[NET]: Revert sk_buff walker cleanups.

This reverts eefa390

The simplification made in that change works with the assumption that
the 'offset' parameter to these functions is always positive or zero,
which is not true.  It can be and often is negative in order to access
SKB header values in front of skb->data.

Signed-off-by: David S. Miller <davem@davemloft.net>

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 end = skb_headlen(skb);
+	int start = skb_headlen(skb);
 	int i, copy;
 
 	/* checksum stuff in header space */
-	if ((copy = end - offset) > 0) {
+	if ( (copy = start - offset) > 0) {
 		if (copy > len)
 			copy = len;
 		sum = atalk_sum_partial(skb->data + offset, copy, sum);
@@ -953,41 +953,48 @@ 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++) {
-		BUG_TRAP(len >= 0);
+		int end;
 
-		end = offset + skb_shinfo(skb)->frags[i].size;
+		BUG_TRAP(start <= offset + len);
+
+		end = start + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			u8 *vaddr;
 			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
 			if (copy > len)
 				copy = len;
 			vaddr = kmap_skb_frag(frag);
-			sum = atalk_sum_partial(vaddr + frag->page_offset,
-						copy, sum);
+			sum = atalk_sum_partial(vaddr + frag->page_offset +
+						  offset - start, 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) {
-			BUG_TRAP(len >= 0);
+			int end;
+
+			BUG_TRAP(start <= offset + len);
 
-			end = offset + list->len;
+			end = start + list->len;
 			if ((copy = end - offset) > 0) {
 				if (copy > len)
 					copy = len;
-				sum = atalk_sum_skb(list, 0, copy, sum);
+				sum = atalk_sum_skb(list, offset - start,
+						    copy, sum);
 				if ((len -= copy) == 0)
 					return sum;
 				offset += copy;
 			}
+			start = end;
 		}
 	}
 

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 end = skb_headlen(skb);
-	int i, copy = end - offset;
+	int start = skb_headlen(skb);
+	int i, copy = start - offset;
 
 	/* Copy header. */
 	if (copy > 0) {
@@ -263,9 +263,11 @@ 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++) {
-		BUG_TRAP(len >= 0);
+		int end;
 
-		end = offset + skb_shinfo(skb)->frags[i].size;
+		BUG_TRAP(start <= offset + len);
+
+		end = start + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			int err;
 			u8  *vaddr;
@@ -275,33 +277,39 @@ 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,
-					     copy);
+			err = memcpy_toiovec(to, vaddr + frag->page_offset +
+					     offset - start, copy);
 			kunmap(page);
 			if (err)
 				goto fault;
 			if (!(len -= copy))
 				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) {
-			BUG_TRAP(len >= 0);
+			int end;
+
+			BUG_TRAP(start <= offset + len);
 
-			end = offset + list->len;
+			end = start + list->len;
 			if ((copy = end - offset) > 0) {
 				if (copy > len)
 					copy = len;
-				if (skb_copy_datagram_iovec(list, 0, to, copy))
+				if (skb_copy_datagram_iovec(list,
+							    offset - start,
+							    to, copy))
 					goto fault;
 				if ((len -= copy) == 0)
 					return 0;
 				offset += copy;
 			}
+			start = end;
 		}
 	}
 	if (!len)
@@ -315,9 +323,9 @@ static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
 				      u8 __user *to, int len,
 				      __wsum *csump)
 {
-	int end = skb_headlen(skb);
+	int start = skb_headlen(skb);
 	int pos = 0;
-	int i, copy = end - offset;
+	int i, copy = start - offset;
 
 	/* Copy header. */
 	if (copy > 0) {
@@ -336,9 +344,11 @@ static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
 	}
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		BUG_TRAP(len >= 0);
+		int end;
 
-		end = offset + skb_shinfo(skb)->frags[i].size;
+		BUG_TRAP(start <= offset + len);
+
+		end = start + skb_shinfo(skb)->frags[i].size;
 		if ((copy = end - offset) > 0) {
 			__wsum csum2;
 			int err = 0;
@@ -350,7 +360,8 @@ 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,
+							frag->page_offset +
+							offset - start,
 						      to, copy, 0, &err);
 			kunmap(page);
 			if (err)
@@ -362,20 +373,24 @@ 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) {
-			BUG_TRAP(len >= 0);
+			int end;
+
+			BUG_TRAP(start <= offset + len);
 
-			end = offset + list->len;
+			end = start + list->len;
 			if ((copy = end - offset) > 0) {
 				__wsum csum2 = 0;
 				if (copy > len)
 					copy = len;
-				if (skb_copy_and_csum_datagram(list, 0,
+				if (skb_copy_and_csum_datagram(list,
+							       offset - start,
 							       to, copy,
 							       &csum2))
 					goto fault;
@@ -386,6 +401,7 @@ static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
 				to += copy;
 				pos += copy;
 			}
+			start = end;
 		}
 	}
 	if (!len)