netlink: support dumping IPv4 multicast addresses

Extended RTM_GETMULTICAST to support dumping joined IPv4 multicast
addresses, in addition to the existing IPv6 functionality. This allows
userspace applications to retrieve both IPv4 and IPv6 multicast
addresses through similar netlink command and then monitor future
changes by registering to RTNLGRP_IPV4_MCADDR and RTNLGRP_IPV6_MCADDR.

Cc: Maciej Żenczykowski <[e-mail]>
Cc: Lorenzo Colitti <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Signed-off-by: Yuyang Huang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/ipv4/devinet.c

···
 #include <linux/notifier.h>
 #include <linux/inetdevice.h>
 #include <linux/igmp.h>
+#include "igmp_internal.h"
 #include <linux/slab.h>
 #include <linux/hash.h>
 #ifdef CONFIG_SYSCTL
···
 	[IFA_RT_PRIORITY]	= { .type = NLA_U32 },
 	[IFA_TARGET_NETNSID]	= { .type = NLA_S32 },
 	[IFA_PROTO]		= { .type = NLA_U8 },
-};
-
-struct inet_fill_args {
-	u32 portid;
-	u32 seq;
-	int event;
-	unsigned int flags;
-	int netnsid;
-	int ifindex;
 };
 
 #define IN4_ADDR_HSIZE_SHIFT	8
···
 	return 0;
 }
 
-static int in_dev_dump_addr(struct in_device *in_dev, struct sk_buff *skb,
-			    struct netlink_callback *cb, int *s_ip_idx,
-			    struct inet_fill_args *fillargs)
+static int in_dev_dump_ifmcaddr(struct in_device *in_dev, struct sk_buff *skb,
+				struct netlink_callback *cb, int *s_ip_idx,
+				struct inet_fill_args *fillargs)
+{
+	struct ip_mc_list *im;
+	int ip_idx = 0;
+	int err;
+
+	for (im = rcu_dereference(in_dev->mc_list);
+	     im;
+	     im = rcu_dereference(im->next_rcu)) {
+		if (ip_idx < *s_ip_idx) {
+			ip_idx++;
+			continue;
+		}
+		err = inet_fill_ifmcaddr(skb, in_dev->dev, im, fillargs);
+		if (err < 0)
+			goto done;
+
+		nl_dump_check_consistent(cb, nlmsg_hdr(skb));
+		ip_idx++;
+	}
+	err = 0;
+	ip_idx = 0;
+done:
+	*s_ip_idx = ip_idx;
+	return err;
+}
+
+static int in_dev_dump_ifaddr(struct in_device *in_dev, struct sk_buff *skb,
+			      struct netlink_callback *cb, int *s_ip_idx,
+			      struct inet_fill_args *fillargs)
 {
 	struct in_ifaddr *ifa;
 	int ip_idx = 0;
···
 	return err;
 }
 
+static int in_dev_dump_addr(struct in_device *in_dev, struct sk_buff *skb,
+			    struct netlink_callback *cb, int *s_ip_idx,
+			    struct inet_fill_args *fillargs)
+{
+	switch (fillargs->event) {
+	case RTM_NEWADDR:
+		return in_dev_dump_ifaddr(in_dev, skb, cb, s_ip_idx, fillargs);
+	case RTM_GETMULTICAST:
+		return in_dev_dump_ifmcaddr(in_dev, skb, cb, s_ip_idx,
+					    fillargs);
+	default:
+		return -EINVAL;
+	}
+}
+
 /* Combine dev_addr_genid and dev_base_seq to detect changes.
  */
 static u32 inet_base_seq(const struct net *net)
···
 	return res;
 }
 
