Atomically accessed boolean. We don't need to use cached value in
ip_output(). While following through "reroute" label the `ifp' pointer
is NULL, so we don't reach the "if (atomic_load_int(&ipmforwarding) &&"
check.

Index: sys/netinet/ip_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_input.c,v
diff -u -p -r1.418 ip_input.c
--- sys/netinet/ip_input.c	15 Jul 2025 08:27:47 -0000	1.418
+++ sys/netinet/ip_input.c	15 Jul 2025 09:21:42 -0000
@@ -92,7 +92,7 @@
 
 /* values controllable via sysctl */
 int	ip_forwarding = 0;			/* [a] */
-int	ipmforwarding = 0;
+int	ipmforwarding = 0;			/* [a] */
 int	ipmultipath = 0;			/* [a] */
 int	ip_sendredirects = 1;			/* [a] */
 int	ip_dosourceroute = 0;			/* [a] */
@@ -124,10 +124,10 @@ const struct sysctl_bounded_args ipctl_v
 	{ IPCTL_IPPORT_HIFIRSTAUTO, &ipport_hifirstauto, 0, 65535 },
 	{ IPCTL_IPPORT_HILASTAUTO, &ipport_hilastauto, 0, 65535 },
 	{ IPCTL_IPPORT_MAXQUEUE, &ip_maxqueue, 0, 10000 },
+	{ IPCTL_MFORWARDING, &ipmforwarding, 0, 1 },
 };
 
 const struct sysctl_bounded_args ipctl_vars[] = {
-	{ IPCTL_MFORWARDING, &ipmforwarding, 0, 1 },
 	{ IPCTL_ARPTIMEOUT, &arpt_keep, 0, INT_MAX },
 	{ IPCTL_ARPDOWN, &arpt_down, 0, INT_MAX },
 };
@@ -529,7 +529,8 @@ ip_input_if(struct mbuf **mp, int *offp,
 		m->m_flags |= M_MCAST;
 
 #ifdef MROUTING
-		if (ipmforwarding && ip_mrouter[ifp->if_rdomain]) {
+		if (atomic_load_int(&ipmforwarding) &&
+		    ip_mrouter[ifp->if_rdomain]) {
 			int error;
 
 			if (m->m_flags & M_EXT) {
@@ -1843,6 +1844,7 @@ ip_sysctl(int *name, u_int namelen, void
 	case IPCTL_IPPORT_HIFIRSTAUTO:
 	case IPCTL_IPPORT_HILASTAUTO:
 	case IPCTL_IPPORT_MAXQUEUE:
+	case IPCTL_MFORWARDING:
 		return (sysctl_bounded_arr(
 		    ipctl_vars_unlocked, nitems(ipctl_vars_unlocked),
 		    name, namelen, oldp, oldlenp, newp, newlen));
Index: sys/netinet/ip_output.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_output.c,v
diff -u -p -r1.412 ip_output.c
--- sys/netinet/ip_output.c	8 Jul 2025 00:47:41 -0000	1.412
+++ sys/netinet/ip_output.c	15 Jul 2025 09:21:42 -0000
@@ -324,7 +324,8 @@ reroute:
 			 * above, will be forwarded by the ip_input() routine,
 			 * if necessary.
 			 */
-			if (ipmforwarding && ip_mrouter[ifp->if_rdomain] &&
+			if (atomic_load_int(&ipmforwarding) &&
+			    ip_mrouter[ifp->if_rdomain] &&
 			    (flags & IP_FORWARDING) == 0) {
 				int rv;