mctpd: Set initial route MTU to interface minimum

This allows better compatibility with devices that have a low
initial allowed packet size and require application negotiation
to increase that packet size.

Previously the MTU was left at 0, so matching the currently
set MTU of the interface.

Signed-off-by: Matt Johnston <[email protected]>

CHANGELOG.md

@@ -40,6 +40,13 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
    now associated with the interface object, they no longer take the
    interface name as their first argument.
 
+6. In mctpd the initial route MTU for an endpoint is now set to the minimum MTU
+   of the interface. This allows better compatibility with devices that 
+   have a low initial allowed packet size and require application negotiation
+   to increase that packet size. Previously the initial MTU was left as the
+   interface default (normally the maximum MTU).
+   The .SetMTU method can be used to set the endpoint route MTU.
+
 ### Fixed
 
 1. mctpd: EID assignments now work in the case where a new endpoint has a

src/mctp-netlink.c

@@ -26,6 +26,9 @@ struct linkmap_entry {
 	int	net;
 	bool 	up;
 
+	uint32_t min_mtu;
+	uint32_t max_mtu;
+
 	mctp_eid_t *local_eids;
 	size_t num_local;
 
@@ -54,7 +57,7 @@ static int fill_linkmap(mctp_nl *nl);
 static void sort_linkmap(mctp_nl *nl);
 static int linkmap_add_entry(mctp_nl *nl, struct ifinfomsg *info,
 		const char *ifname, size_t ifname_len, int net,
-		bool up);
+		bool up, uint32_t min_mtu, uint32_t max_mtu);
 static struct linkmap_entry *entry_byindex(const mctp_nl *nl,
 	int index);
 
@@ -678,10 +681,10 @@ static int parse_getlink_dump(mctp_nl *nl, struct nlmsghdr *nlh, uint32_t len)
 	struct ifinfomsg *info;
 
 	for (; NLMSG_OK(nlh, len); nlh = NLMSG_NEXT(nlh, len)) {
-		struct rtattr *rta, *rt_nest, *rt_mctp;
-		char *ifname;
+		struct rtattr *rta = NULL, *rt_nest = NULL, *rt_mctp = NULL;
+		char *ifname = NULL;
 		size_t ifname_len, rlen, nlen, mlen;
-		uint32_t net;
+		uint32_t net, min_mtu = 0, max_mtu = 0;
 		bool up;
 
 		if (nlh->nlmsg_type == NLMSG_DONE)
@@ -700,7 +703,6 @@ static int parse_getlink_dump(mctp_nl *nl, struct nlmsghdr *nlh, uint32_t len)
 		rta = (void *)(info + 1);
 		rlen = NLMSG_PAYLOAD(nlh, sizeof(*info));
 
-		rt_mctp = false;
 		rt_nest = mctp_get_rtnlmsg_attr(IFLA_AF_SPEC, rta, rlen, &nlen);
 		if (rt_nest) {
 			rt_mctp = mctp_get_rtnlmsg_attr(AF_MCTP, rt_nest, nlen, &mlen);
@@ -709,21 +711,33 @@ static int parse_getlink_dump(mctp_nl *nl, struct nlmsghdr *nlh, uint32_t len)
 			/* Skip non-MCTP interfaces */
 			continue;
 		}
-		if (!mctp_get_rtnlmsg_attr_u32(IFLA_MCTP_NET, rt_mctp, mlen, &net)) {
-			warnx("Missing IFLA_MCTP_NET");
-			continue;
-		}
-
-		/* TODO: media type */
 
