bgpd: Limit size of sent NLRIs to flowspec peers (issue 18557)

See issue 18557 for detailed description of the problem. When announcing flowspec routes, frr currently sends NLRIs up to max_packet_size.
However, the maximum size of flowspec NLRIs is limited to a much lower value here.
Because of this, past as certain amount of flowspec routes, the peer will drop the session.
The proposed change reduces the size of the buffer for the NLRI to the maximum value between nlri_max_length and either FLOWSPEC_NLRI_SIZELIMIT_EXTENDED (if the peer advertised support for extended messages) or FLOWSPEC_NLRI_SIZELIMIT (if it did not).

Signed-off-by: Stephane Poignant <[email protected]>

Author: spoignant-proton

Diff for: bgpd/bgp_updgrp.c

@@ -41,6 +41,7 @@
 #include "bgpd/bgp_route.h"
 #include "bgpd/bgp_filter.h"
 #include "bgpd/bgp_io.h"
+#include "bgpd/bgp_flowspec_private.h"
 
 /********************
  * PRIVATE FUNCTIONS
@@ -69,6 +70,8 @@ static void sync_init(struct update_subgroup *subgrp,
 		      struct update_group *updgrp)
 {
 	struct peer *peer = UPDGRP_PEER(updgrp);
+	safi_t safi = UPDGRP_SAFI(updgrp);
+	size_t nlri_max_length;
 
 	subgrp->sync =
 		XCALLOC(MTYPE_BGP_SYNCHRONISE, sizeof(struct bgp_synchronize));
@@ -95,7 +98,20 @@ static void sync_init(struct update_subgroup *subgrp,
 	 */
 	subgrp->work = stream_new(peer->max_packet_size
 				  + BGP_MAX_PACKET_SIZE_OVERFLOW);
-	subgrp->scratch = stream_new(peer->max_packet_size);
+	if (safi == SAFI_FLOWSPEC) {
+		/* Issue 18557: for flowspec, need to make sure we never send
+		 * SAFIs larger than FLOWSPEC_NLRI_SIZELIMIT_EXTENDED or
+		 * FLOWSPEC_NLRI_SIZELIMIT, as those would be rejected
+		 */
+		nlri_max_length = (CHECK_FLAG(peer->cap, PEER_CAP_EXTENDED_MESSAGE_RCV))
+					  ? FLOWSPEC_NLRI_SIZELIMIT_EXTENDED
+					  : FLOWSPEC_NLRI_SIZELIMIT;
+		subgrp->scratch = stream_new((peer->max_packet_size <= nlri_max_length)
+						     ? peer->max_packet_size
+						     : nlri_max_length);
+	} else {
+		subgrp->scratch = stream_new(peer->max_packet_size);
+	}
 }
 
 static void sync_delete(struct update_subgroup *subgrp)