Re: [PATCH net-next v5 0/2] af_mpls: fix undefined reference to ip6_route_output with CONFIG_IPV6=n

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

On Thu, 2015-07-30 at 06:22 -0700, roopa wrote:
> On 7/29/15, 10:42 PM, David Miller wrote:
> > From: Roopa Prabhu <roopa@cumulusnetworks.com>
> > Date: Tue, 28 Jul 2015 15:27:39 -0700
> > 
> > > v4 - v5: Use ipv6_stub_impl.ipv6_dst_lookup as suggested by Hannes
> > I think this might not work.
> > 
> > The ipv6_stub pointer is NULL until the ipv6 module is loaded.
> > 
> > VXLAN can safely call through ipv6_stub->foo() because it _knows_ 
> > the
> > ipv6 module has been loaded, and will not go away, because it is
> > working with an ipv6 socket.
> > 
> > You don't have that set of preconditions here, and thus it is pretty
> > easy to generate a scenerio where ipv6_stub will be NULL at your 
> > call
> > sites.
> 
> v4 seems to be a better/safe option in that case. I can resubmit v4 if 
> that is acceptable.
> 
> > 
> > This is starting to get really messy, to be honest.  And we've been
> > harboring this build failure for several days now.
> > 
> please revert the commit that introduced the IPV6 dependency. I will 
> resubmit if needed.
> I don't feel good about the build failure either.
> 
> Thanks and apologies.

Of course you have to guard the NULL pointer dereference. Something simple like that will do:

--- a/net/ipv6/addrconf_core.c
+++ b/net/ipv6/addrconf_core.c
@@ -107,7 +107,15 @@ int inet6addr_notifier_call_chain(unsigned long val, void *v)
 }
 EXPORT_SYMBOL(inet6addr_notifier_call_chain);
 
-const struct ipv6_stub *ipv6_stub __read_mostly;
+static int eafnosupport_ipv6_dst_lookup(struct sock *u1, struct dst_entry **u2,
+                                       struct flowi6 *u3)
+{
+       return -EAFNOSUPPORT;
+}
+
+const struct ipv6_stub *ipv6_stub __read_mostly = &(struct ipv6_stub){
+       .ipv6_dst_lookup = eafnosupport_ipv6_dst_lookup,
+};
 EXPORT_SYMBOL_GPL(ipv6_stub);
 
 /* IPv6 Wildcard Address and Loopback Address defined by RFC2553 */


Bye,
Hannes