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.

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>
---
v2: no changes
---
 include/net/ip6_fib.h   | 31 +++++++++++++++++++++++++++++++
 include/net/ip6_route.h | 26 ++++++++++++++++++++++++++
 include/net/ipv6.h      | 10 ++++++++++
 include/net/ndisc.h     | 13 ++++++++++++-
 net/ipv6/ip6_output.c   |  1 +
 net/ipv6/ndisc.c        |  1 +
 net/ipv6/route.c        |  1 +
 7 files changed, 82 insertions(+), 1 deletion(-)

diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
index 39c98c3d8ac5..62a1e96bbdd4 100644
--- a/include/net/ip6_fib.h
+++ b/include/net/ip6_fib.h
@@ -486,11 +486,29 @@ 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)
+{
+	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 +520,15 @@ 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);
 
@@ -568,7 +593,13 @@ unsigned int fib6_tables_seq_read(const struct net *net);
 int fib6_tables_dump(struct net *net, struct notifier_block *nb,
 		     struct netlink_ext_ack *extack);
 
+#if IS_ENABLED(CONFIG_IPV6)
 void fib6_update_sernum(struct net *net, struct fib6_info *rt);
+#else
+static inline void fib6_update_sernum(struct net *net, struct fib6_info *rt)
+{
+}
+#endif
 void fib6_update_sernum_upto_root(struct net *net, struct fib6_info *rt);
 void fib6_update_sernum_stub(struct net *net, struct fib6_info *f6i);
 
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 1c0ce5151275..06d1fa54cbf7 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..6152401f29f1 100644
--- a/include/net/ndisc.h
+++ b/include/net/ndisc.h
@@ -69,6 +69,12 @@ struct prefix_info;
 
 extern struct neigh_table nd_tbl;
 
+#if IS_ENABLED(CONFIG_IPV6)
+#define ipv6_get_nd_tbl() (&nd_tbl)
+#else
+#define ipv6_get_nd_tbl() NULL
+#endif
+
 struct nd_msg {
         struct icmp6hdr	icmph;
         struct in6_addr	target;
@@ -356,7 +362,8 @@ static inline u32 ndisc_hashfn(const void *pkey, const struct net_device *dev, _
 
 static inline struct neighbour *__ipv6_neigh_lookup_noref(struct net_device *dev, const void *pkey)
 {
-	return ___neigh_lookup_noref(&nd_tbl, neigh_key_eq128, ndisc_hashfn, pkey, dev);
+	return ___neigh_lookup_noref(ipv6_get_nd_tbl(), neigh_key_eq128,
+				     ndisc_hashfn, pkey, dev);
 }
 
 static inline
@@ -406,6 +413,7 @@ 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);
@@ -413,6 +421,9 @@ static inline struct neighbour *ip_neigh_gw6(struct net_device *dev,
 		neigh = __neigh_create(ipv6_stub->nd_tbl, addr, dev, false);
 
 	return neigh;
+#else
+	return ERR_PTR(-EAFNOSUPPORT);
+#endif
 }
 
 int ndisc_init(void);
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 8e2a6b28cea7..315808dc2a07 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1045,6 +1045,7 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 	kfree_skb(skb);
 	return err;
 }
+EXPORT_SYMBOL_GPL(ip6_fragment);
 
 static inline int ip6_rt_check(const struct rt6key *rt_key,
 			       const struct in6_addr *fl_addr,
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index f6a5d8c73af9..f76fb8a85452 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -576,6 +576,7 @@ void ndisc_send_na(struct net_device *dev, const struct in6_addr *daddr,
 
 	ndisc_send_skb(skb, daddr, src_addr);
 }
+EXPORT_SYMBOL_GPL(ndisc_send_na);
 
 static void ndisc_send_unsol_na(struct net_device *dev)
 {
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index e206777785d4..b66a2f791e3f 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2655,6 +2655,7 @@ void ip6_route_input(struct sk_buff *skb)
 	skb_dst_set_noref(skb, ip6_route_input_lookup(net, skb->dev,
 						      &fl6, skb, flags));
 }
+EXPORT_SYMBOL_GPL(ip6_route_input);
 
 INDIRECT_CALLABLE_SCOPE struct rt6_info *ip6_pol_route_output(struct net *net,
 					     struct fib6_table *table,
-- 
2.53.0

On 3/10/26 9:34 AM, Fernando Fernandez Mancera wrote:
> diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
> index 39c98c3d8ac5..62a1e96bbdd4 100644
> --- a/include/net/ip6_fib.h
> +++ b/include/net/ip6_fib.h
> @@ -486,11 +486,29 @@ 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");

let's keep the helpful, user facing message

> +	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,