net: Add inet_proto_csum_replace_by_diff utility function

This function updates a checksum field value and skb->csum based on
a value which is the difference between the old and new checksum.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 0 deletions

diff --git a/net/core/utils.c b/net/core/utils.c
index cd7d202f340e..3dffce953c39 100644
--- a/net/core/utils.c
+++ b/net/core/utils.c
@@ -336,6 +336,19 @@ void inet_proto_csum_replace16(__sum16 *sum, struct sk_buff *skb,
 }
 EXPORT_SYMBOL(inet_proto_csum_replace16);
 
+void inet_proto_csum_replace_by_diff(__sum16 *sum, struct sk_buff *skb,
+                                     __wsum diff, bool pseudohdr)
+{
+        if (skb->ip_summed != CHECKSUM_PARTIAL) {
+                *sum = csum_fold(csum_add(diff, ~csum_unfold(*sum)));
+                if (skb->ip_summed == CHECKSUM_COMPLETE && pseudohdr)
+                        skb->csum = ~csum_add(diff, ~skb->csum);
+        } else if (pseudohdr) {
+                *sum = ~csum_fold(csum_add(diff, csum_unfold(*sum)));
+        }
+}
+EXPORT_SYMBOL(inet_proto_csum_replace_by_diff);
+
 struct __net_random_once_work {
         struct work_struct work;
         struct static_key *key;