zwave_api: Harden zwapi_protocol_rx_dispatch.c by checking snprintf

Checking snprintf results, reminder :

    If the output was truncated due to this limit, then the return
    value is the number of characters (excluding the terminating
    null byte) which would have been written to the final string if
    enough space had been available

This was found using CodeQL:

    Potential fix for code scanning alert no. 23:
    Potentially overflowing call to snprintf

Relate-to: SiliconLabsSoftware#100
Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>
Signed-off-by: Philippe Coval <[email protected]>

Author: rzr

applications/zpc/components/zwave_api/src/zwapi_protocol_rx_dispatch.c

@@ -12,6 +12,7 @@
  *****************************************************************************/
 
 // Generic includes
+#include <assert.h>
 #include <string.h>
 #include <stdio.h>
 
@@ -111,15 +112,25 @@ static const char *zwapi_frame_to_string(const uint8_t *buffer,
   static char message[1000] = {'\0'};
   uint16_t index            = 0;
   for (uint16_t i = 0; i < buffer_length; i++) {
+    int written = 0;
     if (i == IDX_LEN) {
-      index += snprintf(message + index, sizeof(message) - index, "Length=");
+      written = snprintf(message + index, sizeof(message) - index, "Length=");
     } else if (i == IDX_TYPE) {
-      index += snprintf(message + index, sizeof(message) - index, "Type=");
+      written = snprintf(message + index, sizeof(message) - index, "Type=");
     } else if (i == IDX_CMD) {
-      index += snprintf(message + index, sizeof(message) - index, "Cmd=");
+      written = snprintf(message + index, sizeof(message) - index, "Cmd=");
+    } else {
+      written = snprintf(message + index,
+                         sizeof(message) - index,
+                         "%02X ",
+                         buffer[i]);
     }
-    index
-      += snprintf(message + index, sizeof(message) - index, "%02X ", buffer[i]);
+    if (written < 0 || written >= (int)(sizeof(message) - index)) {
+      assert(false);
+      sl_log_error(LOG_TAG, "Overflow in zwapi_frame_to_string\n");
+      return NULL;
+    }
+    index += written;
   }
   return message;
 }
@@ -650,7 +661,7 @@ void zwave_api_protocol_rx_dispatch(uint8_t *pData, uint16_t len)
     case FUNC_ID_ZW_REQUEST_PROTOCOL_CC_ENCRYPTION:
       if (zwave_api_get_callbacks()->protocol_cc_encryption_request != NULL) {
         // ZW->HOST: REQ | 0x6C | destination_node_id | payload_length | payload | protocol_metadata_length | protocol_metadata | use_supervision | session_id
-        uint8_t current_index               = IDX_DATA;
+        uint8_t current_index = IDX_DATA;
         const zwave_node_id_t destination_node_id
           = zwapi_read_node_id(pData, &current_index);
         const uint8_t payload_length = pData[current_index++];