fix RLC AM issue where an invalid status PDU was generated

happens when very small grant is provided and the status
PDU generation fails because of a too small grant

add extra check to prevent that ACK_SN is also in NACKS list

lib/include/srslte/upper/rlc_am.h

@@ -306,6 +306,7 @@ int         rlc_am_write_status_pdu(rlc_status_pdu_t *status, uint8_t *payload);
 uint32_t    rlc_am_packed_length(rlc_amd_pdu_header_t *header);
 uint32_t    rlc_am_packed_length(rlc_status_pdu_t *status);
 uint32_t    rlc_am_packed_length(rlc_amd_retx_t retx);
+bool        rlc_am_is_valid_status_pdu(const rlc_status_pdu_t& status);
 bool        rlc_am_is_control_pdu(byte_buffer_t *pdu);
 bool        rlc_am_is_control_pdu(uint8_t *payload);
 bool        rlc_am_is_pdu_segment(uint8_t *payload);
@@ -318,5 +319,4 @@ bool        rlc_am_not_start_aligned(const uint8_t fi);
 
 } // namespace srslte
 
-
 #endif // SRSLTE_RLC_AM_H

lib/include/srslte/upper/rlc_common.h

@@ -154,7 +154,7 @@ struct rlc_status_nack_t{
 
 // STATUS PDU
 struct rlc_status_pdu_t{
-  uint16_t          ack_sn;
+  uint16_t          ack_sn; // SN of the next not received RLC Data PDU
   uint32_t          N_nack;
   rlc_status_nack_t nacks[RLC_AM_WINDOW_SIZE];
 

lib/src/upper/rlc_am.cc

@@ -1629,6 +1629,14 @@ int rlc_am::rlc_am_rx::get_status_pdu(rlc_status_pdu_t* status, const uint32_t m
       if (status->N_nack >= 1) {
         log->debug("Removing last NACK SN=%d\n", status->nacks[status->N_nack].nack_sn);
         status->N_nack--;
+        // make sure we don't have the current ACK_SN in the NACK list
+        if (rlc_am_is_valid_status_pdu(*status) == false) {
+          // No space to send any NACKs
+          log->debug("Resetting N_nack to zero\n");
+          status->N_nack = 0;
+        }
+      } else {
+        log->error("Failed to generate small enough status PDU\n");
       }
       break;
     }
@@ -2024,6 +2032,16 @@ int rlc_am_write_status_pdu(rlc_status_pdu_t *status, uint8_t *payload)
   return tmp.N_bits/8;
 }
 
+bool rlc_am_is_valid_status_pdu(const rlc_status_pdu_t& status)
+{
+  for (uint16_t i = 0; i < status.N_nack; ++i) {
+    if (status.nacks[i].nack_sn == status.ack_sn) {
+      return false;
+    }
+  }
+  return true;
+}
+
 uint32_t rlc_am_packed_length(rlc_amd_pdu_header_t *header)
 {
   uint32_t len = 2;                 // Fixed part is 2 bytes

lib/test/upper/rlc_am_test.cc

@@ -1586,11 +1586,18 @@ bool status_pdu_test()
 
   // Read status PDU from RLC2
   byte_buffer_t status_buf;
-  len                = rlc2.read_pdu(status_buf.msg, 3); // provide only small grant
+  len                = rlc2.read_pdu(status_buf.msg, 5); // provide only small grant
   status_buf.N_bytes = len;
 
   assert(status_buf.N_bytes != 0);
 
+  // check status PDU doesn't contain ACK_SN in NACK list
+  rlc_status_pdu_t status_pdu = {};
+  rlc_am_read_status_pdu(status_buf.msg, status_buf.N_bytes, &status_pdu);
+  if (rlc_am_is_valid_status_pdu(status_pdu) == false) {
+    return -1;
+  }
+
   // Write status PDU to RLC1
   rlc1.write_pdu(status_buf.msg, status_buf.N_bytes);