nanocoap: add coap_build_reply_header()

sys/include/net/coap.h

@@ -85,7 +85,7 @@
  * Used in a CoAP request to signal that the request payload conveys both an
  * EDHOC message_3 and OSCORE protected data, combined together.
  *
  * @see [draft-ietf-core-oscore-edhoc-02](https://datatracker.ietf.org/doc/draft-ietf-core-oscore-edhoc/02/)
  */
 #define COAP_OPT_EDHOC          (21)
 #define COAP_OPT_BLOCK2         (23)
@@ -354,7 +354,7 @@
 #define COAP_FORMAT_SWID_CBOR               (258)
 /**
  * @brief   Content-Type `application/pkixcmp`
  * @see     [draft-ietf-ace-cmpv2-coap-transport](https://datatracker.ietf.org/doc/draft-ietf-ace-cmpv2-coap-transport/)
  * @see     [RFC 4210](https://datatracker.ietf.org/doc/html/rfc4210)
  */
 #define COAP_FORMAT_PKIXCMP                 (259)
@@ -441,22 +441,22 @@
 #define COAP_FORMAT_YAML_DATA_CBOR_ID_NAME  (341)
 /**
  * @brief   Content-Type `application/td+json`
  * @see     [Web of Things (WoT) Thing Description 1.1](https://www.w3.org/TR/wot-thing-description11/)
  */
 #define COAP_FORMAT_TD_JSON                 (432)
 /**
  * @brief   Content-Type `application/tm+json`
  * @see     [Web of Things (WoT) Thing Description 1.1](https://www.w3.org/TR/wot-thing-description11/)
  */
 #define COAP_FORMAT_TM_JSON                 (433)
 /**
  * @brief   Content-Type `application/dns-message`
  * @see     [draft-ietf-core-dns-over-coap](https://datatracker.ietf.org/doc/draft-ietf-core-dns-over-coap/)
  */
 #define COAP_FORMAT_DNS_MESSAGE             (553)
 /**
  * @brief   Content-Type `application/voucher-cose+cbor`
  * @see     [draft-ietf-anima-constrained-voucher](https://datatracker.ietf.org/doc/draft-ietf-anima-constrained-voucher/)
  * @note    Temporary registration until April 12, 2024.
  */
 #define COAP_FORMAT_VOUCER_COSE_CBOR        (836)
@@ -488,17 +488,17 @@
 #define COAP_FORMAT_CBOR_DEFLATE          (11060)
 /**
  * @brief   Content-Type `application/vnd.oma.lwm2m+tlv`
  * @see     [OMA-TS-LightweightM2M-V1_0](https://www.openmobilealliance.org/release/LightweightM2M/V1_0-20170208-A/OMA-TS-LightweightM2M-V1_0-20170208-A.pdf)
  */
 #define COAP_FORMAT_VND_OMA_LWM2M_TLV     (11542)
 /**
  * @brief   Content-Type `application/vnd.oma.lwm2m+json`
  * @see     [OMA-TS-LightweightM2M-V1_0](https://www.openmobilealliance.org/release/LightweightM2M/V1_0-20170208-A/OMA-TS-LightweightM2M-V1_0-20170208-A.pdf)
  */
 #define COAP_FORMAT_VND_OMA_LWM2M_JSON    (11543)
 /**
  * @brief   Content-Type `application/vnd.oma.lwm2m+cbor`
  * @see     [OMA-TS-LightweightM2M-V1_2](https://www.openmobilealliance.org/release/LightweightM2M/V1_2-20201110-A/HTML-Version/OMA-TS-LightweightM2M_Core-V1_2-20201110-A.html)
  */
 #define COAP_FORMAT_VND_OMA_LWM2M_CBOR    (11544)
 /**
@@ -535,6 +535,11 @@
 #define COAP_ETAG_LENGTH_MAX     (8U)   /**< maximum length of the ETag option */
 /** @} */
 
+/**
+ * @brief   Marks the boundary between header and payload
+ */
+#define COAP_PAYLOAD_MARKER      (0xFF)
+
 /**
  * @defgroup net_coap_conf    CoAP compile configurations
  * @ingroup  net_coap

sys/include/net/nanocoap.h

@@ -1843,7 +1843,7 @@
  *
  * @returns     amount of bytes written to @p buf
  */
 size_t coap_put_option(uint8_t *buf, uint16_t lastonum, uint16_t onum, const void *odata, size_t olen);
 
 /**
  * @brief   Insert block1 option into buffer
@@ -2115,6 +2115,36 @@
  */
 ssize_t coap_payload_put_char(coap_pkt_t *pkt, char c);
 
+/**
+ * @brief   Create CoAP reply header (convenience function)
+ *
+ * This function generates the reply CoAP header and sets
+ * the payload pointer inside the response buffer to point to
+ * the start of the payload, so that it can be written directly
+ * after the header.
+ *
+ * @param[in]   pkt         packet to reply to
+ * @param[in]   code        reply code (e.g., COAP_CODE_204)
+ * @param[out]  buf         buffer to write reply to
+ * @param[in]   len         size of @p buf
+ * @param[in]   ct          content type of payload
+ *                          if ct < 0 this will be ignored
+ * @param[out]  payload     Will be set to the start of the payload inside
+ *                          @p buf.
+ *                          May be set to NULL if no payload response is
+ *                          wanted (no-reply option)
+ * @param[out]  payload_len_max max length of payload left in @p buf
+ *
+ * @returns     size of reply header on success
+ * @returns     0 if no reply should be sent
+ * @returns     <0 on error
+ * @returns     -ENOSPC if @p buf too small
+ */
+ssize_t coap_build_reply_header(coap_pkt_t *pkt, unsigned code,
+                                void *buf, size_t len,
+                                int ct,
+                                void **payload, size_t *payload_len_max);
+
 /**
  * @brief   Create CoAP reply (convenience function)
  *

