[PATCH iproute2-next] ipmaddr: use RTM_GETMULTICAST to list multicast addresses

[PATCH iproute2-next] ipmaddr: use RTM_GETMULTICAST to list multicast addresses

[Yuyang Huang]

Replace /proc/net/igmp and /proc/net/igmp6 parsing in "ip maddr show"
with RTM_GETMULTICAST dumps. The kernel dumps IPv6 multicast addresses
via netlink since the beginning, IPv4 since v6.15 (eb4e17a1d915), and
reports the group users count via IFA_MC_USERS since kernel commits
7cb8198761e6 and e1d0f3f08391.

The netlink result is only used when it carries the same information
as procfs: if the dump fails (e.g. no IPv4 dump support) or any entry
lacks IFA_MC_USERS, the result is discarded and the procfs parsers
run as before, so output is unchanged on older kernels.

Link-layer multicast addresses are still read from
/proc/net/dev_mcast as there is no netlink API for them.

When a device is given, its ifindex is passed in the dump request so
strict-check kernels filter the dump server side; received entries
are checked against the ifindex again for kernels that ignore the
request field. An unknown device keeps printing an empty list.

Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
---
 include/libnetlink.h |   3 ++
 ip/ipmaddr.c         | 110 +++++++++++++++++++++++++++++++++++++++++--
 lib/libnetlink.c     |  26 ++++++++++
 3 files changed, 135 insertions(+), 4 deletions(-)

diff --git a/include/libnetlink.h b/include/libnetlink.h
index e91505d9..518b8714 100644
--- a/include/libnetlink.h
+++ b/include/libnetlink.h
@@ -62,6 +62,9 @@ typedef int (*req_filter_fn_t)(struct nlmsghdr *nlh, int reqlen);
 int rtnl_addrdump_req(struct rtnl_handle *rth, int family,
 		      req_filter_fn_t filter_fn)
 	__attribute__((warn_unused_result));
+int rtnl_mcaddrdump_req(struct rtnl_handle *rth, int family,
+			req_filter_fn_t filter_fn)
+	__attribute__((warn_unused_result));
 int rtnl_addrlbldump_req(struct rtnl_handle *rth, int family)
 	__attribute__((warn_unused_result));
 int rtnl_routedump_req(struct rtnl_handle *rth, int family,
diff --git a/ip/ipmaddr.c b/ip/ipmaddr.c
index 462b409e..7d8a3a09 100644
--- a/ip/ipmaddr.c
+++ b/ip/ipmaddr.c
@@ -27,6 +27,7 @@
 
 static struct {
 	char *dev;
+	int  index;
 	int  family;
 } filter;
 
@@ -207,6 +208,94 @@ static void read_igmp6(struct ma_info **result_p)
 	fclose(fp);
 }
 
