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

From: Vladimir Oltean

Sent: 02 December 2021 13:11

...

quoted

--- 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,

quoted

 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.
...

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.

	David

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