sys/net/application_layer/nanocoap/nanocoap.c

@@ -119,7 +119,7 @@ int coap_parse(coap_pkt_t *pkt, uint8_t *buf, size_t len)
     while (pkt_pos < pkt_end) {
         uint8_t *option_start = pkt_pos;
         uint8_t option_byte = *pkt_pos++;
-        if (option_byte == 0xff) {
+        if (option_byte == COAP_PAYLOAD_MARKER) {
             pkt->payload = pkt_pos;
             pkt->payload_len = buf + len - pkt_pos;
             DEBUG("payload len = %u\n", pkt->payload_len);
@@ -232,7 +232,7 @@ static uint8_t *_parse_option(const coap_pkt_t *pkt,
     uint8_t *hdr_end = pkt->payload;
 
     if ((pkt_pos >= hdr_end)
-            || (((pkt_pos + 1) == hdr_end) && (*pkt_pos == 0xFF))) {
+            || (((pkt_pos + 1) == hdr_end) && (*pkt_pos == COAP_PAYLOAD_MARKER))) {
         return NULL;
     }
 
@@ -545,29 +545,85 @@ ssize_t coap_tree_handler(coap_pkt_t *pkt, uint8_t *resp_buf, unsigned resp_buf_
     return coap_build_reply(pkt, COAP_CODE_404, resp_buf, resp_buf_len, 0);
 }
 
+ssize_t coap_build_reply_header(coap_pkt_t *pkt, unsigned code,
+                                void *buf, size_t len,
+                                int ct,
+                                void **payload, size_t *payload_len_max)
+{
+    uint8_t *bufpos = buf;
+    uint32_t no_response;
+    unsigned tkl = coap_get_token_len(pkt);
+    size_t hdr_len = sizeof(coap_hdr_t) + tkl;
+    uint8_t type = coap_get_type(pkt) == COAP_TYPE_CON
+                 ? COAP_TYPE_ACK
+                 : COAP_TYPE_NON;
+
+    if (hdr_len > len) {
+        return -ENOBUFS;
+    }
+
+    bufpos += coap_build_hdr(buf, type, coap_get_token(pkt), tkl,
+                             code, ntohs(pkt->hdr->id));
+
+    if (coap_opt_get_uint(pkt, COAP_OPT_NO_RESPONSE, &no_response) == 0) {
+        const uint8_t no_response_index = (code >> 5) - 1;
+        /* If the handler code misbehaved here, we'd face UB otherwise */
+        assume(no_response_index < 7);
+
+        const uint8_t mask = 1 << no_response_index;
+        if (no_response & mask) {
+            if (payload) {
+                *payload = NULL;
+                *payload_len_max = 0;
+                payload = NULL;
+            }
+
+            /* no-response requested, only send empty ACK or nothing */
+            if (type != COAP_TYPE_ACK) {
+                return 0;
+            }
+        }
+    }
+
+    if (payload) {
+        if (ct >= 0) {
+            bufpos += coap_put_option_ct(bufpos, 0, ct);
+        }
+        *bufpos++ = COAP_PAYLOAD_MARKER;
+        *payload = bufpos;
+        hdr_len  = bufpos - (uint8_t *)buf;
+        *payload_len_max = len - hdr_len;
+    }
+
+    /* with the nanoCoAP API we can't detect the overflow before it happens */
+    assert(hdr_len <= len);
+
+    return hdr_len;
+}
+
 ssize_t coap_reply_simple(coap_pkt_t *pkt,
                           unsigned code,
                           uint8_t *buf, size_t len,
                           unsigned ct,
                           const void *payload, size_t payload_len)
 {
-    uint8_t *payload_start = buf + coap_get_total_hdr_len(pkt);
-    uint8_t *bufpos = payload_start;
+    void *payload_start;
+    size_t payload_len_max;
 
-    if (payload_len) {
-        bufpos += coap_put_option_ct(bufpos, 0, ct);
-        *bufpos++ = 0xff;
+    ssize_t header_len = coap_build_reply_header(pkt, code, buf, len, ct,
+                                                payload ? &payload_start : NULL,
+                                                &payload_len_max);
+    if (payload == NULL || header_len <= 0) {
+        return header_len;
     }
 
-    ssize_t res = coap_build_reply(pkt, code, buf, len,
-                                   bufpos - payload_start + payload_len);
-
-    if (payload_len && (res > 0)) {
-        assert(payload);
-        memcpy(bufpos, payload, payload_len);
+    if (payload_len > payload_len_max) {
+        return -ENOBUFS;
     }
 
-    return res;
+    memcpy(payload_start, payload, payload_len);
+
+    return header_len + payload_len;
 }
 
 ssize_t coap_build_reply(coap_pkt_t *pkt, unsigned code,
@@ -1112,7 +1168,7 @@ ssize_t coap_opt_finish(coap_pkt_t *pkt, uint16_t flags)
             return -ENOSPC;
         }
 
-        *pkt->payload++ = 0xFF;
+        *pkt->payload++ = COAP_PAYLOAD_MARKER;
         pkt->payload_len--;
     }
     else {
@@ -1337,7 +1393,7 @@ ssize_t coap_well_known_core_default_handler(coap_pkt_t *pkt, uint8_t *buf, \
     bufpos += coap_put_option_ct(bufpos, 0, COAP_FORMAT_LINK);
     bufpos += coap_opt_put_block2(bufpos, COAP_OPT_CONTENT_FORMAT, &slicer, 1);
 
-    *bufpos++ = 0xff;
+    *bufpos++ = COAP_PAYLOAD_MARKER;
 
     for (unsigned i = 0; i < coap_resources_numof; i++) {
         if (i) {