net: bareudp: avoid uninitialized variable warning

clang points out that building without IPv6 would lead to returning
an uninitialized variable if a packet with family!=AF_INET is
passed into bareudp_udp_encap_recv():

drivers/net/bareudp.c:139:6: error: variable 'err' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
        if (family == AF_INET)
            ^~~~~~~~~~~~~~~~~
drivers/net/bareudp.c:146:15: note: uninitialized use occurs here
        if (unlikely(err)) {
                     ^~~
include/linux/compiler.h:78:42: note: expanded from macro 'unlikely'
 # define unlikely(x)    __builtin_expect(!!(x), 0)
                                            ^
drivers/net/bareudp.c:139:2: note: remove the 'if' if its condition is always true
        if (family == AF_INET)
        ^~~~~~~~~~~~~~~~~~~~~~

This cannot happen in practice, so change the condition in a way that
gcc sees the IPv4 case as unconditionally true here.
For consistency, change all the similar constructs in this file the
same way, using "if(IS_ENABLED())" instead of #if IS_ENABLED()".

Fixes: 571912c6 ("net: UDP tunnel encapsulation module for tunnelling different protocols like MPLS, IP, NSH etc.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bareudp.c

@@ -136,25 +136,21 @@ static int bareudp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
 	oiph = skb_network_header(skb);
 	skb_reset_network_header(skb);
 
-	if (family == AF_INET)
+	if (!IS_ENABLED(CONFIG_IPV6) || family == AF_INET)
 		err = IP_ECN_decapsulate(oiph, skb);
-#if IS_ENABLED(CONFIG_IPV6)
 	else
 		err = IP6_ECN_decapsulate(oiph, skb);
-#endif
 
 	if (unlikely(err)) {
 		if (log_ecn_error) {
-			if  (family == AF_INET)
+			if  (!IS_ENABLED(CONFIG_IPV6) || family == AF_INET)
 				net_info_ratelimited("non-ECT from %pI4 "
 						     "with TOS=%#x\n",
 						     &((struct iphdr *)oiph)->saddr,
 						     ((struct iphdr *)oiph)->tos);
-#if IS_ENABLED(CONFIG_IPV6)
 			else
 				net_info_ratelimited("non-ECT from %pI6\n",
 						     &((struct ipv6hdr *)oiph)->saddr);
-#endif
 		}
 		if (err > 1) {
 			++bareudp->dev->stats.rx_frame_errors;
@@ -350,7 +346,6 @@ static int bareudp_xmit_skb(struct sk_buff *skb, struct net_device *dev,
 	return err;
 }
 
-#if IS_ENABLED(CONFIG_IPV6)
 static int bareudp6_xmit_skb(struct sk_buff *skb, struct net_device *dev,
 			     struct bareudp_dev *bareudp,
 			     const struct ip_tunnel_info *info)
@@ -411,7 +406,6 @@ static int bareudp6_xmit_skb(struct sk_buff *skb, struct net_device *dev,
 	dst_release(dst);
 	return err;
 }
-#endif
 
 static netdev_tx_t bareudp_xmit(struct sk_buff *skb, struct net_device *dev)
 {
@@ -435,11 +429,9 @@ static netdev_tx_t bareudp_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	rcu_read_lock();
-#if IS_ENABLED(CONFIG_IPV6)
-	if (info->mode & IP_TUNNEL_INFO_IPV6)
+	if (IS_ENABLED(CONFIG_IPV6) && info->mode & IP_TUNNEL_INFO_IPV6)
 		err = bareudp6_xmit_skb(skb, dev, bareudp, info);
 	else
-#endif
 		err = bareudp_xmit_skb(skb, dev, bareudp, info);
 
 	rcu_read_unlock();
@@ -467,7 +459,7 @@ static int bareudp_fill_metadata_dst(struct net_device *dev,
 
 	use_cache = ip_tunnel_dst_cache_usable(skb, info);
 
-	if (ip_tunnel_info_af(info) == AF_INET) {
+	if (!IS_ENABLED(CONFIG_IPV6) || ip_tunnel_info_af(info) == AF_INET) {
 		struct rtable *rt;
 		__be32 saddr;
 
@@ -478,7 +470,6 @@ static int bareudp_fill_metadata_dst(struct net_device *dev,
 
 		ip_rt_put(rt);
 		info->key.u.ipv4.src = saddr;
-#if IS_ENABLED(CONFIG_IPV6)
 	} else if (ip_tunnel_info_af(info) == AF_INET6) {
 		struct dst_entry *dst;
 		struct in6_addr saddr;
@@ -492,7 +483,6 @@ static int bareudp_fill_metadata_dst(struct net_device *dev,
 
 		dst_release(dst);
 		info->key.u.ipv6.src = saddr;
-#endif
 	} else {
 		return -EINVAL;
 	}

include/net/udp_tunnel.h

@@ -143,14 +143,12 @@ void udp_tunnel_xmit_skb(struct rtable *rt, struct sock *sk, struct sk_buff *skb
 			 __be16 df, __be16 src_port, __be16 dst_port,
 			 bool xnet, bool nocheck);
 
-#if IS_ENABLED(CONFIG_IPV6)
 int udp_tunnel6_xmit_skb(struct dst_entry *dst, struct sock *sk,
 			 struct sk_buff *skb,
 			 struct net_device *dev, struct in6_addr *saddr,
 			 struct in6_addr *daddr,
 			 __u8 prio, __u8 ttl, __be32 label,
 			 __be16 src_port, __be16 dst_port, bool nocheck);
-#endif
 
 void udp_tunnel_sock_release(struct socket *sock);