From mboxrd@z Thu Jan  1 00:00:00 1970
From: Edward Cree <ecree@solarflare.com>
Subject: Re: [PATCH net-next 6/8] net: gre: Implement LCO for GRE over IPv4
Date: Mon, 11 Jan 2016 13:21:29 +0000
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Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"linux-net-drivers@solarflare.com" <linux-net-drivers@solarflare.com>,
	"tom@herbertland.com" <tom@herbertland.com>,
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To: David Laight <David.Laight@ACULAB.COM>,
	David Miller <davem@davemloft.net>
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On 11/01/16 10:09, David Laight wrote:
> From: Edward Cree
>> Sent: 08 January 2016 19:47
> ...
>> +	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