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Subject[PATCH 4.9 65/71] net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

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

--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2954,6 +2954,7 @@ static inline unsigned char *skb_push_rc
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
{
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)
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1578,6 +1578,20 @@ done:
}
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

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