-static int inet_dump_ifaddr(struct sk_buff *skb, struct netlink_callback *cb)
+static int inet_dump_addr(struct sk_buff *skb, struct netlink_callback *cb,
+			  int event)
 {
 	const struct nlmsghdr *nlh = cb->nlh;
 	struct inet_fill_args fillargs = {
 		.portid = NETLINK_CB(cb->skb).portid,
 		.seq = nlh->nlmsg_seq,
-		.event = RTM_NEWADDR,
+		.event = event,
 		.flags = NLM_F_MULTI,
 		.netnsid = -1,
 	};
···
 		put_net(tgt_net);
 	rcu_read_unlock();
 	return err;
+}
+
+static int inet_dump_ifaddr(struct sk_buff *skb, struct netlink_callback *cb)
+{
+	return inet_dump_addr(skb, cb, RTM_NEWADDR);
+}
+
+static int inet_dump_ifmcaddr(struct sk_buff *skb, struct netlink_callback *cb)
+{
+	return inet_dump_addr(skb, cb, RTM_GETMULTICAST);
 }
 
 static void rtmsg_ifa(int event, struct in_ifaddr *ifa, struct nlmsghdr *nlh,
···
 	{.protocol = PF_INET, .msgtype = RTM_GETNETCONF,
 	 .doit = inet_netconf_get_devconf, .dumpit = inet_netconf_dump_devconf,
 	 .flags = RTNL_FLAG_DOIT_UNLOCKED | RTNL_FLAG_DUMP_UNLOCKED},
+	{.owner = THIS_MODULE, .protocol = PF_INET, .msgtype = RTM_GETMULTICAST,
+	 .dumpit = inet_dump_ifmcaddr, .flags = RTNL_FLAG_DUMP_UNLOCKED},
 };
 
 void __init devinet_init(void)

net/ipv4/igmp.c

···
 #include <linux/skbuff.h>
 #include <linux/inetdevice.h>
 #include <linux/igmp.h>
+#include "igmp_internal.h"
 #include <linux/if_arp.h>
 #include <linux/rtnetlink.h>
 #include <linux/times.h>
···
 	*mc_hash = im->next_hash;
 }
 
-static int inet_fill_ifmcaddr(struct sk_buff *skb, struct net_device *dev,
-			      const struct ip_mc_list *im, int event)
+int inet_fill_ifmcaddr(struct sk_buff *skb, struct net_device *dev,
+		       const struct ip_mc_list *im,
+		       struct inet_fill_args *args)
 {
 	struct ifa_cacheinfo ci;
 	struct ifaddrmsg *ifm;
 	struct nlmsghdr *nlh;
 
-	nlh = nlmsg_put(skb, 0, 0, event, sizeof(struct ifaddrmsg), 0);
+	nlh = nlmsg_put(skb, args->portid, args->seq, args->event,
+			sizeof(struct ifaddrmsg), args->flags);
 	if (!nlh)
 		return -EMSGSIZE;
 
···
 static void inet_ifmcaddr_notify(struct net_device *dev,
 				 const struct ip_mc_list *im, int event)
 {
+	struct inet_fill_args fillargs = {
+		.event = event,
+	};
 	struct net *net = dev_net(dev);
 	struct sk_buff *skb;
 	int err = -ENOMEM;
···
 	if (!skb)
 		goto error;
 
-	err = inet_fill_ifmcaddr(skb, dev, im, event);
+	err = inet_fill_ifmcaddr(skb, dev, im, &fillargs);
 	if (err < 0) {
 		WARN_ON_ONCE(err == -EMSGSIZE);
 		nlmsg_free(skb);

net/ipv4/igmp_internal.h

···
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _LINUX_IGMP_INTERNAL_H
+#define _LINUX_IGMP_INTERNAL_H
+
+struct inet_fill_args {
+	u32 portid;
+	u32 seq;
+	int event;
+	unsigned int flags;
+	int netnsid;
+	int ifindex;
+};
+
+int inet_fill_ifmcaddr(struct sk_buff *skb, struct net_device *dev,
+		       const struct ip_mc_list *im,
+		       struct inet_fill_args *args);
+#endif