filter_log_to_metrics: Process validations for range tightly

Signed-off-by: Hiroshi Hatake <hiroshi@chronosphere.io>

Author: cosmo0920

plugins/filter_log_to_metrics/log_to_metrics.c

@@ -434,12 +434,12 @@ static int set_labels(struct log_to_metrics_ctx *ctx,
     mk_list_foreach(head, &f_ins->properties) {
         kv = mk_list_entry(head, struct flb_kv, _head);
 
-        if (counter > ctx->label_counter) {
-            flb_plg_error(ctx->ins, "internal label counter overflow");
-            return -1;
-        }
-
         if (strcasecmp(kv->key, "label_field") == 0) {
+            if (counter >= ctx->label_counter) {
+                flb_plg_error(ctx->ins, "internal label counter overflow");
+                return -1;
+            }
+
             /* name and accessor are the same string */
             ctx->label_keys[counter] = flb_strdup(kv->val);
             if (!ctx->label_keys[counter]) {
@@ -454,6 +454,10 @@ static int set_labels(struct log_to_metrics_ctx *ctx,
             counter++;
         }
         else if (strcasecmp(kv->key, "add_label") == 0) {
+            if (counter >= ctx->label_counter) {
+                flb_plg_error(ctx->ins, "internal label counter overflow");
+                return -1;
+            }
             split = flb_utils_split(kv->val, ' ', 1);
             if (mk_list_size(split) != 2) {
                 flb_plg_error(ctx->ins, "invalid label, expected name and key");