net: nexthop: add NHA_DST_PORT for fdb nexthops

Commit 1274e1cc4226 ("vxlan: ecmp support for mac fdb entries") lets a
single inner MAC be reached through a group of remote VTEPs, with the
kernel flow-hashing across the group members.  Each member carries its
own remote IP, but the UDP destination port is always taken from the
VXLAN device (vxlan->cfg.dst_port) and cannot be set per member.

Some deployments pack several receivers behind one underlay IP and tell
them apart by UDP port, so they need a per-nexthop destination port to
spread flows across (IP, port) tuples rather than IP alone.

Add a netlink attribute NHA_DST_PORT (__be16, mirroring NDA_PORT) that
carries an optional UDP destination port on an fdb nexthop.  It is only
accepted together with NHA_FDB and NHA_GATEWAY; it is stored in struct
nh_info and echoed back on dump.  The attribute is named generically
rather than fdb-specific so it can be reused should another nexthop type
ever need a destination port.  This patch is control-plane plumbing
only; the VXLAN datapath is wired up in a follow-up patch, so behaviour
is unchanged for now.

Signed-off-by: Jack Ma <jack4it@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260724-b4-vxlan-fdb-port-v5-1-cd1c6aeee058@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 19 insertions, 1 deletions

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index 0f1e21a5c812..af1dcb8ea427 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -39,6 +39,7 @@ static const struct nla_policy rtm_nh_policy_new[] = {
 	[NHA_ENCAP_TYPE]	= { .type = NLA_U16 },
 	[NHA_ENCAP]		= { .type = NLA_NESTED },
 	[NHA_FDB]		= { .type = NLA_FLAG },
+	[NHA_DST_PORT]		= NLA_POLICY_MIN(NLA_BE16, 1),
 	[NHA_RES_GROUP]		= { .type = NLA_NESTED },
 	[NHA_HW_STATS_ENABLE]	= NLA_POLICY_MAX(NLA_U32, true),
 };
@@ -956,6 +957,9 @@ static int nh_fill_node(struct sk_buff *skb, struct nexthop *nh,
 	} else if (nhi->fdb_nh) {
 		if (nla_put_flag(skb, NHA_FDB))
 			goto nla_put_failure;
+		if (nhi->dst_port &&
+		    nla_put_be16(skb, NHA_DST_PORT, nhi->dst_port))
+			goto nla_put_failure;
 	} else {
 		const struct net_device *dev;
 
@@ -1055,6 +1059,9 @@ static size_t nh_nlmsg_size_single(struct nexthop *nh)
 		break;
 	}
 
+	if (nhi->dst_port)
+		sz += nla_total_size(2);	/* NHA_DST_PORT */
+
 	if (nhi->fib_nhc.nhc_lwtstate) {
 		sz += lwtunnel_get_encap_size(nhi->fib_nhc.nhc_lwtstate);
 		sz += nla_total_size(2);  /* NHA_ENCAP_TYPE */
@@ -2965,8 +2972,10 @@ static struct nexthop *nexthop_create(struct net *net, struct nh_config *cfg,
 	nhi->family = cfg->nh_family;
 	nhi->fib_nhc.nhc_scope = RT_SCOPE_LINK;
 
-	if (cfg->nh_fdb)
+	if (cfg->nh_fdb) {
 		nhi->fdb_nh = 1;
+		nhi->dst_port = cfg->nh_dst_port;
+	}
 
 	if (cfg->nh_blackhole) {
 		nhi->reject_nh = 1;
@@ -3156,6 +3165,15 @@ static int rtm_to_nh_config(struct net *net, struct sk_buff *skb,
 		cfg->nh_fdb = nla_get_flag(tb[NHA_FDB]);
 	}
 
+	if (tb[NHA_DST_PORT]) {
+		if (!tb[NHA_FDB] || !tb[NHA_GATEWAY]) {
+			NL_SET_ERR_MSG(extack,
+				       "Destination port can only be set on fdb nexthops that have a gateway");
+			goto out;
+		}
+		cfg->nh_dst_port = nla_get_be16(tb[NHA_DST_PORT]);
+	}
+
 	if (tb[NHA_GROUP]) {
 		if (nhm->nh_family != AF_UNSPEC) {
 			NL_SET_ERR_MSG(extack, "Invalid family for group");