[PATCH net-next v3] inet_ecn: Use csum16_add() helper for IP_ECN_set_* helpers

[Toke Høiland-Jørgensen]

Jakub pointed out that the IP_ECN_set* helpers basically open-code
csum16_add(), so let's switch them over to using the helper instead.

v2:
- Use __be16 for check_add stack variable in IP_ECN_set_ce() (kbot)
v3:
- Turns out we need __force casts to do arithmetic on __be16 types

Reported-by: Jakub Kicinski <k...@kernel.org>
Tested-by: Jonathan Morton <chromati...@gmail.com>
Tested-by: Pete Heist <p...@heistp.net>
Signed-off-by: Toke Høiland-Jørgensen <t...@redhat.com>
---
 include/net/inet_ecn.h | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/include/net/inet_ecn.h b/include/net/inet_ecn.h
index 563457fec557..ba77f47ef61e 100644
--- a/include/net/inet_ecn.h
+++ b/include/net/inet_ecn.h
@@ -8,6 +8,7 @@
 
 #include <net/inet_sock.h>
 #include <net/dsfield.h>
+#include <net/checksum.h>
 
 enum {
        INET_ECN_NOT_ECT = 0,
@@ -75,8 +76,8 @@ 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;
+       __be16 check_add;
 
        /*
         * After the last operation we have (in binary):
@@ -93,23 +94,20 @@ static inline int IP_ECN_set_ce(struct iphdr *iph)
         * INET_ECN_ECT_1 => check += htons(0xFFFD)
         * INET_ECN_ECT_0 => check += htons(0xFFFE)
         */
-       check += (__force u16)htons(0xFFFB) + (__force u16)htons(ecn);
+       check_add = (__force __be16)((__force u16)htons(0xFFFB) +
+                                    (__force u16)htons(ecn));
 
-       iph->check = (__force __sum16)(check + (check>=0xFFFF));
+       iph->check = csum16_add(iph->check, check_add);
        iph->tos |= INET_ECN_CE;
        return 1;
 }
 
 static inline int IP_ECN_set_ect1(struct iphdr *iph)
 {
-       u32 check = (__force u32)iph->check;
-
        if ((iph->tos & INET_ECN_MASK) != INET_ECN_ECT_0)
                return 0;
 
-       check += (__force u16)htons(0x1);
-
-       iph->check = (__force __sum16)(check + (check>=0xFFFF));
+       iph->check = csum16_add(iph->check, htons(0x1));
        iph->tos ^= INET_ECN_MASK;
        return 1;
 }
-- 
2.29.2