From: Alexander Bluhm <bluhm@openbsd.org>
Subject: Re: Unlock IPV6CTL_NEIGHBORGCTHRESH case of ip6_sysctl()
To: Vitaliy Makkoveev <mvs@openbsd.org>
Cc: tech@openbsd.org
Date: Sat, 26 Jul 2025 02:57:20 +0200

On Sat, Jul 26, 2025 at 02:08:19AM +0300, Vitaliy Makkoveev wrote:
> Both sysctl(2) and IPv6 stack accept negative value for
> `ip6_neighborgcthresh' to disable garbage collector of NDP entries. No
> limits is bad design, however I don't want to mix unlocking with
> behavior change.
> 
> Not used in ramdisk.

OK bluhm@

> Index: sys/netinet6/in6_proto.c
> ===================================================================
> RCS file: /cvs/src/sys/netinet6/in6_proto.c,v
> retrieving revision 1.146
> diff -u -p -r1.146 in6_proto.c
> --- sys/netinet6/in6_proto.c	25 Jul 2025 22:24:06 -0000	1.146
> +++ sys/netinet6/in6_proto.c	25 Jul 2025 23:02:15 -0000
> @@ -363,7 +363,7 @@ int	ip6_dad_pending;	/* number of curren
>  int	ip6_auto_flowlabel = 1;	/* [a] */
>  int	ip6_use_deprecated = 1;	/* [a] allow deprecated addr (RFC2462 5.5.4) */
>  int	ip6_mcast_pmtu = 0;	/* [a] enable pMTU discovery for multicast? */
> -int	ip6_neighborgcthresh = 2048; /* Threshold # of NDP entries for GC */
> +int	ip6_neighborgcthresh = 2048; /* [a] Threshold # of NDP entries for GC */
>  int	ip6_maxdynroutes = 4096; /* Max # of routes created via redirect */
>  time_t	ip6_log_time = (time_t)0L;
>  
> Index: sys/netinet6/ip6_input.c
> ===================================================================
> RCS file: /cvs/src/sys/netinet6/ip6_input.c,v
> retrieving revision 1.291
> diff -u -p -r1.291 ip6_input.c
> --- sys/netinet6/ip6_input.c	25 Jul 2025 22:24:06 -0000	1.291
> +++ sys/netinet6/ip6_input.c	25 Jul 2025 23:02:15 -0000
> @@ -1458,10 +1458,10 @@ const struct sysctl_bounded_args ipv6ctl
>  	{ IPV6CTL_MAXFRAGS, &ip6_maxfrags, 0, 1000 },
>  	{ IPV6CTL_MFORWARDING, &ip6_mforwarding, 0, 1 },
>  	{ IPV6CTL_MCAST_PMTU, &ip6_mcast_pmtu, 0, 1 },
> +	{ IPV6CTL_NEIGHBORGCTHRESH, &ip6_neighborgcthresh, -1, 5 * 2048 },
>  };
>  
>  const struct sysctl_bounded_args ipv6ctl_vars[] = {
> -	{ IPV6CTL_NEIGHBORGCTHRESH, &ip6_neighborgcthresh, -1, 5 * 2048 },
>  	{ IPV6CTL_MAXDYNROUTES, &ip6_maxdynroutes, -1, 5 * 4096 },
>  };
>  
> @@ -1578,6 +1578,7 @@ ip6_sysctl(int *name, u_int namelen, voi
>  	case IPV6CTL_MAXFRAGS:
>  	case IPV6CTL_MFORWARDING:
>  	case IPV6CTL_MCAST_PMTU:
> +	case IPV6CTL_NEIGHBORGCTHRESH:
>  		return (sysctl_bounded_arr(
>  		    ipv6ctl_vars_unlocked, nitems(ipv6ctl_vars_unlocked),
>  		    name, namelen, oldp, oldlenp, newp, newlen));
> Index: sys/netinet6/nd6.c
> ===================================================================
> RCS file: /cvs/src/sys/netinet6/nd6.c,v
> retrieving revision 1.292
> diff -u -p -r1.292 nd6.c
> --- sys/netinet6/nd6.c	8 Jul 2025 00:47:41 -0000	1.292
> +++ sys/netinet6/nd6.c	25 Jul 2025 23:02:15 -0000
> @@ -729,6 +729,7 @@ nd6_rtrequest(struct ifnet *ifp, int req
>  	struct llinfo_nd6 *ln;
>  	struct ifaddr *ifa;
>  	struct in6_ifaddr *ifa6;
> +	int ip6_neighborgcthresh_local;
>  
>  	if (ISSET(rt->rt_flags, RTF_GATEWAY|RTF_MULTICAST|RTF_MPLS))
>  		return;
> @@ -829,8 +830,11 @@ nd6_rtrequest(struct ifnet *ifp, int req
>  		 * cause re-entering rtable related routines triggering
>  		 * lock-order-reversal problems.
>  		 */
> -		if (ip6_neighborgcthresh >= 0 &&
> -		    nd6_inuse >= ip6_neighborgcthresh) {
> +		ip6_neighborgcthresh_local =
> +		    atomic_load_int(&ip6_neighborgcthresh);
> +
> +		if (ip6_neighborgcthresh_local >= 0 &&
> +		    nd6_inuse >= ip6_neighborgcthresh_local) {
>  			int i;
>  
>  			for (i = 0; i < 10; i++) {
>