core: Fix `libwifi_frame_verify()` and clarify usage.

2 files changed, 12 insertions, 3 deletions

@@ -49,15 +49,17 @@ uint32_t libwifi_calculate_fcs(const unsigned char *frame, size_t frame_len) {
 }
 
 /*
- * Verify a frame containing a FCS at the end to the FCS calculated
+ * Verify a raw frame containing a FCS at the end to the FCS calculated
  * by libwifi.
  */
 int libwifi_frame_verify(void *frame, size_t frame_len) {
-    uint32_t oCRC = *((uint32_t *) ((char *) frame + frame_len));
+    // A frame with a CRC will have the CRC placed at the end, and is 4 bytes long.
+    uint32_t oCRC = *((uint32_t *) ((char *) frame + (frame_len - 4)));
     uint32_t rCRC = libwifi_calculate_fcs(frame, frame_len);
 
     if (rCRC == oCRC) {
         return 1;
     }
+
     return 0;
 }
@@ -38,7 +38,14 @@ uint32_t libwifi_crc32(const unsigned char *message, int message_len);
 uint32_t libwifi_calculate_fcs(const unsigned char *frame, size_t frame_len);
 
 /**
- * Check if the given 802.11 frame has a valid FCS.
+ * Check if the given raw 802.11 frame has a valid FCS.
+ *
+ * This function relies on an assumption that the last 4 bytes of the supplied
+ * frame is the CRC, as stated in the Radiotap specification.
+ *
+ * You can check if the frame data has this field by using libwifi_get_wifi_frame()
+ * and then checking if the libwifi_frame's flags has the LIBWIFI_FLAGS_FCS_PRESENT
+ * bit set.
  *
  * @param frame An 802.11 frame with an FCS
  * @param frame_len Length of the frame