Improve MTU handling

by:
- Correcting Contiki-NG's value of COAP_DEFAULT_MAX_PDU_RX_SIZE
- Denying to create PDUs longer than COAP_DEFAULT_MAX_PDU_RX_SIZE
- Using the MTU setter, thereby not overriding stack-specific limits
- Using existing constants for L4-specific header sizes

include/coap3/coap_pdu_internal.h

@@ -77,7 +77,7 @@
 #if defined(WITH_LWIP)
 #define COAP_DEFAULT_MAX_PDU_RX_SIZE (COAP_MAX_MESSAGE_SIZE_TCP16+4UL)
 #elif defined(WITH_CONTIKI)
-#define COAP_DEFAULT_MAX_PDU_RX_SIZE (sizeof(coap_packet_t) + UIP_APPDATA_SIZE)
+#define COAP_DEFAULT_MAX_PDU_RX_SIZE (UIP_APPDATA_SIZE)
 #elif (UINT_MAX < (8UL*1024*1024+256))
 #define COAP_DEFAULT_MAX_PDU_RX_SIZE (1500UL)
 #elif defined(RIOT_VERSION) && defined(COAP_DISABLE_TCP)

src/coap_pdu.c

@@ -118,14 +118,16 @@ coap_pdu_init(coap_pdu_type_t type, coap_pdu_code_t code, coap_mid_t mid,
  if (!pdu)
  return NULL;
 
-#if defined(WITH_CONTIKI) || defined(WITH_LWIP)
- assert(size <= COAP_DEFAULT_MAX_PDU_RX_SIZE);
- if (size > COAP_DEFAULT_MAX_PDU_RX_SIZE)
- return NULL;
+#if COAP_DEFAULT_MAX_PDU_RX_SIZE <= COAP_MAX_MESSAGE_SIZE_TCP16
+ /* on TCP, the CoAP header will also have a maximum length of 4 bytes */
  pdu->max_hdr_size = COAP_PDU_MAX_UDP_HEADER_SIZE;
 #else
  pdu->max_hdr_size = COAP_PDU_MAX_TCP_HEADER_SIZE;
 #endif
+ if (size > ((size_t)COAP_DEFAULT_MAX_PDU_RX_SIZE - pdu->max_hdr_size)) {
+ coap_free_type(COAP_PDU, pdu);
+ return NULL;
+ }
 
 #ifdef WITH_LWIP
  pdu->pbuf = pbuf_alloc(PBUF_TRANSPORT, size + pdu->max_hdr_size, PBUF_RAM);

src/coap_session.c

@@ -443,7 +443,7 @@ coap_make_session(coap_proto_t proto, coap_session_type_t type,
  session->mtu = endpoint->default_mtu;
  else
 #endif /* COAP_SERVER_SUPPORT */
- session->mtu = COAP_DEFAULT_MTU;
+ coap_session_set_mtu(session, COAP_DEFAULT_MTU);
  session->block_mode = context->block_mode;
  if (proto == COAP_PROTO_DTLS) {
  session->tls_overhead = 29;
@@ -604,10 +604,14 @@ coap_session_max_pdu_size_internal(const coap_session_t *session,
  size_t max_with_header) {
 #if COAP_DISABLE_TCP
  (void)session;
- return max_with_header > 4 ? max_with_header - 4 : 0;
+ return max_with_header > COAP_PDU_MAX_UDP_HEADER_SIZE
+ ? max_with_header - COAP_PDU_MAX_UDP_HEADER_SIZE
+ : 0;
 #else /* !COAP_DISABLE_TCP */
  if (COAP_PROTO_NOT_RELIABLE(session->proto))
- return max_with_header > 4 ? max_with_header - 4 : 0;
+ return max_with_header > COAP_PDU_MAX_UDP_HEADER_SIZE
+ ? max_with_header - COAP_PDU_MAX_UDP_HEADER_SIZE
+ : 0;
  /* we must assume there is no token to be on the safe side */
  if (max_with_header <= 2)
  return 0;
@@ -618,7 +622,7 @@ coap_session_max_pdu_size_internal(const coap_session_t *session,
  else if (max_with_header <= COAP_MAX_MESSAGE_SIZE_TCP16 + 4)
  return max_with_header - 4;
  else
- return max_with_header - 6;
+ return max_with_header - COAP_PDU_MAX_TCP_HEADER_SIZE;
 #endif /* !COAP_DISABLE_TCP */
 }
 
@@ -741,7 +745,7 @@ coap_session_send_csm(coap_session_t *session) {
  session->state = COAP_SESSION_STATE_CSM;
  session->partial_write = 0;
  if (session->mtu == 0)
- session->mtu = COAP_DEFAULT_MTU; /* base value */
+ coap_session_set_mtu(session, COAP_DEFAULT_MTU); /* base value */
  pdu = coap_pdu_init(COAP_MESSAGE_CON, COAP_SIGNALING_CODE_CSM, 0, 20);
  if (pdu == NULL
  || coap_add_option_internal(pdu, COAP_SIGNALING_OPTION_MAX_MESSAGE_SIZE,