misc todo's and related fixes

mongoose.c

@@ -2142,7 +2142,6 @@ static void handle_mdns_query(struct mg_connection *c) {
     MG_VERBOSE(("RR %u %u %s", (unsigned int) rr.atype,
                 (unsigned int) rr.aclass, name));
     if (rr.atype == MG_DNS_RTYPE_A) {
-      // TODO(): ensure c->fn_data ends in \0
       // if we have a name to match, go; otherwise users will match and fill
       // req.r.name and set req.is_resp
       if (c->fn_data != NULL && mg_casecmp((char *) c->fn_data, name) != 0)
@@ -5653,6 +5652,13 @@ size_t mg_l2_eth_trailer(struct mg_tcpip_if *ifp, size_t len, uint8_t *cur) {
 struct mg_l2addr *mg_l2_eth_mapip(enum mg_l2addrtype addrtype,
                                   struct mg_addr *addr);
 
+// Read an unaligned little-endian value from byte pointer p into a native integer.
+// Safe on architectures that forbid unaligned access (e.g. Cortex-M0).
+#define MG_LOAD_LE32(p)                           \
+  ((uint32_t) (((uint32_t) MG_U8P(p)[3] << 24U) | \
+               ((uint32_t) MG_U8P(p)[2] << 16U) | \
+               ((uint32_t) MG_U8P(p)[1] << 8U) | MG_U8P(p)[0]))
+
 bool mg_l2_eth_rx(struct mg_tcpip_if *ifp, enum mg_l2proto *proto,
                   struct mg_str *pay, struct mg_str *raw) {
   struct eth *eth = (struct eth *) raw->buf;
@@ -5678,10 +5684,11 @@ bool mg_l2_eth_rx(struct mg_tcpip_if *ifp, enum mg_l2proto *proto,
              sizeof(eth->dst)) != 0)
     return false;  // TODO(): add multicast addresses
   if (ifp->enable_fcs_check && len > hdrlen + 4) {
-    uint32_t crc;
+    uint32_t crc, crc_rx;
-    len -= 4;  // TODO(scaprile): check on bigendian
+    len -= 4;
     crc = mg_crc32(0, (const char *) raw->buf, len);
-    if (memcmp((void *) ((size_t) raw->buf + len), &crc, sizeof(crc)))
+    crc_rx = MG_LOAD_LE32(raw->buf + len);
+    if (crc_rx != crc)
       return false;
   }
   pay->buf = ((char *) eth) + hdrlen;

mongoose.h

@@ -4208,6 +4208,7 @@ size_t mg_dns_parse_rr(const uint8_t *buf, size_t len, size_t ofs,
 // Creates an mDNS listener on UDP multicast 224.0.0.251:5353. Registers it as
 // the manager's mDNS resolver (mgr->mdns). Fires MG_EV_MDNS_REQ for incoming
 // queries and MG_EV_MDNS_RESP for incoming responses. Returns NULL on error.
+// fn_data is a NUL-terminated server name, or NULL to pass all requests to fn.
 struct mg_connection *mg_mdns_listen(struct mg_mgr *mgr, mg_event_handler_t fn,
                                      void *fn_data);
 

src/dns.c

@@ -463,7 +463,6 @@ static void handle_mdns_query(struct mg_connection *c) {
     MG_VERBOSE(("RR %u %u %s", (unsigned int) rr.atype,
                 (unsigned int) rr.aclass, name));
     if (rr.atype == MG_DNS_RTYPE_A) {
-      // TODO(): ensure c->fn_data ends in \0
       // if we have a name to match, go; otherwise users will match and fill
       // req.r.name and set req.is_resp
       if (c->fn_data != NULL && mg_casecmp((char *) c->fn_data, name) != 0)

