Fix ATS handling in tags that NAK RATS

pcd_14a_reader_ats_request didn't check for NAK so in case the tag NAKd
the RATS it would return a zero length ATS which would cause an
underflow in pcd_14a_reader_scan_once, that in turn resulted in
HF_ERR_ATS being returned due to an invalid ATS length.

CHANGELOG.md

@@ -3,6 +3,7 @@ All notable changes to this project will be documented in this file.
 This project uses the changelog in accordance with [keepchangelog](http://keepachangelog.com/). Please use this to write notable changes, which is not the same as git commit log...
 
 ## [unreleased][unreleased]
+ - Fixed ATS handling in tags that NAK RATS (@augustozanellato)
  - Changed battery level curves based on experimental measures (@spp2000)
  - Added multithreading on Nested and StaticNested (@xianglin1998)
  - Fixed factory reset hanging (@augustozanellato)

firmware/application/src/rfid/reader/hf/rc522.c

@@ -645,20 +645,23 @@ uint8_t pcd_14a_reader_scan_once(picc_14a_tag_t *tag) {
         // Tag supports 14443-4, sending RATS
         uint16_t ats_size;
         status = pcd_14a_reader_ats_request(tag->ats, &ats_size, 0xFF * 8);
-        ats_size -= 2;  // size returned by pcd_14a_reader_ats_request includes CRC
-        if (ats_size > 254) {
-            NRF_LOG_INFO("Invalid ATS > 254!");
-            return HF_ERR_ATS;
-        }
-        tag->ats_len = ats_size;
-        // We do not validate ATS here as we want to report ATS as it is without breaking 14a scan
-        if (tag->ats[0] != ats_size - 1) {
-            NRF_LOG_INFO("Invalid ATS! First byte doesn't match received length");
-            // return HF_ERR_ATS;
-        }
+        NRF_LOG_INFO("ats status %d, length %d", status, ats_size);
         if (status != HF_TAG_OK) {
             NRF_LOG_INFO("Tag SAK claimed to support ATS but tag NAKd RATS");
+            tag->ats_len = 0;
             // return HF_ERR_ATS;
+        } else {
+            ats_size -= 2;  // size returned by pcd_14a_reader_ats_request includes CRC
+            if (ats_size > 254) {
+                NRF_LOG_INFO("Invalid ATS > 254!");
+                return HF_ERR_ATS;
+            }
+            tag->ats_len = ats_size;
+            // We do not validate ATS here as we want to report ATS as it is without breaking 14a scan
+            if (tag->ats[0] != ats_size - 1) {
+                NRF_LOG_INFO("Invalid ATS! First byte doesn't match received length");
+                // return HF_ERR_ATS;
+            }
         }
         /*
         * FIXME: If there is an issue here, it will cause the label to lose its selected state.
@@ -706,11 +709,14 @@ uint8_t pcd_14a_reader_ats_request(uint8_t *pAts, uint16_t *szAts, uint16_t szAt
 
     if (status != HF_TAG_OK) {
         *szAts = 0;
-        NRF_LOG_INFO("Err at ats receive.\n");
+        NRF_LOG_ERROR("ATS rx error: %d", status);
         return status;
+    } else if (*szAts == 7 && pAts[0] == 0x4) { // tag replied with NAK
+        *szAts = 0;
+        return HF_ERR_ATS;
     }
 
-    // NRF_LOG_INFO("Length: %d\n", *szAts);
+    NRF_LOG_INFO("Received ATS length: %d\n", *szAts);
 
     if (*szAts > 0) { *szAts = *szAts / 8; }
     return HF_TAG_OK;