[RFC PATCH] fix IP_ECN_set_ce

[roy.qing.li@gmail.com]

From: Li RongQing <roy.qing.li@gmail.com>

1. ECN uses the two least significant (right-most) bits of the DiffServ
field in the IPv4, so it should be in iph->tos, not in (iph->tos+1)

2. When setting CE, we should check if ECN Capable Transport supports,
both 10 and 01 mean ECN Capable Transport, so only check 10 is not enough
    00: Non ECN-Capable Transport — Non-ECT
    10: ECN Capable Transport — ECT(0)
    01: ECN Capable Transport — ECT(1)
    11: Congestion Encountered — CE

3. Remove the misunderstand comment

4. fix the checksum computation

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
---
 include/net/inet_ecn.h |   22 ++++------------------
 1 file changed, 4 insertions(+), 18 deletions(-)

diff --git a/include/net/inet_ecn.h b/include/net/inet_ecn.h
index aab7375..545a683 100644
--- a/include/net/inet_ecn.h
+++ b/include/net/inet_ecn.h
@@ -73,27 +73,13 @@ static inline void INET_ECN_dontxmit(struct sock *sk)
 
 static inline int IP_ECN_set_ce(struct iphdr *iph)
 {
-	u32 check = (__force u32)iph->check;
-	u32 ecn = (iph->tos + 1) & INET_ECN_MASK;
-
-	/*
-	 * After the last operation we have (in binary):
-	 * INET_ECN_NOT_ECT => 01
-	 * INET_ECN_ECT_1   => 10
-	 * INET_ECN_ECT_0   => 11
-	 * INET_ECN_CE      => 00
-	 */
-	if (!(ecn & 2))
+	u32 ecn = iph->tos & INET_ECN_MASK;
+
+	if (INET_ECN_is_ce(ecn) || INET_ECN_is_not_ect(ecn))
 		return !ecn;
 
-	/*
-	 * The following gives us:
-	 * INET_ECN_ECT_1 => check += htons(0xFFFD)
-	 * INET_ECN_ECT_0 => check += htons(0xFFFE)
-	 */
-	check += (__force u16)htons(0xFFFB) + (__force u16)htons(ecn);
+	csum_replace2(&iph->check, iph->tos, iph->tos|INET_ECN_CE);
 
-	iph->check = (__force __sum16)(check + (check>=0xFFFF));
 	iph->tos |= INET_ECN_CE;
 	return 1;
 }
-- 
1.7.10.4