src/dns.h

@@ -82,6 +82,7 @@ size_t mg_dns_parse_rr(const uint8_t *buf, size_t len, size_t ofs,
 // Creates an mDNS listener on UDP multicast 224.0.0.251:5353. Registers it as
 // the manager's mDNS resolver (mgr->mdns). Fires MG_EV_MDNS_REQ for incoming
 // queries and MG_EV_MDNS_RESP for incoming responses. Returns NULL on error.
+// fn_data is a NUL-terminated server name, or NULL to pass all requests to fn.
 struct mg_connection *mg_mdns_listen(struct mg_mgr *mgr, mg_event_handler_t fn,
                                      void *fn_data);
 

src/l2_eth.c

@@ -94,6 +94,13 @@ size_t mg_l2_eth_trailer(struct mg_tcpip_if *ifp, size_t len, uint8_t *cur) {
 struct mg_l2addr *mg_l2_eth_mapip(enum mg_l2addrtype addrtype,
                                   struct mg_addr *addr);
 
+// Read an unaligned little-endian value from byte pointer p into a native integer.
+// Safe on architectures that forbid unaligned access (e.g. Cortex-M0).
+#define MG_LOAD_LE32(p)                           \
+  ((uint32_t) (((uint32_t) MG_U8P(p)[3] << 24U) | \
+               ((uint32_t) MG_U8P(p)[2] << 16U) | \
+               ((uint32_t) MG_U8P(p)[1] << 8U) | MG_U8P(p)[0]))
+
 bool mg_l2_eth_rx(struct mg_tcpip_if *ifp, enum mg_l2proto *proto,
                   struct mg_str *pay, struct mg_str *raw) {
   struct eth *eth = (struct eth *) raw->buf;
@@ -119,10 +126,11 @@ bool mg_l2_eth_rx(struct mg_tcpip_if *ifp, enum mg_l2proto *proto,
              sizeof(eth->dst)) != 0)
     return false;  // TODO(): add multicast addresses
   if (ifp->enable_fcs_check && len > hdrlen + 4) {
-    uint32_t crc;
+    uint32_t crc, crc_rx;
-    len -= 4;  // TODO(scaprile): check on bigendian
+    len -= 4;
     crc = mg_crc32(0, (const char *) raw->buf, len);
-    if (memcmp((void *) ((size_t) raw->buf + len), &crc, sizeof(crc)))
+    crc_rx = MG_LOAD_LE32(raw->buf + len);
+    if (crc_rx != crc)
       return false;
   }
   pay->buf = ((char *) eth) + hdrlen;

test/eth_test.c

@@ -63,8 +63,11 @@ static size_t mk_vlan(uint8_t *buf, const uint8_t *dst, uint16_t vlan,
 
 static size_t add_fcs(uint8_t *buf, size_t len) {
   uint32_t crc = mg_crc32(0, (const char *) buf, len);
-  memcpy(buf + len, &crc, sizeof(crc));
+  buf[len++] = (uint8_t) crc;       // store as LE32
-  return len + sizeof(crc);
+  buf[len++] = (uint8_t) (crc >> 8);
+  buf[len++] = (uint8_t) (crc >> 16);
+  buf[len++] = (uint8_t) (crc >> 24);
+  return len;
 }
 
 #define DASHBOARD(x) \
@@ -150,6 +153,14 @@ static void test_fcs(void) {
   size_t len;
   uint8_t frame[64];
 
+  {  // verify add_fcs() stores the CRC bytes in the proper order
+    // (internal consistency test)
+    uint8_t payload[] = {1, 2, 3, 4};
+    uint8_t fcs[] = {0xf4, 0xd4, 0x37, 0xf6};
+    len = add_fcs(frame, mk_eth(frame, s_mac, 0x0800, payload,
+                                sizeof(payload)));
+    ASSERT(memcmp(frame + len - sizeof(fcs), fcs, sizeof(fcs)) == 0);
+  }
   {  // FCS check strips a valid CRC
     uint8_t payload[] = {1, 2, 3, 4};
     reset_if(&ifp);