fix (NGAP): encode optional QoS list when not empty

Only build QoSFlowAddOrModifyResponseList when at least one QFI is present
and fail fast if PDUSessionResourceModifyResponseTransfer encoding does not
produce a valid buffer. This avoids sending malformed modify responses and
makes encoding failures explicit in the NGAP modify response path.

Changes:
- do QoS add/modify response list allocation when nb_of_qos_flow > 0
- add ASN.1 encode result validation
Signed-off-by: Guido Casati <guido.casati@openairinterface.org>

openair3/NGAP/ngap_gNB_pdu_session_management.c

@@ -220,17 +220,24 @@ int ngap_gNB_pdusession_modify_resp(instance_t instance, ngap_pdusession_modify_
       item->pDUSessionID = pdusession_modify_resp_p->pdusessions[i].pdusession_id;
 
       NGAP_PDUSessionResourceModifyResponseTransfer_t transfer = {0};
-      asn1cCalloc(transfer.qosFlowAddOrModifyResponseList, tmp);
-
-      for (int qos_flow_index = 0; qos_flow_index < pdusession_modify_resp_p->pdusessions[i].nb_of_qos_flow; qos_flow_index++) {
-        asn1cSequenceAdd(tmp->list, NGAP_QosFlowAddOrModifyResponseItem_t, qos);
-        qos->qosFlowIdentifier = pdusession_modify_resp_p->pdusessions[i].qos[qos_flow_index].qfi;
+      if (pdusession_modify_resp_p->pdusessions[i].nb_of_qos_flow > 0) {
+        asn1cCalloc(transfer.qosFlowAddOrModifyResponseList, tmp);
+        for (int qos_flow_index = 0; qos_flow_index < pdusession_modify_resp_p->pdusessions[i].nb_of_qos_flow; qos_flow_index++) {
+          asn1cSequenceAdd(tmp->list, NGAP_QosFlowAddOrModifyResponseItem_t, qos);
+          qos->qosFlowIdentifier = pdusession_modify_resp_p->pdusessions[i].qos[qos_flow_index].qfi;
+        }
       }
       asn_encode_to_new_buffer_result_t res = {0};
       res = asn_encode_to_new_buffer(NULL,
                                      ATS_ALIGNED_CANONICAL_PER,
                                      &asn_DEF_NGAP_PDUSessionResourceModifyResponseTransfer,
                                      &transfer);
+      if (res.buffer == NULL || res.result.encoded <= 0) {
+        NGAP_ERROR("Failed to encode PDUSessionResourceModifyResponseTransfer for PDU session %ld\n", item->pDUSessionID);
+        ASN_STRUCT_FREE_CONTENTS_ONLY(asn_DEF_NGAP_PDUSessionResourceModifyResponseTransfer, &transfer);
+        ASN_STRUCT_FREE_CONTENTS_ONLY(asn_DEF_NGAP_NGAP_PDU, &pdu);
+        return -1;
+      }
       item->pDUSessionResourceModifyResponseTransfer.buf = res.buffer;
       item->pDUSessionResourceModifyResponseTransfer.size = res.result.encoded;