Re: [PATCH 00/11 net-next v4] Convert CONFIG_IPV6 to built-in and remove stubs

[Ido Schimmel]

On Fri, Mar 20, 2026 at 07:55:52PM +0100, Fernando Fernandez Mancera wrote:
> Changes:
>   v4:
>     Patch 2: new patch introduced on this revision
>     Patch 3: drop some IS_BUILTIN(IPV6) on files compiled only when IPV6=y
>     Patch 5: introduce ipv6_mod_enabled() checks on fib6_nh_init(), ip6_dst_lookup_flow() and ip6_fragment() to avoid crash when ipv6.disable=1 during boot
>     Patch 6: use &nd_tbl on code guarded by IS_ENABLED(CONFIG_IPV6)

The IPv6 code is still using ipv6_get_nd_tbl() for some reason. If you
remove it from there (it's not needed), then we are only left with
handful of users that can be converted to use '&nd_tbl' when IPv6 is
enabled. Something like the patch below.

diff --git a/include/net/ndisc.h b/include/net/ndisc.h
index 9043ec00c050..6794ce58acca 100644
--- a/include/net/ndisc.h
+++ b/include/net/ndisc.h
@@ -67,12 +67,6 @@ 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;
@@ -360,7 +354,7 @@ 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(ipv6_get_nd_tbl(), neigh_key_eq128,
+	return ___neigh_lookup_noref(&nd_tbl, neigh_key_eq128,
 				     ndisc_hashfn, pkey, dev);
 }
 
diff --git a/net/core/filter.c b/net/core/filter.c
index 4b0c98f0d994..d55525cc5540 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6093,9 +6093,9 @@ static int bpf_fib_set_fwd_params(struct bpf_fib_lookup *params, u32 mtu)
 static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 			       u32 flags, bool check_mtu)
 {
+	struct neighbour *neigh = NULL;
 	struct fib_nh_common *nhc;
 	struct in_device *in_dev;
-	struct neighbour *neigh;
 	struct net_device *dev;
 	struct fib_result res;
 	struct flowi4 fl4;
@@ -6215,7 +6215,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	if (likely(nhc->nhc_gw_family != AF_INET6))
 		neigh = __ipv4_neigh_lookup_noref(dev,
 						  (__force u32)params->ipv4_dst);
-	else
+	else if (IS_ENABLED(CONFIG_IPV6))
 		neigh = __ipv6_neigh_lookup_noref(dev, params->ipv6_dst);
 
 	if (!neigh || !(READ_ONCE(neigh->nud_state) & NUD_VALID))
diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index b576dd74d678..3e8fadc28798 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -585,8 +585,8 @@ static int fib_detect_death(struct fib_info *fi, int order,
 
 	if (likely(nhc->nhc_gw_family == AF_INET))
 		n = neigh_lookup(&arp_tbl, &nhc->nhc_gw.ipv4, nhc->nhc_dev);
-	else if (nhc->nhc_gw_family == AF_INET6)
-		n = neigh_lookup(ipv6_get_nd_tbl(), &nhc->nhc_gw.ipv6, nhc->nhc_dev);
+	else if (IS_ENABLED(CONFIG_IPV6) && nhc->nhc_gw_family == AF_INET6)
+		n = neigh_lookup(&nd_tbl, &nhc->nhc_gw.ipv6, nhc->nhc_dev);
 	else
 		n = NULL;
 
@@ -2146,7 +2146,8 @@ static bool fib_good_nh(const struct fib_nh *nh)
 		if (likely(nh->fib_nh_gw_family == AF_INET))
 			n = __ipv4_neigh_lookup_noref(nh->fib_nh_dev,
 						   (__force u32)nh->fib_nh_gw4);
-		else if (nh->fib_nh_gw_family == AF_INET6)
+		else if (IS_ENABLED(CONFIG_IPV6) &&
+			 nh->fib_nh_gw_family == AF_INET6)
 			n = __ipv6_neigh_lookup_noref(nh->fib_nh_dev,
 						      &nh->fib_nh_gw6);
 		else
diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index c65f88b253b9..9703e67d552a 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -1401,7 +1401,7 @@ static bool nexthop_is_good_nh(const struct nexthop *nh)
 	case AF_INET:
 		return ipv4_good_nh(&nhi->fib_nh);
 	case AF_INET6:
-		return ipv6_good_nh(&nhi->fib6_nh);
+		return IS_ENABLED(CONFIG_IPV6) && ipv6_good_nh(&nhi->fib6_nh);
 	}
 
 	return false;
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 802c68a1b446..bc1296f0ea69 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -446,7 +446,7 @@ static void ipv4_confirm_neigh(const struct dst_entry *dst, const void *daddr)
 
 	if (rt->rt_gw_family == AF_INET) {
 		pkey = (const __be32 *)&rt->rt_gw4;
-	} else if (rt->rt_gw_family == AF_INET6) {
+	} else if (IS_ENABLED(CONFIG_IPV6) && rt->rt_gw_family == AF_INET6) {
 		return __ipv6_confirm_neigh(dev, &rt->rt_gw6);
 	} else if (!daddr ||
 		 (rt->rt_flags &