+struct maddr_dump_ctx {
+	struct ma_info *list;
+	bool mc_users_missing;
+};
+
+static int maddr_dump_filter(struct nlmsghdr *nlh, int reqlen)
+{
+	struct ifaddrmsg *ifm = NLMSG_DATA(nlh);
+
+	ifm->ifa_index = filter.index;
+
+	return 0;
+}
+
+static int accept_maddr(struct nlmsghdr *n, void *arg)
+{
+	struct maddr_dump_ctx *ctx = arg;
+	struct ifaddrmsg *ifm = NLMSG_DATA(n);
+	int len = n->nlmsg_len - NLMSG_LENGTH(sizeof(*ifm));
+	struct rtattr *tb[IFA_MAX + 1];
+	struct ma_info *ma;
+
+	if (n->nlmsg_type != RTM_GETMULTICAST &&
+	    n->nlmsg_type != RTM_NEWMULTICAST)
+		return 0;
+
+	if (len < 0)
+		return -1;
+
+	if (filter.index && filter.index != ifm->ifa_index)
+		return 0;
+
+	parse_rtattr(tb, IFA_MAX, IFA_RTA(ifm), len);
+
+	if (!tb[IFA_MULTICAST] ||
+	    RTA_PAYLOAD(tb[IFA_MULTICAST]) > sizeof(ma->addr.data))
+		return 0;
+
+	if (!tb[IFA_MC_USERS]) {
+		ctx->mc_users_missing = true;
+		return 0;
+	}
+
+	ma = calloc(1, sizeof(*ma));
+	if (ma == NULL)
+		return -1;
+
+	ma->index = ifm->ifa_index;
+	strlcpy(ma->name, ll_index_to_name(ifm->ifa_index), sizeof(ma->name));
+	ma->addr.family = ifm->ifa_family;
+	ma->addr.bytelen = RTA_PAYLOAD(tb[IFA_MULTICAST]);
+	ma->addr.bitlen = ma->addr.bytelen << 3;
+	memcpy(ma->addr.data, RTA_DATA(tb[IFA_MULTICAST]), ma->addr.bytelen);
+	ma->users = rta_getattr_u32(tb[IFA_MC_USERS]);
+	maddr_ins(&ctx->list, ma);
+
+	return 0;
+}
+
+static int read_maddr_netlink(int family, struct ma_info **result_p)
+{
+	struct maddr_dump_ctx ctx = {};
+	struct ma_info *ma;
+	int err;
+
+	rth.flags |= RTNL_HANDLE_F_SUPPRESS_NLERR;
+	err = rtnl_mcaddrdump_req(&rth, family,
+				  filter.index ? maddr_dump_filter : NULL);
+	if (err >= 0)
+		err = rtnl_dump_filter(&rth, accept_maddr, &ctx);
+	rth.flags &= ~RTNL_HANDLE_F_SUPPRESS_NLERR;
+
+	/* Kernels that dump multicast addresses but do not report the
+	 * users count via IFA_MC_USERS cannot replace procfs.
+	 */
+	if (err < 0 || ctx.mc_users_missing) {
+		maddr_clear(ctx.list);
+		return -1;
+	}
+
+	while ((ma = ctx.list) != NULL) {
+		ctx.list = ma->next;
+		maddr_ins(result_p, ma);
+	}
+
+	return 0;
+}
+
 static void print_maddr(FILE *fp, struct ma_info *list)
 {
 	print_string(PRINT_FP, NULL, "\t", NULL);
@@ -291,12 +380,25 @@ static int multiaddr_list(int argc, char **argv)
 		argv++; argc--;
 	}
 
+	if (filter.dev) {
+		filter.index = ll_name_to_index(filter.dev);
+		/* an unknown device has no multicast addresses */
+		if (!filter.index) {
+			print_mlist(stdout, NULL);
+			return 0;
+		}
+	}
+
 	if (!filter.family || filter.family == AF_PACKET)
 		read_dev_mcast(&list);
-	if (!filter.family || filter.family == AF_INET)
-		read_igmp(&list);
-	if (!filter.family || filter.family == AF_INET6)
-		read_igmp6(&list);
+	if (!filter.family || filter.family == AF_INET) {
+		if (read_maddr_netlink(AF_INET, &list) < 0)
+			read_igmp(&list);
+	}
+	if (!filter.family || filter.family == AF_INET6) {
+		if (read_maddr_netlink(AF_INET6, &list) < 0)
+			read_igmp6(&list);
+	}
 	print_mlist(stdout, list);
 	maddr_clear(list);
 	return 0;
diff --git a/lib/libnetlink.c b/lib/libnetlink.c
index 8905e297..edf3a8ba 100644
--- a/lib/libnetlink.c
+++ b/lib/libnetlink.c
@@ -336,6 +336,32 @@ int rtnl_addrdump_req(struct rtnl_handle *rth, int family,
 	return send(rth->fd, &req, sizeof(req), 0);
 }
 
+int rtnl_mcaddrdump_req(struct rtnl_handle *rth, int family,
+			req_filter_fn_t filter_fn)
+{
+	struct {
+		struct nlmsghdr nlh;
+		struct ifaddrmsg ifm;
+		char buf[128];
+	} req = {
+		.nlh.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifaddrmsg)),
+		.nlh.nlmsg_type = RTM_GETMULTICAST,
+		.nlh.nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST,
+		.nlh.nlmsg_seq = rth->dump = ++rth->seq,
+		.ifm.ifa_family = family,
+	};
+
+	if (filter_fn) {
+		int err;
+
+		err = filter_fn(&req.nlh, sizeof(req));
+		if (err)
+			return err;
+	}
+
+	return send(rth->fd, &req, sizeof(req), 0);
+}
+
 int rtnl_addrlbldump_req(struct rtnl_handle *rth, int family)
 {
 	struct {
-- 
2.43.0