[PATCH net-next v4 3/4] net: Add inet_proto_csum_replace_by_diff utility function

[Tom Herbert]

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 <t...@herbertland.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 619f344..9fcaedf 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,
                               bool pseudohdr);
+void inet_proto_csum_replace_by_diff(__sum16 *sum, struct sk_buff *skb,
+                                    __wsum diff, bool 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 cd7d202..3dffce9 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;
-- 
1.8.1

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