commit 8d70d82cdcc6da0a77440c6d860957a1f50b8089
Author: Patrick McHardy <kaber@trash.net>
Date:   Mon Sep 20 19:29:40 2010 +0200

    netfilter: nf_nat_snmp: fix checksum calculation (v4)
    
    Fix checksum calculation in nf_nat_snmp_basic.
    
    Based on patches by Clark Wang <wtweeker@163.com> and
    Stephen Hemminger <shemminger@vyatta.com>.
    
    https://bugzilla.kernel.org/show_bug.cgi?id=17622
    
    Signed-off-by: Patrick McHardy <kaber@trash.net>

diff --git a/net/ipv4/netfilter/nf_nat_snmp_basic.c b/net/ipv4/netfilter/nf_nat_snmp_basic.c
index 1679e2c..ee5f419 100644
--- a/net/ipv4/netfilter/nf_nat_snmp_basic.c
+++ b/net/ipv4/netfilter/nf_nat_snmp_basic.c
@@ -893,13 +893,15 @@ static void fast_csum(__sum16 *csum,
 	unsigned char s[4];
 
 	if (offset & 1) {
-		s[0] = s[2] = 0;
+		s[0] = ~0;
 		s[1] = ~*optr;
+		s[2] = 0;
 		s[3] = *nptr;
 	} else {
-		s[1] = s[3] = 0;
 		s[0] = ~*optr;
+		s[1] = ~0;
 		s[2] = *nptr;
+		s[3] = 0;
 	}
 
 	*csum = csum_fold(csum_partial(s, 4, ~csum_unfold(*csum)));