Re: [PATCH net-next 6/8] net: gre: Implement LCO for GRE over IPv4

On 11/01/16 10:09, David Laight wrote:

From: Edward Cree

quoted

Sent: 08 January 2016 19:47

...

quoted

+	if (skb->ip_summed == CHECKSUM_PARTIAL) {
+		csum = csum_fold(lco_csum(skb));
+		if (csum == 0)
+			csum = CSUM_MANGLED_0;
+		return csum;
+	} else {
+		return csum_fold(skb_checksum(skb, 0, skb->len, 0));
+	}

You see to be worried about csum_fold() returning 0 in one
path, but not in the other.
I'm guessing that 0 can only happen if all the bytes that have
been checksummed are zero.

csum_fold complements, so if the sum comes to (say) 0x1fffe, it will return ~0xffff which is 0.

Next version of patch will mangle 0 for both branches.

-ed