ipv6: prepare headers for ipv6_stub removal

In preparation for dropping ipv6_stub and converting its users to direct
function calls, introduce static inline dummy functions and fallback
macros in the IPv6 networking headers. In addition, introduce checks on
fib6_nh_init(), ip6_dst_lookup_flow() and ip6_fragment() to avoid a
crash due to ipv6.disable=1 set during booting. The other functions are
safe as they cannot be called with ipv6.disable=1 set.

These fallbacks ensure that when CONFIG_IPV6 is completely disabled,
there are no compiling or linking errors due to code paths not guarded
by preprocessor macro IS_ENABLED(CONFIG_IPV6).

In addition, export ndisc_send_na(), ip6_route_input() and
ip6_fragment().

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Tested-by: Ricardo B. Marlière <rbm@suse.com>
Link: https://patch.msgid.link/20260325120928.15848-6-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

4 files changed, 73 insertions, 1 deletions

diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
index 10f30d158340..20e1231262d6 100644
--- a/include/net/ip6_fib.h
+++ b/include/net/ip6_fib.h
@@ -486,11 +486,30 @@ void rt6_get_prefsrc(const struct rt6_info *rt, struct in6_addr *addr)
 	rcu_read_unlock();
 }
 
+#if IS_ENABLED(CONFIG_IPV6)
 int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh,
 		 struct fib6_config *cfg, gfp_t gfp_flags,
 		 struct netlink_ext_ack *extack);
 void fib6_nh_release(struct fib6_nh *fib6_nh);
 void fib6_nh_release_dsts(struct fib6_nh *fib6_nh);
+#else
+static inline int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh,
+			       struct fib6_config *cfg, gfp_t gfp_flags,
+			       struct netlink_ext_ack *extack)
+{
+	NL_SET_ERR_MSG(extack, "IPv6 support not enabled in kernel");
+	return -EAFNOSUPPORT;
+}
+
+static inline void fib6_nh_release(struct fib6_nh *fib6_nh)
+{
+}
+
+static inline void fib6_nh_release_dsts(struct fib6_nh *fib6_nh)
+{
+}
+#endif
+
 
 int call_fib6_entry_notifiers(struct net *net,
 			      enum fib_event_type event_type,
@@ -502,8 +521,15 @@ int call_fib6_multipath_entry_notifiers(struct net *net,
 					unsigned int nsiblings,
 					struct netlink_ext_ack *extack);
 int call_fib6_entry_notifiers_replace(struct net *net, struct fib6_info *rt);
+#if IS_ENABLED(CONFIG_IPV6)
 void fib6_rt_update(struct net *net, struct fib6_info *rt,
 		    struct nl_info *info);
+#else
+static inline void fib6_rt_update(struct net *net, struct fib6_info *rt,
+				  struct nl_info *info)
+{
+}
+#endif
 void inet6_rt_notify(int event, struct fib6_info *rt, struct nl_info *info,
 		     unsigned int flags);
 
@@ -588,7 +614,13 @@ int fib6_tables_dump(struct net *net, struct notifier_block *nb,
 		     struct netlink_ext_ack *extack);
 
 void fib6_update_sernum(struct net *net, struct fib6_info *rt);
+#if IS_ENABLED(CONFIG_IPV6)
 void fib6_update_sernum_upto_root(struct net *net, struct fib6_info *rt);
+#else
+static inline void fib6_update_sernum_upto_root(struct net *net, struct fib6_info *rt)
+{
+}
+#endif
 void fib6_update_sernum_stub(struct net *net, struct fib6_info *f6i);
 
 void fib6_metric_set(struct fib6_info *f6i, int metric, u32 val);
diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 0c8eeb6abe7a..09ffe0f13ce7 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -77,7 +77,14 @@ static inline bool rt6_qualify_for_ecmp(const struct fib6_info *f6i)
 		f6i->fib6_nh->fib_nh_gw_family;
 }
 
