page_offset,
+					     copy);
 			kunmap(page);
 			if (err)
 				goto fault;
@@ -286,30 +284,24 @@ int skb_copy_datagram_iovec(const struct sk_buff *skb, int offset,
 				return 0;
 			offset += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
 		struct sk_buff *list = skb_shinfo(skb)->frag_list;
 
 		for (; list; list = list->next) {
-			int end;
-
-			BUG_TRAP(start <= offset + len);
+			BUG_TRAP(len >= 0);
 
-			end = start + list->len;
+			end = offset + list->len;
 			if ((copy = end - offset) > 0) {
 				if (copy > len)
 					copy = len;
-				if (skb_copy_datagram_iovec(list,
-							    offset - start,
-							    to, copy))
+				if (skb_copy_datagram_iovec(list, 0, to, copy))
 					goto fault;
 				if ((len -= copy) == 0)
 					return 0;
 				offset += copy;
 			}
-			start = end;
 		}
 	}
 	if (!len)
@@ -323,9 +315,9 @@ static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
 				      u8 __user *to, int len,
 				      __wsum *csump)
 {
-	int start = skb_headlen(skb);
+	int end = skb_headlen(skb);
 	int pos = 0;
-	int i, copy = start - offset;
+	int i, copy = end - offset;
 
 	/* Copy header. */
 	if (copy > 0) {
@@ -344,11 +336,9 @@ static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
 	}
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		int end;
+		BUG_TRAP(len >= 0);
 
-		BUG_TRAP(start <= offset + len);
-
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = offset + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			__wsum csum2;
 			int err = 0;
@@ -360,8 +350,7 @@ static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
 				copy = len;
 			vaddr = kmap(page);
 			csum2 = csum_and_copy_to_user(vaddr +
-							frag->page_offset +
-							offset - start,
+							frag->page_offset,
 						      to, copy, 0, &err);
 			kunmap(page);
 			if (err)
@@ -373,24 +362,20 @@ static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
 			to += copy;
 			pos += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
 		struct sk_buff *list = skb_shinfo(skb)->frag_list;
 
 		for (; list; list=list->next) {
-			int end;
-
-			BUG_TRAP(start <= offset + len);
+			BUG_TRAP(len >= 0);
 
-			end = start + list->len;
+			end = offset + list->len;
 			if ((copy = end - offset) > 0) {
 				__wsum csum2 = 0;
 				if (copy > len)
 					copy = len;
-				if (skb_copy_and_csum_datagram(list,
-							       offset - start,
+				if (skb_copy_and_csum_datagram(list, 0,
 							       to, copy,
 							       &csum2))
 					goto fault;
@@ -401,7 +386,6 @@ static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
 				to += copy;
 				pos += copy;
 			}
-			start = end;
 		}
 	}
 	if (!len)
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 142257307fa2..32f087b5233e 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1045,13 +1045,13 @@ pull_pages:
 int skb_copy_bits(const struct sk_buff *skb, int offset, void *to, int len)
 {
 	int i, copy;
-	int start = skb_headlen(skb);
+	int end = skb_headlen(skb);
 
 	if (offset > (int)skb->len - len)
 		goto fault;
 
 	/* Copy header. */
-	if ((copy = start - offset) > 0) {
+	if ((copy = end - offset) > 0) {
 		if (copy > len)
 			copy = len;
 		skb_copy_from_linear_data_offset(skb, offset, to, copy);
@@ -1062,11 +1062,9 @@ int skb_copy_bits(const struct sk_buff *skb, int offset, void *to, int len)
 	}
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		int end;
+		BUG_TRAP(len >= 0);
 
-		BUG_TRAP(start <= offset + len);
-
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = offset + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			u8 *vaddr;
 
@@ -1075,8 +1073,8 @@ int skb_copy_bits(const struct sk_buff *skb, int offset, void *to, int len)
 
 			vaddr = kmap_skb_frag(&skb_shinfo(skb)->frags[i]);
 			memcpy(to,
-			       vaddr + skb_shinfo(skb)->frags[i].page_offset+
-			       offset - start, copy);
+			       vaddr + skb_shinfo(skb)->frags[i].page_offset,
+			       copy);
 			kunmap_skb_frag(vaddr);
 
 			if ((len -= copy) == 0)
@@ -1084,30 +1082,25 @@ int skb_copy_bits(const struct sk_buff *skb, int offset, void *to, int len)
 			offset += copy;
 			to     += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
 		struct sk_buff *list = skb_shinfo(skb)->frag_list;
 
 		for (; list; list = list->next) {
-			int end;
-
-			BUG_TRAP(start <= offset + len);
+			BUG_TRAP(len >= 0);
 
