geoip: stricter validation of CNAME usage

The geoip module does not validate the semantic validity of views, such
as that a CNAME cannot occur along with any other record. It will take
care to only return the right type in any given response, but this can
still lead to inconsistent results. Furthermore, one can supply two
CNAMEs in a view, and the module will write both of them into a
response.

This adds some input validation to tighten the rules of what the module
will accept as valid configuration, specifically that a CNAME cannot
occur along any other record in a view. This prevents ambiguities that
might otherwise arise in the query processing.

The additional storing of the domain name in case of a CNAME was chosen
in anticipation of using it for enabling CNAME chaing resolution,
implemented seperately.

src/knot/modules/geoip/geoip.c

@@ -115,6 +115,8 @@ typedef struct {
 	size_t count, avail;
 	knot_rrset_t *rrsets;
 	knot_rrset_t *rrsigs;
+
+	knot_dname_t *cname;
 } geo_view_t;
 
 typedef struct {
@@ -298,6 +300,7 @@ static int init_geo_view(geo_view_t *view)
 	if (view->rrsets == NULL) {
 		return KNOT_ENOMEM;
 	}
+	view->cname = NULL;
 	return KNOT_EOK;
 }
 
@@ -320,6 +323,8 @@ static void clear_geo_view(geo_view_t *view)
 	view->rrsets = NULL;
 	free(view->rrsigs);
 	view->rrsigs = NULL;
+	free(view->cname);
+	view->cname = NULL;
 }
 
 static int parse_origin(yp_parser_t *yp, zs_scanner_t *scanner)
@@ -435,6 +440,18 @@ static int parse_rr(knotd_mod_t *mod, yp_parser_t *yp, zs_scanner_t *scanner,
 		return KNOT_EINVAL;
 	}
 
+	if (rr_type == KNOT_RRTYPE_CNAME && view->count > 0) {
+		knotd_mod_log(mod, LOG_ERR, "cannot add CNAME to view with other RRs on line %zu",
+		              yp->line_count);
+		return KNOT_EINVAL;
+	}
+
+	if (view->cname != NULL) {
+		knotd_mod_log(mod, LOG_ERR, "cannot add RR to view with CNAME on line %zu",
+		              yp->line_count);
+		return KNOT_EINVAL;
+	}
+
 	if (knot_rrtype_is_dnssec(rr_type)) {
 		knotd_mod_log(mod, LOG_ERR, "DNSSEC record (%s) not allowed on line %zu",
 		              yp->key, yp->line_count);
@@ -481,6 +498,10 @@ static int parse_rr(knotd_mod_t *mod, yp_parser_t *yp, zs_scanner_t *scanner,
 	}
 	free(input_string);
 
+	if (rr_type == KNOT_RRTYPE_CNAME) {
+		view->cname = knot_dname_from_str_alloc(yp->data);
+	}
+
 	// Add new rdata to current rrset.
 	return knot_rrset_add_rdata(add_rr, scanner->r_data, scanner->r_data_length, NULL);
 }