gre: do not assign header_ops in collect metadata mode

BugLink: https://bugs.launchpad.net/bugs/1878232

commit a64b04d8 upstream.

In ipgre mode (i.e. not gretap) with collect metadata flag set, the tunnel
is incorrectly assumed to be mGRE in NBMA mode (see commit 6a5f44d7).
This is not the case, we're controlling the encapsulation addresses by
lwtunnel metadata. And anyway, assigning dev->header_ops in collect metadata
mode does not make sense.

Although it would be more user firendly to reject requests that specify
both the collect metadata flag and a remote/local IP address, this would
break current users of gretap or introduce ugly code and differences in
handling ipgre and gretap configuration. Keep the current behavior of
remote/local IP address being ignored in such case.

v3: Back to v1, added explanation paragraph.
v2: Reject configuration specifying both remote/local address and collect
    metadata flag.

Fixes: 2e15ea39 ("ip_gre: Add support to collect tunnel metadata.")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/ipv4/ip_gre.c

@@ -882,7 +882,7 @@ static int ipgre_tunnel_init(struct net_device *dev)
 	netif_keep_dst(dev);
 	dev->addr_len		= 4;
 
-	if (iph->daddr) {
+	if (iph->daddr && !tunnel->collect_md) {
 #ifdef CONFIG_NET_IPGRE_BROADCAST
 		if (ipv4_is_multicast(iph->daddr)) {
 			if (!iph->saddr)
@@ -891,8 +891,9 @@ static int ipgre_tunnel_init(struct net_device *dev)
 			dev->header_ops = &ipgre_header_ops;
 		}
 #endif
-	} else
+	} else if (!tunnel->collect_md) {
 		dev->header_ops = &ipgre_header_ops;
+	}
 
 	return ip_tunnel_init(dev);
 }