+#if IS_ENABLED(CONFIG_IPV6)
 void ip6_route_input(struct sk_buff *skb);
+#else
+static inline void ip6_route_input(struct sk_buff *skb)
+{
+}
+#endif
+
 struct dst_entry *ip6_route_input_lookup(struct net *net,
 					 struct net_device *dev,
 					 struct flowi6 *fl6,
@@ -119,7 +126,15 @@ int ipv6_route_ioctl(struct net *net, unsigned int cmd,
 int ip6_route_add(struct fib6_config *cfg, gfp_t gfp_flags,
 		  struct netlink_ext_ack *extack);
 int ip6_ins_rt(struct net *net, struct fib6_info *f6i);
+#if IS_ENABLED(CONFIG_IPV6)
 int ip6_del_rt(struct net *net, struct fib6_info *f6i, bool skip_notify);
+#else
+static inline int ip6_del_rt(struct net *net, struct fib6_info *f6i,
+			     bool skip_notify)
+{
+	return -EAFNOSUPPORT;
+}
+#endif
 
 void rt6_flush_exceptions(struct fib6_info *f6i);
 void rt6_age_exceptions(struct fib6_info *f6i, struct fib6_gc_args *gc_args,
@@ -270,8 +285,19 @@ static inline bool ipv6_anycast_destination(const struct dst_entry *dst,
 	return __ipv6_anycast_destination(&rt->rt6i_dst, rt->rt6i_flags, daddr);
 }
 
+#if IS_ENABLED(CONFIG_IPV6)
 int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 		 int (*output)(struct net *, struct sock *, struct sk_buff *));
+#else
+static inline int ip6_fragment(struct net *net, struct sock *sk,
+			       struct sk_buff *skb,
+			       int (*output)(struct net *, struct sock *,
+					     struct sk_buff *))
+{
+	kfree_skb(skb);
+	return -EAFNOSUPPORT;
+}
+#endif
 
 /* Variant of dst_mtu() for IPv6 users */
 static inline u32 dst6_mtu(const struct dst_entry *dst)
diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index 0958cc5c6ec3..f99f273341f0 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -1044,8 +1044,18 @@ static inline struct sk_buff *ip6_finish_skb(struct sock *sk)
 
 int ip6_dst_lookup(struct net *net, struct sock *sk, struct dst_entry **dst,
 		   struct flowi6 *fl6);
+#if IS_ENABLED(CONFIG_IPV6)
 struct dst_entry *ip6_dst_lookup_flow(struct net *net, const struct sock *sk, struct flowi6 *fl6,
 				      const struct in6_addr *final_dst);
+#else
+static inline struct dst_entry *ip6_dst_lookup_flow(struct net *net, const struct sock *sk,
+						    struct flowi6 *fl6,
+						    const struct in6_addr *final_dst)
+{
+	return ERR_PTR(-EAFNOSUPPORT);
+}
+#endif
+
 struct dst_entry *ip6_sk_dst_lookup_flow(struct sock *sk, struct flowi6 *fl6,
 					 const struct in6_addr *final_dst,
 					 bool connected);
diff --git a/include/net/ndisc.h b/include/net/ndisc.h
index d38783a2ce57..19e2a177bd29 100644
--- a/include/net/ndisc.h
+++ b/include/net/ndisc.h
@@ -406,13 +406,17 @@ static inline void __ipv6_confirm_neigh_stub(struct net_device *dev,
 static inline struct neighbour *ip_neigh_gw6(struct net_device *dev,
 					     const void *addr)
 {
+#if IS_ENABLED(CONFIG_IPV6)
 	struct neighbour *neigh;
 
 	neigh = __ipv6_neigh_lookup_noref_stub(dev, addr);
 	if (unlikely(!neigh))
-		neigh = __neigh_create(ipv6_stub->nd_tbl, addr, dev, false);
+		neigh = __neigh_create(&nd_tbl, addr, dev, false);
 
 	return neigh;
+#else
+	return ERR_PTR(-EAFNOSUPPORT);
+#endif
 }
 
 int ndisc_init(void);