Re: git: f9cdaeeeb9ad - main - netinet6: Pass IPv4-mapped ASM multicast joins/leaves to netinet.

On Mon, Aug 03, 2026 at 11:28:38AM +0000, Bruce M Simpson wrote:
> The branch main has been updated by bms:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=f9cdaeeeb9ad45a5e8c0cd32ff9443d245b64185
> 
> commit f9cdaeeeb9ad45a5e8c0cd32ff9443d245b64185
> Author:     Bruce M Simpson <bms@FreeBSD.org>
> AuthorDate: 2026-08-01 10:14:57 +0000
> Commit:     Bruce M Simpson <bms@FreeBSD.org>
> CommitDate: 2026-08-03 11:27:45 +0000
> 
>     netinet6: Pass IPv4-mapped ASM multicast joins/leaves to netinet.
>     
>     Add support for allowing IPv4 multicast groups to be joined on IPv6 sockets,
>     as a number of applications began to rely on this over the years, despite it
>     only ever having been a convenience which appeared in Solaris & Linux over
>     the course of the 00s decade. It is limited to any-source joins (ASM).
>     
>     To avoid further quibbling over the meaning of the term "undocumented" as it
>     applies to this change, I have chosen to use the wording "non-IETF-ratified
>     extension" in comments, with reference to the updated ip6(4) man page.
>     
>     PR:             https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=193246
> ---
>  sys/netinet/in_mcast.c   |   6 +--
>  sys/netinet/in_var.h     |   5 +++
>  sys/netinet6/in6_mcast.c | 112 ++++++++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 118 insertions(+), 5 deletions(-)
> 
> diff --git a/sys/netinet/in_mcast.c b/sys/netinet/in_mcast.c
> index 934cc71e745b..ad908b72c340 100644
> --- a/sys/netinet/in_mcast.c
> +++ b/sys/netinet/in_mcast.c
> @@ -158,8 +158,6 @@ static void inm_release(struct in_multi *);
>  static struct ip_moptions *
>  		inp_findmoptions(struct inpcb *);
>  static int	inp_get_source_filters(struct inpcb *, struct sockopt *);
> -static int	inp_join_group(struct inpcb *, struct sockopt *);
> -static int	inp_leave_group(struct inpcb *, struct sockopt *);
>  static int	inp_block_unblock_source(struct inpcb *, struct sockopt *);
>  static int	inp_set_multicast_if(struct inpcb *, struct sockopt *);
>  static int	inp_set_source_filters(struct inpcb *, struct sockopt *);
> @@ -1884,7 +1882,7 @@ const struct in_addr *ina, const u_int index)
>  /*
>   * Join an IPv4 multicast group, possibly with a source.
>   */
> -static int
> +int
>  inp_join_group(struct inpcb *inp, struct sockopt *sopt)
>  {
>  	struct group_source_req		 gsr;
> @@ -2208,7 +2206,7 @@ out_inp_unlocked:
>  /*
>   * Leave an IPv4 multicast group on an inpcb, possibly with a source.
>   */
> -static int
> +int
>  inp_leave_group(struct inpcb *inp, struct sockopt *sopt)
>  {
>  	struct epoch_tracker		 et;
> diff --git a/sys/netinet/in_var.h b/sys/netinet/in_var.h
> index 99a628477155..e9db1efd9fe9 100644
> --- a/sys/netinet/in_var.h
> +++ b/sys/netinet/in_var.h
> @@ -459,6 +459,11 @@ void	in_ifattach(void *, struct ifnet *);
>  void	in_detachhead(struct rib_head *rh);
>  #endif
>  
> +struct sockopt;
> +
> +int	inp_join_group(struct inpcb *, struct sockopt *);
> +int	inp_leave_group(struct inpcb *, struct sockopt *);
> +
>  #endif /* _KERNEL */
>  
>  /* INET6 stuff */
> diff --git a/sys/netinet6/in6_mcast.c b/sys/netinet6/in6_mcast.c
> index fad47cb0e69b..a204ad88a9a5 100644
> --- a/sys/netinet6/in6_mcast.c
> +++ b/sys/netinet6/in6_mcast.c
> @@ -1,7 +1,7 @@
>  /*-
>   * SPDX-License-Identifier: BSD-3-Clause
>   *
> - * Copyright (c) 2009 Bruce Simpson.
> + * Copyright (c) 2009-2026 Bruce Simpson.
>   * All rights reserved.
>   *
>   * Redistribution and use in source and binary forms, with or without
> @@ -34,6 +34,7 @@
>   * Normative references: RFC 2292, RFC 3492, RFC 3542, RFC 3678, RFC 3810.
