[PATCH stable 4.9 v2 23/29] net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends

[Florian Fainelli]

From: Eric Dumazet <eduma...@google.com>

After working on IP defragmentation lately, I found that some large
packets defeat CHECKSUM_COMPLETE optimization because of NIC adding
zero paddings on the last (small) fragment.

While removing the padding with pskb_trim_rcsum(), we set skb->ip_summed
to CHECKSUM_NONE, forcing a full csum validation, even if all prior
fragments had CHECKSUM_COMPLETE set.

We can instead compute the checksum of the part we are trimming,
usually smaller than the part we keep.

Signed-off-by: Eric Dumazet <eduma...@google.com>
Signed-off-by: David S. Miller <da...@davemloft.net>
(cherry picked from commit 88078d98d1bb085d72af8437707279e203524fa5)
---
 include/linux/skbuff.h |  5 ++---
 net/core/skbuff.c      | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 724c6abdb9e6..11974e63a41d 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2954,6 +2954,7 @@ static inline unsigned char *skb_push_rcsum(struct 
sk_buff *skb,
        return skb->data;
 }
 
+int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len);
 /**
  *     pskb_trim_rcsum - trim received skb and update checksum
  *     @skb: buffer to trim
@@ -2967,9 +2968,7 @@ static inline int pskb_trim_rcsum(struct sk_buff *skb, 
unsigned int len)
 {
        if (likely(len >= skb->len))
                return 0;
-       if (skb->ip_summed == CHECKSUM_COMPLETE)
-               skb->ip_summed = CHECKSUM_NONE;
-       return __pskb_trim(skb, len);
+       return pskb_trim_rcsum_slow(skb, len);
 }
 
 static inline int __skb_trim_rcsum(struct sk_buff *skb, unsigned int len)
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 4e545e4432eb..038ec74fa131 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1578,6 +1578,20 @@ int ___pskb_trim(struct sk_buff *skb, unsigned int len)
 }
 EXPORT_SYMBOL(___pskb_trim);
 
+/* Note : use pskb_trim_rcsum() instead of calling this directly
+ */
+int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
+{
+       if (skb->ip_summed == CHECKSUM_COMPLETE) {
+               int delta = skb->len - len;
+
+               skb->csum = csum_sub(skb->csum,
+                                    skb_checksum(skb, len, delta, 0));
+       }
+       return __pskb_trim(skb, len);
+}
+EXPORT_SYMBOL(pskb_trim_rcsum_slow);
+
 /**
  *     __pskb_pull_tail - advance tail of skb header
  *     @skb: buffer to reallocate
-- 
2.17.1