 		ifname = mctp_get_rtnlmsg_attr(IFLA_IFNAME, rta, rlen, &ifname_len);
 		if (!ifname) {
 			warnx("no ifname?");
 			continue;
 		}
+
 		ifname_len = strnlen(ifname, ifname_len);
+		if (!mctp_get_rtnlmsg_attr_u32(IFLA_MCTP_NET, rt_mctp, mlen, &net)) {
+			warnx("Missing IFLA_MCTP_NET for %s", ifname);
+			continue;
+		}
+
+		if (!mctp_get_rtnlmsg_attr_u32(IFLA_MIN_MTU, rta, rlen, &min_mtu)) {
+			warnx("Missing IFLA_MIN_MTU for %s", ifname);
+			continue;
+		}
+
+		if (!mctp_get_rtnlmsg_attr_u32(IFLA_MAX_MTU, rta, rlen, &max_mtu)) {
+			warnx("Missing IFLA_MAX_MTU for %s", ifname);
+			continue;
+		}
+
+		/* TODO: media type */
+
 		up = info->ifi_flags & IFF_UP;
-		linkmap_add_entry(nl, info, ifname, ifname_len, net, up);
+		linkmap_add_entry(nl, info, ifname, ifname_len, net, up, min_mtu, max_mtu);
 	}
 	// Not done.
 	return 1;
@@ -973,6 +987,22 @@ bool mctp_nl_up_byindex(const mctp_nl *nl, int index)
 	return false;
 }
 
+uint32_t mctp_nl_min_mtu_byindex(const mctp_nl *nl, int index)
+{
+	struct linkmap_entry *entry = entry_byindex(nl, index);
+	if (entry)
+		return entry->min_mtu;
+	return 0;
+}
+
+uint32_t mctp_nl_max_mtu_byindex(const mctp_nl *nl, int index)
+{
+	struct linkmap_entry *entry = entry_byindex(nl, index);
+	if (entry)
+		return entry->max_mtu;
+	return 0;
+}
+
 mctp_eid_t *mctp_nl_addrs_byindex(const mctp_nl *nl, int index,
 	size_t *ret_num)
 {
@@ -1055,7 +1085,7 @@ int *mctp_nl_if_list(const mctp_nl *nl, size_t *ret_num_ifs)
 
 static int linkmap_add_entry(mctp_nl *nl, struct ifinfomsg *info,
 		const char *ifname, size_t ifname_len, int net,
-		bool up)
+		bool up, uint32_t min_mtu, uint32_t max_mtu)
 {
 	struct linkmap_entry *entry;
 	size_t newsz;
@@ -1091,6 +1121,8 @@ static int linkmap_add_entry(mctp_nl *nl, struct ifinfomsg *info,
 	entry->ifindex = info->ifi_index;
 	entry->net = net;
 	entry->up = up;
+	entry->max_mtu = max_mtu;
+	entry->min_mtu = min_mtu;
 	return 0;
 }
 

src/mctp-netlink.h

@@ -64,6 +64,10 @@ int mctp_nl_ifindex_byname(const mctp_nl *nl, const char *ifname);
 const char* mctp_nl_if_byindex(const mctp_nl *nl, int index);
 int mctp_nl_net_byindex(const mctp_nl *nl, int index);
 bool mctp_nl_up_byindex(const mctp_nl *nl, int index);
+/* Returns interface min_mtu, or 0 if bad index */
+uint32_t mctp_nl_min_mtu_byindex(const mctp_nl *nl, int index);
+/* Returns interface max_mtu, or 0 if bad index */
+uint32_t mctp_nl_max_mtu_byindex(const mctp_nl *nl, int index);
 /* Caller to free */
 mctp_eid_t *mctp_nl_addrs_byindex(const mctp_nl *nl, int index,
 	size_t *ret_num);

src/mctpd.c

@@ -146,7 +146,8 @@ struct peer {
 	bool have_neigh;
 	bool have_route;
 
-	// set by SetMTU method, 0 otherwise
+	// MTU for the route. Set to the interface's minimum MTU initially,
+	// or changed by .SetMTU method
 	uint32_t mtu;
 
 	// malloc()ed list of supported message types, from Get Message Type
@@ -2161,6 +2162,10 @@ static int setup_added_peer(peer *peer)
 {
 	int rc;
 
+	// Set minimum MTU by default for compatibility. Clients can increase
+	// this with .SetMTU as needed
+	peer->mtu = mctp_nl_min_mtu_byindex(peer->ctx->nl_query, peer->phys.ifindex);
+
 	// add route before querying
 	add_peer_route(peer);