>   */
>  
> +#include "opt_inet.h"
>  #include "opt_inet6.h"
>  
>  #include <sys/param.h>
> @@ -162,6 +163,9 @@ static struct ifnet *
>  static int	in6p_block_unblock_source(struct inpcb *, struct sockopt *);
>  static int	in6p_set_multicast_if(struct inpcb *, struct sockopt *);
>  static int	in6p_set_source_filters(struct inpcb *, struct sockopt *);
> +#ifdef INET
> +static int	in6_v6_mreq_to_v4(struct ipv6_mreq *, struct ip_mreq *);
> +#endif
>  static int	sysctl_ip6_mcast_filters(SYSCTL_HANDLER_ARGS);
>  
>  SYSCTL_DECL(_net_inet6_ip6);	/* XXX Not in any common header. */
> @@ -1881,6 +1885,54 @@ in6p_lookup_mcast_ifp(const struct inpcb *inp, const struct sockaddr_in6 *gsin6)
>  	return (nh ? nh->nh_ifp : NULL);
>  }
>  
> +#ifdef INET
> +/*
> + * Perform sockopt mreq argument conversion for IPv4-mapped groups.
> + *
> + * This function is required to support an extension to the behaviour
> + * in RFC 3493 Sec 3.7, which was never formally proposed by any
> + * contemporary IPv6 normative reference, but which is now required
> + * by much application software using IPv6 sockets as a convenience.
> + * Refer to manual page ip6(4) for further information.
> + *
> + * FUTURE: Use IPv4 source-address selection.
> + */
> +static int
> +in6_v6_mreq_to_v4(struct ipv6_mreq *mreq, struct ip_mreq *mreq_v4)
> +{
> +	int			 error;
> +	struct epoch_tracker	 et;
> +	struct ifnet		*ifp;
> +	struct in_ifaddr	*ia;
> +
> +	NET_EPOCH_ENTER(et);
> +
> +	ifp = ifnet_byindex(mreq->ipv6mr_interface);
> +	if (ifp == NULL) {
> +		error = EADDRNOTAVAIL;
> +		goto out;
> +	}
> +
> +	/*
> +	 * Here, we do not compare the ifnet's primary IPv4 address with
> +	 * INADDR_ANY, to permit its use during system initialization.
> +	 * If this is not required, an appropriate check to screen it out
> +	 * should be added, e.g. in_nullhost(ia->ia_addr.sin_addr.s_addr).
> +	 */
> +	ia = in_ifprimaryaddr(ifp);
> +	if (ia == NULL) {
> +		error = EADDRNOTAVAIL;
> +		goto out;
> +	}
> +	mreq_v4->imr_interface.s_addr = IA_SIN(ia)->sin_addr.s_addr;
> +	error = 0;
> +
> +out:
> +	NET_EPOCH_EXIT(et);
> +	return (error);
> +}
> +#endif /* INET */
> +
>  /*
>   * Join an IPv6 multicast group, possibly with a source.
>   *
> @@ -1929,6 +1981,35 @@ in6p_join_group(struct inpcb *inp, struct sockopt *sopt)
>  		    sizeof(struct ipv6_mreq));
>  		if (error)
>  			return (error);
> +#ifdef INET
> +		/*
> +		 * Support for the non-IETF-ratified extension to RFC 3493 to
> +		 * join IPv4 groups as IPv4 mapped addresses on IPv6 sockets.
> +		 */
> +		if (IN6_IS_ADDR_V4MAPPED(&mreq.ipv6mr_multiaddr)) {
> +			struct ip_mreq mreq_v4;
> +			struct sockopt sopt_v4 = {
> +				.sopt_dir     = SOPT_SET,
> +				.sopt_level   = sopt->sopt_level,
> +				.sopt_name    = IP_ADD_MEMBERSHIP,
> +				.sopt_val     = &mreq_v4,
> +				.sopt_valsize = sizeof(mreq_v4),
> +				.sopt_rights  = sopt->sopt_rights,
> +				.sopt_td      = sopt->sopt_td
> +			};
> +
> +			mreq_v4.imr_multiaddr.s_addr =
> +			    mreq.ipv6mr_multiaddr.s6_addr32[3];
> +			if (mreq.ipv6mr_interface == 0)
> +				mreq_v4.imr_interface.s_addr = INADDR_ANY;
> +			else
> +				error = in6_v6_mreq_to_v4(&mreq, &mreq_v4);
> +			if (error)
> +				return error;
> +
> +			return (inp_join_group(inp, &sopt_v4));

I'm not sure how this can work: inp_join_group() will invoke
sooptcopyin() with sopt_val pointing to kernel memory and sopt_td
non-NULL, so sooptcopyin() will try to copy in from a kernel address and
inevitably return EFAULT.