-			end = start + list->len;
+			end = offset + list->len;
 			if ((copy = end - offset) > 0) {
 				if (copy > len)
 					copy = len;
-				if (skb_copy_bits(list, offset - start,
-						  to, copy))
+				if (skb_copy_bits(list, 0, to, copy))
 					goto fault;
 				if ((len -= copy) == 0)
 					return 0;
 				offset += copy;
 				to     += copy;
 			}
-			start = end;
 		}
 	}
 	if (!len)
@@ -1132,12 +1125,12 @@ fault:
 int skb_store_bits(struct sk_buff *skb, int offset, const void *from, int len)
 {
 	int i, copy;
-	int start = skb_headlen(skb);
+	int end = skb_headlen(skb);
 
 	if (offset > (int)skb->len - len)
 		goto fault;
 
-	if ((copy = start - offset) > 0) {
+	if ((copy = end - offset) > 0) {
 		if (copy > len)
 			copy = len;
 		skb_copy_to_linear_data_offset(skb, offset, from, copy);
@@ -1149,11 +1142,9 @@ int skb_store_bits(struct sk_buff *skb, int offset, const void *from, int len)
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
 		skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
-		int end;
-
-		BUG_TRAP(start <= offset + len);
+		BUG_TRAP(len >= 0);
 
-		end = start + frag->size;
+		end = offset + frag->size;
 		if ((copy = end - offset) > 0) {
 			u8 *vaddr;
 
@@ -1161,8 +1152,7 @@ int skb_store_bits(struct sk_buff *skb, int offset, const void *from, int len)
 				copy = len;
 
 			vaddr = kmap_skb_frag(frag);
-			memcpy(vaddr + frag->page_offset + offset - start,
-			       from, copy);
+			memcpy(vaddr + frag->page_offset, from, copy);
 			kunmap_skb_frag(vaddr);
 
 			if ((len -= copy) == 0)
@@ -1170,30 +1160,25 @@ int skb_store_bits(struct sk_buff *skb, int offset, const void *from, int len)
 			offset += copy;
 			from += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
 		struct sk_buff *list = skb_shinfo(skb)->frag_list;
 
 		for (; list; list = list->next) {
-			int end;
+			BUG_TRAP(len >= 0);
 
-			BUG_TRAP(start <= offset + len);
-
-			end = start + list->len;
+			end = offset + list->len;
 			if ((copy = end - offset) > 0) {
 				if (copy > len)
 					copy = len;
-				if (skb_store_bits(list, offset - start,
-						   from, copy))
+				if (skb_store_bits(list, 0, from, copy))
 					goto fault;
 				if ((len -= copy) == 0)
 					return 0;
 				offset += copy;
 				from += copy;
 			}
-			start = end;
 		}
 	}
 	if (!len)
@@ -1210,8 +1195,8 @@ EXPORT_SYMBOL(skb_store_bits);
 __wsum skb_checksum(const struct sk_buff *skb, int offset,
 			  int len, __wsum csum)
 {
-	int start = skb_headlen(skb);
-	int i, copy = start - offset;
+	int end = skb_headlen(skb);
+	int i, copy = end - offset;
 	int pos = 0;
 
 	/* Checksum header. */
@@ -1226,11 +1211,9 @@ __wsum skb_checksum(const struct sk_buff *skb, int offset,
 	}
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		int end;
-
-		BUG_TRAP(start <= offset + len);
+		BUG_TRAP(len >= 0);
 
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = offset + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			__wsum csum2;
 			u8 *vaddr;
@@ -1239,8 +1222,8 @@ __wsum skb_checksum(const struct sk_buff *skb, int offset,
 			if (copy > len)
 				copy = len;
 			vaddr = kmap_skb_frag(frag);
-			csum2 = csum_partial(vaddr + frag->page_offset +
-					     offset - start, copy, 0);
+			csum2 = csum_partial(vaddr + frag->page_offset,
+					     copy, 0);
 			kunmap_skb_frag(vaddr);
 			csum = csum_block_add(csum, csum2, pos);
 			if (!(len -= copy))
@@ -1248,31 +1231,26 @@ __wsum skb_checksum(const struct sk_buff *skb, int offset,
 			offset += copy;
 			pos    += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
 		struct sk_buff *list = skb_shinfo(skb)->frag_list;
 
 		for (; list; list = list->next) {
-			int end;
+			BUG_TRAP(len >= 0);
 
