Subject: Re: [PATCH net v3 2/3] bpf: bpf_out_neigh_v6: Fix nd_tbl NULL dereference when IPv6 is disabled

From: Jakub Kicinski <[email-masked]>
Date: Fri, 6 Mar 2026 18:39:14 -0800

On Fri, 6 Mar 2026 10:58:44 +0100 Fernando Fernandez Mancera wrote:
> >> +	if (unlikely(!ipv6_stub->nd_tbl))
> >> +		goto out_drop;  
> > 
> > So neither Fernando nor you checked whether this code can be built
> > as a module? ipv6_mod_enabled() should work just fine
> 
> I checked it. If IPV6=m this is not fine.
> 
> ld: vmlinux.o: in function `bpf_out_neigh_v6':
> filter.c:(.text+0x3231d60): undefined reference to `ipv6_mod_enabled'

Guess I got it backwards :/ Should we add this to the series?

-->8--------
Subject: ipv6: move the disable_ipv6_mod knob to core code

Make sure disable_ipv6_mod itself is not part of the IPv6 module,
in case core code wants to refer to it. We will remove support
for IPv6=m soon, this change helps make fixes we commit before
that less messy.

Signed-off-by: Jakub Kicinski <[email-masked]>
---
 include/linux/ipv6.h | 7 ++++++-
 net/ipv4/af_inet.c   | 6 ++++++
 net/ipv6/af_inet6.c  | 8 --------
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 443053a76dcf..e5b6853d517a 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -333,7 +333,12 @@ struct tcp6_timewait_sock {
 };
 
 #if IS_ENABLED(CONFIG_IPV6)
-bool ipv6_mod_enabled(void);
+extern int disable_ipv6_mod;
+
+static inline bool ipv6_mod_enabled(void)
+{
+        return disable_ipv6_mod == 0;
+}
 
 static inline struct ipv6_pinfo *inet6_sk(const struct sock *__sk)
 {
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index babcd75a08e2..f1944e8fd9d3 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -124,6 +124,12 @@
 
 #include <trace/events/sock.h>
 
+/* Keep the definition of IPv6 disable here for now, to avoid annoying linker
+ * issues in case IPv6=m
+ */
+int disable_ipv6_mod;
+EXPORT_SYMBOL(disable_ipv6_mod);
+
 /* The inetsw table contains everything that inet_create needs to
  * build a new socket.
  */
diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index 0b995a961359..03c175cbbdb6 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -86,8 +86,6 @@ struct ipv6_params ipv6_defaults = {
 	.autoconf = 1,
 };
 
-static int disable_ipv6_mod;
-
 module_param_named(disable, disable_ipv6_mod, int, 0444);
 MODULE_PARM_DESC(disable, "Disable IPv6 module such that it is non-functional");
 
@@ -97,12 +95,6 @@ MODULE_PARM_DESC(disable_ipv6, "Disable IPv6 on all interfaces");
 module_param_named(autoconf, ipv6_defaults.autoconf, int, 0444);
 MODULE_PARM_DESC(autoconf, "Enable IPv6 address autoconfiguration on all interfaces");
 
-bool ipv6_mod_enabled(void)
-{
-	return disable_ipv6_mod == 0;
-}
-EXPORT_SYMBOL_GPL(ipv6_mod_enabled);
-
 static struct ipv6_pinfo *inet6_sk_generic(struct sock *sk)
 {
 	const int offset = sk->sk_prot->ipv6_pinfo_offset;
-- 
2.53.0