ipv4: Revert removal of rt_uses_gateway

Julian noted that rt_uses_gateway has a more subtle use than 'is gateway
set':
    https://lore.kernel.org/netdev/alpine.LFD.2.21.1909151104060.2546@ja.home.ssi.bg/

Revert that part of the commit referenced in the Fixes tag.

Currently, there are no u8 holes in 'struct rtable'. There is a 4-byte hole
in the second cacheline which contains the gateway declaration. So move
rt_gw_family down to the gateway declarations since they are always used
together, and then re-use that u8 for rt_uses_gateway. End result is that
rtable size is unchanged.

Fixes: 1550c171935d ("ipv4: Prepare rtable for IPv6 gateway")
Reported-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

1 files changed, 21 insertions, 15 deletions

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index b6a6f18c3dd1..7dcce724c78b 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -635,6 +635,7 @@ static void fill_route_from_fnhe(struct rtable *rt, struct fib_nh_exception *fnh
 
 	if (fnhe->fnhe_gw) {
 		rt->rt_flags |= RTCF_REDIRECTED;
+		rt->rt_uses_gateway = 1;
 		rt->rt_gw_family = AF_INET;
 		rt->rt_gw4 = fnhe->fnhe_gw;
 	}
@@ -1313,7 +1314,7 @@ static unsigned int ipv4_mtu(const struct dst_entry *dst)
 	mtu = READ_ONCE(dst->dev->mtu);
 
 	if (unlikely(ip_mtu_locked(dst))) {
-		if (rt->rt_gw_family && mtu > 576)
+		if (rt->rt_uses_gateway && mtu > 576)
 			mtu = 576;
 	}
 
@@ -1569,6 +1570,7 @@ static void rt_set_nexthop(struct rtable *rt, __be32 daddr,
 		struct fib_nh_common *nhc = FIB_RES_NHC(*res);
 
 		if (nhc->nhc_gw_family && nhc->nhc_scope == RT_SCOPE_LINK) {
+			rt->rt_uses_gateway = 1;
 			rt->rt_gw_family = nhc->nhc_gw_family;
 			/* only INET and INET6 are supported */
 			if (likely(nhc->nhc_gw_family == AF_INET))
@@ -1634,6 +1636,7 @@ struct rtable *rt_dst_alloc(struct net_device *dev,
 		rt->rt_iif = 0;
 		rt->rt_pmtu = 0;
 		rt->rt_mtu_locked = 0;
+		rt->rt_uses_gateway = 0;
 		rt->rt_gw_family = 0;
 		rt->rt_gw4 = 0;
 		INIT_LIST_HEAD(&rt->rt_uncached);
@@ -2694,6 +2697,7 @@ struct dst_entry *ipv4_blackhole_route(struct net *net, struct dst_entry *dst_or
 		rt->rt_genid = rt_genid_ipv4(net);
 		rt->rt_flags = ort->rt_flags;
 		rt->rt_type = ort->rt_type;
+		rt->rt_uses_gateway = ort->rt_uses_gateway;
 		rt->rt_gw_family = ort->rt_gw_family;
 		if (rt->rt_gw_family == AF_INET)
 			rt->rt_gw4 = ort->rt_gw4;
@@ -2778,21 +2782,23 @@ static int rt_fill_info(struct net *net, __be32 dst, __be32 src,
 		if (nla_put_in_addr(skb, RTA_PREFSRC, fl4->saddr))
 			goto nla_put_failure;
 	}
-	if (rt->rt_gw_family == AF_INET &&
-	    nla_put_in_addr(skb, RTA_GATEWAY, rt->rt_gw4)) {
-		goto nla_put_failure;
-	} else if (rt->rt_gw_family == AF_INET6) {
-		int alen = sizeof(struct in6_addr);
-		struct nlattr *nla;
-		struct rtvia *via;
-
-		nla = nla_reserve(skb, RTA_VIA, alen + 2);
-		if (!nla)
+	if (rt->rt_uses_gateway) {
+		if (rt->rt_gw_family == AF_INET &&
+		    nla_put_in_addr(skb, RTA_GATEWAY, rt->rt_gw4)) {
 			goto nla_put_failure;
-
-		via = nla_data(nla);
-		via->rtvia_family = AF_INET6;
-		memcpy(via->rtvia_addr, &rt->rt_gw6, alen);
+		} else if (rt->rt_gw_family == AF_INET6) {
+			int alen = sizeof(struct in6_addr);
+			struct nlattr *nla;
+			struct rtvia *via;
+
+			nla = nla_reserve(skb, RTA_VIA, alen + 2);
+			if (!nla)
+				goto nla_put_failure;
+
+			via = nla_data(nla);
+			via->rtvia_family = AF_INET6;
+			memcpy(via->rtvia_addr, &rt->rt_gw6, alen);
+		}
 	}
 
 	expires = rt->dst.expires;