-			BUG_TRAP(start <= offset + len);
-
-			end = start + list->len;
+			end = offset + list->len;
 			if ((copy = end - offset) > 0) {
 				__wsum csum2;
 				if (copy > len)
 					copy = len;
-				csum2 = skb_checksum(list, offset - start,
-						     copy, 0);
+				csum2 = skb_checksum(list, 0, copy, 0);
 				csum = csum_block_add(csum, csum2, pos);
 				if ((len -= copy) == 0)
 					return csum;
 				offset += copy;
 				pos    += copy;
 			}
-			start = end;
 		}
 	}
 	BUG_ON(len);
@@ -1285,8 +1263,8 @@ __wsum skb_checksum(const struct sk_buff *skb, int offset,
 __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 				    u8 *to, int len, __wsum csum)
 {
-	int start = skb_headlen(skb);
-	int i, copy = start - offset;
+	int end = skb_headlen(skb);
+	int i, copy = end - offset;
 	int pos = 0;
 
 	/* Copy header. */
@@ -1303,11 +1281,9 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 	}
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		int end;
-
-		BUG_TRAP(start <= offset + len);
+		BUG_TRAP(len >= 0);
 
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = offset + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			__wsum csum2;
 			u8 *vaddr;
@@ -1317,9 +1293,8 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 				copy = len;
 			vaddr = kmap_skb_frag(frag);
 			csum2 = csum_partial_copy_nocheck(vaddr +
-							  frag->page_offset +
-							  offset - start, to,
-							  copy, 0);
+							  frag->page_offset,
+							  to, copy, 0);
 			kunmap_skb_frag(vaddr);
 			csum = csum_block_add(csum, csum2, pos);
 			if (!(len -= copy))
@@ -1328,7 +1303,6 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 			to     += copy;
 			pos    += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
@@ -1336,16 +1310,13 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 
 		for (; list; list = list->next) {
 			__wsum csum2;
-			int end;
-
-			BUG_TRAP(start <= offset + len);
+			BUG_TRAP(len >= 0);
 
-			end = start + list->len;
+			end = offset + list->len;
 			if ((copy = end - offset) > 0) {
 				if (copy > len)
 					copy = len;
-				csum2 = skb_copy_and_csum_bits(list,
-							       offset - start,
+				csum2 = skb_copy_and_csum_bits(list, 0,
 							       to, copy, 0);
 				csum = csum_block_add(csum, csum2, pos);
 				if ((len -= copy) == 0)
@@ -1354,7 +1325,6 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 				to     += copy;
 				pos    += copy;
 			}
-			start = end;
 		}
 	}
 	BUG_ON(len);
@@ -2026,8 +1996,8 @@ void __init skb_init(void)
 int
 skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
 {
-	int start = skb_headlen(skb);
-	int i, copy = start - offset;
+	int end = skb_headlen(skb);
+	int i, copy = end - offset;
 	int elt = 0;
 
 	if (copy > 0) {
@@ -2043,45 +2013,39 @@ skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
 	}
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		int end;
+		BUG_TRAP(len >= 0);
 
-		BUG_TRAP(start <= offset + len);
-
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = offset + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
 			if (copy > len)
 				copy = len;
 			sg[elt].page = frag->page;
-			sg[elt].offset = frag->page_offset+offset-start;
+			sg[elt].offset = frag->page_offset;
 			sg[elt].length = copy;
 			elt++;
 			if (!(len -= copy))
 				return elt;
 			offset += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
 		struct sk_buff *list = skb_shinfo(skb)->frag_list;
 
 		for (; list; list = list->next) {
-			int end;
-
-			BUG_TRAP(start <= offset + len);
+			BUG_TRAP(len >= 0);
 
-			end = start + list->len;
+			end = offset + list->len;
 			if ((copy = end - offset) > 0) {
 				if (copy > len)
 					copy = len;
-				elt += skb_to_sgvec(list, sg+elt, offset - start, copy);
+				elt += skb_to_sgvec(list, sg+elt, 0, copy);
 				if ((len -= copy) == 0)
 					return elt;
 				offset += copy;
 			}
-			start = end;
 		}
 	}
 	BUG_ON(len);
diff --git a/net/core/user_dma.c b/net/core/user_dma.c
index 0ad1cd57bc39..89241cdeea3f 100644
--- a/net/core/user_dma.c
+++ b/net/core/user_dma.c
@@ -49,8 +49,8 @@ int dma_skb_copy_datagram_iovec(struct dma_chan *chan,
 			struct sk_buff *skb, int offset, struct iovec *to,
 			size_t len, struct dma_pinned_list *pinned_list)
 {
-	int start = skb_headlen(skb);
-	int i, copy = start - offset;
+	int end = skb_headlen(skb);
+	int i, copy = end - offset;
 	dma_cookie_t cookie = 0;
 
