[IPV4/IPV6]: Fixup checksums properly when fragmenting.

If we end up with a fraggap, when we copy the data
over to the next frag being built we must compute
a checksum for the bit we copy over for the new
fragment and subtract that checksum from the place
we are copying it from.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -78,6 +78,7 @@
 #include <net/raw.h>
 #include <net/checksum.h>
 #include <net/inetpeer.h>
+#include <net/checksum.h>
 #include <linux/igmp.h>
 #include <linux/netfilter_ipv4.h>
 #include <linux/netfilter_bridge.h>
@@ -892,14 +893,17 @@ int ip_append_data(struct sock *sk,
 			skb->h.raw = data + exthdrlen;
 
 			if (fraggap) {
-				skb_copy_bits(skb_prev, maxfraglen, 
-					      data + transhdrlen, fraggap);
+				skb->csum = skb_copy_and_csum_bits(
+					skb_prev, maxfraglen,
+					data + transhdrlen, fraggap, 0);
+				skb_prev->csum = csum_block_sub(
+					skb_prev->csum, skb->csum, 0);
 				data += fraggap;
 				skb_trim(skb_prev, maxfraglen);
 			}
 
 			copy = datalen - transhdrlen - fraggap;
-			if (copy > 0 && getfrag(from, data + transhdrlen, offset, copy, 0, skb) < 0) {
+			if (copy > 0 && getfrag(from, data + transhdrlen, offset, copy, fraggap, skb) < 0) {
 				err = -EFAULT;
 				kfree_skb(skb);
 				goto error;
@@ -1087,8 +1091,11 @@ ssize_t	ip_append_page(struct sock *sk, struct page *page,
 			skb->h.raw = data;
 
 			if (fraggap) {
-				skb_copy_bits(skb_prev, maxfraglen,
-					      data, fraggap);
+				skb->csum = skb_copy_and_csum_bits(
+					skb_prev, maxfraglen,
+					data, fraggap, 0);
+				skb_prev->csum = csum_block_sub(
+					skb_prev->csum, skb->csum, 0);
 				skb_trim(skb_prev, maxfraglen);
 			}
 

net/ipv6/ip6_output.c

@@ -54,6 +54,7 @@
 #include <net/rawv6.h>
 #include <net/icmp.h>
 #include <net/xfrm.h>
+#include <net/checksum.h>
 
 static int ip6_fragment(struct sk_buff **pskb, int (*output)(struct sk_buff**));
 
@@ -981,8 +982,11 @@ int ip6_append_data(struct sock *sk, int getfrag(void *from, char *to, int offse
 			skb->h.raw = data + exthdrlen;
 
 			if (fraggap) {
-				skb_copy_bits(skb_prev, maxfraglen, 
-					      data + transhdrlen, fraggap);
+				skb->csum = skb_copy_and_csum_bits(
+					skb_prev, maxfraglen,
+					data + transhdrlen, fraggap, 0);
+				skb_prev->csum = csum_block_sub(
+					skb_prev->csum, skb->csum, 0);
 				data += fraggap;
 				skb_trim(skb_prev, maxfraglen);
 			}
@@ -991,7 +995,7 @@ int ip6_append_data(struct sock *sk, int getfrag(void *from, char *to, int offse
 				err = -EINVAL;
 				kfree_skb(skb);
 				goto error;
-			} else if (copy > 0 && getfrag(from, data + transhdrlen, offset, copy, 0, skb) < 0) {
+			} else if (copy > 0 && getfrag(from, data + transhdrlen, offset, copy, fraggap, skb) < 0) {
 				err = -EFAULT;
 				kfree_skb(skb);
 				goto error;