validator: up->bottom chase DS if RRSIG(s) are missing

The original approach was using SOA owner in negative answers
to optimize number of DS queries. This approarch is less realiable with
weird "servers", including pre-DNSSEC servers which reply to DS query
with an SOA owner pointing to the child zone instead of parent zone.

We now walk the tree from root down to find the missing DS or proof of
its non-existance.

lib/layer/validate.c

@@ -551,7 +551,8 @@ static const knot_dname_t *signature_authority(struct kr_request *req)
 	return signer_name;
 }
 
-static int rrsig_not_found(kr_layer_t *ctx, const knot_pkt_t *pkt, const knot_rrset_t *rr)
+static int rrsig_not_found(const kr_layer_t * const ctx, const knot_pkt_t * const pkt,
+				const knot_rrset_t * const rr)
 {
 	/* Signatures are missing.  There might be a zone cut that we've skipped
 	 * and transitions to insecure.  That can commonly happen when iterating
@@ -559,8 +560,8 @@ static int rrsig_not_found(kr_layer_t *ctx, const knot_pkt_t *pkt, const knot_rr
 	 * We'll try proving that the name truly is insecure - by spawning
 	 * a DS sub-query on a suitable QNAME.
 	 */
-	struct kr_request *req = ctx->req;
-	struct kr_query *qry = req->current_query;
+	struct kr_request * const req = ctx->req;
+	struct kr_query * const qry = req->current_query;
 
 	if (qry->flags.FORWARD || qry->flags.STUB) {
 		/* Undiscovered signed cuts can't happen in the current forwarding
@@ -569,27 +570,17 @@ static int rrsig_not_found(kr_layer_t *ctx, const knot_pkt_t *pkt, const knot_rr
 	}
 
 	/* Find cut_next: the name at which to try finding the "missing" zone cut. */
-	const knot_dname_t *cut_next = rr->owner;
 	const knot_dname_t * const cut_top = qry->zone_cut.name;
 	const int next_depth = knot_dname_in_bailiwick(rr->owner, cut_top);
 	if (next_depth <= 0) {
 		return KR_STATE_FAIL; // shouldn't happen, I think
-	} else if (next_depth > 1) {
-		/* Try finding SOA.  That can speed up things in deeper zones. */
-		const knot_pktsection_t *sec = knot_pkt_section(pkt, KNOT_AUTHORITY);
-		for (int i = sec->pos; i < sec->pos + sec->count; ++i) {
-			if (pkt->rr[i].type != KNOT_RRTYPE_SOA) continue;
-			const knot_dname_t *owner = pkt->rr[i].owner;
-			if (knot_dname_in_bailiwick(owner, cut_top) >= 1
-			    && knot_dname_in_bailiwick(cut_next, owner) >= 1) {
-				cut_next = owner;
-				break;
-			}
-		}
 	}
+	/* Add one extra label to cur_top, i.e. descend one level below current zone cut */
+	const knot_dname_t * const cut_next = rr->owner +
+		knot_dname_prefixlen(rr->owner, next_depth - 1, NULL);
 
 	/* Spawn that DS sub-query. */
-	struct kr_query *next = kr_rplan_push(&req->rplan, qry, cut_next,
+	struct kr_query * const next = kr_rplan_push(&req->rplan, qry, cut_next,
 						rr->rclass, KNOT_RRTYPE_DS);
 	if (!next) {
 		return KR_STATE_FAIL;