 	/* Copy header. */
@@ -69,11 +69,9 @@ int dma_skb_copy_datagram_iovec(struct dma_chan *chan,
 
 	/* Copy paged appendix. Hmm... why does this look so complicated? */
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		int end;
+		BUG_TRAP(len >= 0);
 
-		BUG_TRAP(start <= offset + len);
-
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = offset + skb_shinfo(skb)->frags[i].size;
 		copy = end - offset;
 		if ((copy = end - offset) > 0) {
 			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
@@ -82,8 +80,8 @@ int dma_skb_copy_datagram_iovec(struct dma_chan *chan,
 			if (copy > len)
 				copy = len;
 
-			cookie = dma_memcpy_pg_to_iovec(chan, to, pinned_list, page,
-					frag->page_offset + offset - start, copy);
+			cookie = dma_memcpy_pg_to_iovec(chan, to, pinned_list,
+					page, frag->page_offset, copy);
 			if (cookie < 0)
 				goto fault;
 			len -= copy;
@@ -91,25 +89,21 @@ int dma_skb_copy_datagram_iovec(struct dma_chan *chan,
 				goto end;
 			offset += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
 		struct sk_buff *list = skb_shinfo(skb)->frag_list;
 
 		for (; list; list = list->next) {
-			int end;
-
-			BUG_TRAP(start <= offset + len);
+			BUG_TRAP(len >= 0);
 
-			end = start + list->len;
+			end = offset + list->len;
 			copy = end - offset;
 			if (copy > 0) {
 				if (copy > len)
 					copy = len;
 				cookie = dma_skb_copy_datagram_iovec(chan, list,
-						offset - start, to, copy,
-						pinned_list);
+						0, to, copy, pinned_list);
 				if (cookie < 0)
 					goto fault;
 				len -= copy;
@@ -117,7 +111,6 @@ int dma_skb_copy_datagram_iovec(struct dma_chan *chan,
 					goto end;
 				offset += copy;
 			}
-			start = end;
 		}
 	}
 
diff --git a/net/xfrm/xfrm_algo.c b/net/xfrm/xfrm_algo.c
index 6249a9405bb8..be529c4241a6 100644
--- a/net/xfrm/xfrm_algo.c
+++ b/net/xfrm/xfrm_algo.c
@@ -532,8 +532,8 @@ EXPORT_SYMBOL_GPL(xfrm_count_enc_supported);
 int skb_icv_walk(const struct sk_buff *skb, struct hash_desc *desc,
 		 int offset, int len, icv_update_fn_t icv_update)
 {
-	int start = skb_headlen(skb);
-	int i, copy = start - offset;
+	int end = skb_headlen(skb);
+	int i, copy = end - offset;
 	int err;
 	struct scatterlist sg;
 
@@ -556,11 +556,9 @@ int skb_icv_walk(const struct sk_buff *skb, struct hash_desc *desc,
 	}
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		int end;
+		BUG_TRAP(len >= 0);
 
-		BUG_TRAP(start <= offset + len);
-
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = offset + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
@@ -568,7 +566,7 @@ int skb_icv_walk(const struct sk_buff *skb, struct hash_desc *desc,
 				copy = len;
 
 			sg.page = frag->page;
-			sg.offset = frag->page_offset + offset-start;
+			sg.offset = frag->page_offset;
 			sg.length = copy;
 
 			err = icv_update(desc, &sg, copy);
@@ -579,22 +577,19 @@ int skb_icv_walk(const struct sk_buff *skb, struct hash_desc *desc,
 				return 0;
 			offset += copy;
 		}
-		start = end;
 	}
 
 	if (skb_shinfo(skb)->frag_list) {
 		struct sk_buff *list = skb_shinfo(skb)->frag_list;
 
 		for (; list; list = list->next) {
-			int end;
-
-			BUG_TRAP(start <= offset + len);
+			BUG_TRAP(len >= 0);
 
-			end = start + list->len;
+			end = offset + list->len;
 			if ((copy = end - offset) > 0) {
 				if (copy > len)
 					copy = len;
-				err = skb_icv_walk(list, desc, offset-start,
+				err = skb_icv_walk(list, desc, 0,
 						   copy, icv_update);
 				if (unlikely(err))
 					return err;
@@ -602,7 +597,6 @@ int skb_icv_walk(const struct sk_buff *skb, struct hash_desc *desc,
 					return 0;
 				offset += copy;
 			}
-			start = end;
 		}
 	}
 	BUG_ON(len);