[PATCH net 1/3] ip_gre: Make o_seqno start from 0 in native mode

[Peilin Ye <yepeilin.cs@gmail.com>]

From: Peilin Ye <peilin.ye@bytedance.com>

For GRE and GRETAP devices, currently o_seqno starts from 1 in native
mode.  According to RFC 2890 2.2., "The first datagram is sent with a
sequence number of 0."  Fix it.

It is worth mentioning that o_seqno already starts from 0 in collect_md
mode, see gre_fb_xmit(), where tunnel->o_seqno is passed to
gre_build_header() before getting incremented.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
---
 net/ipv4/ip_gre.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 99db2e41ed10..ca70b92e80d9 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -459,14 +459,12 @@ static void __gre_xmit(struct sk_buff *skb, struct net_device *dev,
 		       __be16 proto)
 {
 	struct ip_tunnel *tunnel = netdev_priv(dev);
-
-	if (tunnel->parms.o_flags & TUNNEL_SEQ)
-		tunnel->o_seqno++;
+	__be16 flags = tunnel->parms.o_flags;
 
 	/* Push GRE header. */
 	gre_build_header(skb, tunnel->tun_hlen,
-			 tunnel->parms.o_flags, proto, tunnel->parms.o_key,
-			 htonl(tunnel->o_seqno));
+			 flags, proto, tunnel->parms.o_key,
+			 (flags & TUNNEL_SEQ) ? htonl(tunnel->o_seqno++) : 0);
 
 	ip_tunnel_xmit(skb, dev, tnl_params, tnl_params->protocol);
 }
-- 
2.20.1