misc todo's and related fixes

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;
-    len -= 4;  // TODO(scaprile): check on bigendian
+    uint32_t crc, crc_rx;
+    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;