route-table: Allow parsing routes without nexthop.

A recent commit added condition to "route_table_parse__" function that
causes it to throw an error when parsing route without "nexthop
information" (either RTA_OIF, RTA_GATEWAY, RTA_VIA or RTA_MULTIPATH). While
this requirement is reasonable for regular routes, there are some route types
that don't need nexthop. We intend to use one of these types,
(RTN_BLACKHOLE)[0], in OVN for route advertising .

This change does not enforce the above-mentioned condition for those special
route types that don't require "nexthop information".

[0] https://mail.openvswitch.org/pipermail/ovs-dev/2025-January/419383.html

Fixes: 91fc511 ("route-table: Support parsing multipath routes.")
Signed-off-by: Martin Kalcok <[email protected]>
Signed-off-by: Eelco Chaudron <[email protected]>

lib/route-table.c

@@ -226,6 +226,24 @@ route_table_reset(void)
     }
 }
 
+/* Returns true if the given route requires nexthop information (output
+ * interface, nexthop IP, ...).  Returns false for special route types
+ * that don't need this information. */
+static bool
+route_type_needs_nexthop(unsigned char rtmsg_type)
+{
+    switch (rtmsg_type) {
+    case RTN_BLACKHOLE:
+    case RTN_THROW:
+    case RTN_UNREACHABLE:
+    case RTN_PROHIBIT:
+        return false;
+
+    default:
+        return true;
+    }
+}
+
 static int
 route_table_parse__(struct ofpbuf *buf, size_t ofs,
                     const struct nlmsghdr *nlmsg,
@@ -447,13 +465,20 @@ route_table_parse__(struct ofpbuf *buf, size_t ofs,
                                        &mp_change.rd.nexthops);
             }
         }
-        if (!attrs[RTA_OIF] && !attrs[RTA_GATEWAY]
-                && !attrs[RTA_VIA] && !attrs[RTA_MULTIPATH]) {
+        if (route_type_needs_nexthop(rtm->rtm_type)
+            && !attrs[RTA_OIF] && !attrs[RTA_GATEWAY]
+            && !attrs[RTA_VIA] && !attrs[RTA_MULTIPATH]) {
             VLOG_DBG_RL(&rl, "route message needs an RTA_OIF, RTA_GATEWAY, "
                              "RTA_VIA or RTA_MULTIPATH attribute");
             goto error_out;
         }
         /* Add any additional RTA attribute processing before RTA_MULTIPATH. */
+
+        /* Ensure that the change->rd->nexthops list is cleared in cases when
+         * the route does not need a next hop. */
+        if (!route_type_needs_nexthop(rtm->rtm_type)) {
+            route_data_destroy_nexthops__(&change->rd);
+        }
     } else {
         VLOG_DBG_RL(&rl, "received unparseable rtnetlink route message");
         goto error_out;