Re: [PATCH net-next 2/2] net: optimize skb_postpull_rcsum()

On Thu, Dec 2, 2021 at 7:06 AM David Laight [off-list ref] wrote:

From: Vladimir Oltean

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Sent: 02 December 2021 13:11

...

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--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h

@@ -3485,7 +3485,11 @@ __skb_postpull_rcsum(struct sk_buff *skb, const void *start, unsigned int

len,

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 static inline void skb_postpull_rcsum(struct sk_buff *skb,
                                  const void *start, unsigned int len)
 {
-   __skb_postpull_rcsum(skb, start, len, 0);
+   if (skb->ip_summed == CHECKSUM_COMPLETE)
+           skb->csum = ~csum_partial(start, len, ~skb->csum);

You can't do that, the domain is 1..0xffff (or maybe 0xffffffff).
The invert has to convert ~0 to ~0 not zero.
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There seems to be a disparity when the skb->csum is calculated by
skb_postpull_rcsum as zero. Before, it was calculated as 0xffff.

Which is what that will do for some inputs at least.
Maybe:
                skb->csum = 1 + ~csum_partial(start, len, ~skb->csum + 1);
is right.
I think that is the same as:
                skb->csum = -csum_partial(start, len, -skb->csum);
Although letting the compiler do that transform probably makes
the code easier to read.

Interesting, update_csum_diff4() and update_csum_diff16() seem to both use.

skb->csum = ~csum_partial((char *)diff, sizeof(diff), ~skb->csum);