net: mctp: add gateway routing support

This change allows for gateway routing, where a route table entry
may reference a routable endpoint (by network and EID), instead of
routing directly to a netdevice.

We add support for a RTM_GATEWAY attribute for netlink route updates,
with an attribute format of:

    struct mctp_fq_addr {
        unsigned int net;
        mctp_eid_t eid;
    }

- we need the net here to uniquely identify the target EID, as we no
longer have the device reference directly (which would provide the net
id in the case of direct routes).

This makes route lookups recursive, as a route lookup that returns a
gateway route must be resolved into a direct route (ie, to a device)
eventually. We provide a limit to the route lookups, to prevent infinite
loop routing.

The route lookup populates a new 'nexthop' field in the dst structure,
which now specifies the key for the neighbour table lookup on device
output, rather than using the packet destination address directly.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20250702-dev-forwarding-v5-13-1468191da8a4@codeconstruct.com.au
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

2 files changed, 20 insertions, 1 deletions

diff --git a/include/net/mctp.h b/include/net/mctp.h
index b3af0690f607..ac4f4ecdfc24 100644
--- a/include/net/mctp.h
+++ b/include/net/mctp.h
@@ -237,8 +237,18 @@ struct mctp_route {
 	mctp_eid_t		min, max;
 
 	unsigned char		type;
+
 	unsigned int		mtu;
-	struct mctp_dev		*dev;
+
+	enum {
+		MCTP_ROUTE_DIRECT,
+		MCTP_ROUTE_GATEWAY,
+	} dst_type;
+	union {
+		struct mctp_dev	*dev;
+		struct mctp_fq_addr gateway;
+	};
+
 	int			(*output)(struct mctp_dst *dst,
 					  struct sk_buff *skb);
 
@@ -256,6 +266,7 @@ struct mctp_route {
 struct mctp_dst {
 	struct mctp_dev *dev;
 	unsigned int mtu;
+	mctp_eid_t nexthop;
 
 	/* set for direct addressing */
 	unsigned char halen;
diff --git a/include/uapi/linux/mctp.h b/include/uapi/linux/mctp.h
index e1db65df9359..19ad12a0cd4b 100644
--- a/include/uapi/linux/mctp.h
+++ b/include/uapi/linux/mctp.h
@@ -37,6 +37,14 @@ struct sockaddr_mctp_ext {
 	__u8			smctp_haddr[MAX_ADDR_LEN];
 };
 
+/* A "fully qualified" MCTP address, which includes the system-local network ID,
+ * required to uniquely resolve a routable EID.
+ */
+struct mctp_fq_addr {
+	unsigned int	net;
+	mctp_eid_t	eid;
+};
+
 #define MCTP_NET_ANY		0x0
 
 #define MCTP_ADDR_NULL		0x00