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Date:	Thu, 13 Aug 2015 09:54:02 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	<davem@...emloft.net>, <netdev@...r.kernel.org>
CC:	<tgraf@...g.ch>, <roopa@...ulusnetworks.com>, <kernel-team@...com>
Subject: [PATCH net-next 2/3] 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@...bertland.com>
---
 include/net/checksum.h |  2 ++
 net/core/utils.c       | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/net/checksum.h b/include/net/checksum.h
index 2d1d73c..0e0c987 100644
--- a/include/net/checksum.h
+++ b/include/net/checksum.h
@@ -144,6 +144,8 @@ void inet_proto_csum_replace4(__sum16 *sum, struct sk_buff *skb,
 void inet_proto_csum_replace16(__sum16 *sum, struct sk_buff *skb,
 			       const __be32 *from, const __be32 *to,
 			       int pseudohdr);
+void inet_proto_csum_replace_by_diff(__sum16 *sum, struct sk_buff *skb,
+				     __wsum diff, int pseudohdr);
 
 static inline void inet_proto_csum_replace2(__sum16 *sum, struct sk_buff *skb,
 					    __be16 from, __be16 to,
diff --git a/net/core/utils.c b/net/core/utils.c
index a7732a0..89ccfb1 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, int 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;
-- 
1.8.1

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