Fix handling of byte count fields

The various byte count fields are all unsigned byte however the incoming
data are loaded in a QByteArray which returns signed char. This patch
fixes this by converting the field value to quint8.

[ChangeLog][Modbus] Fixed handling of packets with payloads longer than
127 bytes.

Task-number: QTBUG-54834
Change-Id: I699c4cf0cea9933bef6c040c573d2d6e6a113920
Reviewed-by: André Hartmann <aha_1980@gmx.de>

2 files changed, 23 insertions, 4 deletions

diff --git a/src/serialbus/qmodbuspdu.cpp b/src/serialbus/qmodbuspdu.cpp
index 1615c48..cdba047 100644
--- a/src/serialbus/qmodbuspdu.cpp
+++ b/src/serialbus/qmodbuspdu.cpp
@@ -540,18 +540,18 @@ int QModbusRequest::calculateDataSize(const QModbusRequest &request)
     case QModbusPdu::WriteMultipleCoils:
         minimum -= 1; // first payload payload byte
         if (request.dataSize() >= minimum)
-            size = minimum + request.data()[minimum - 1] /*byte count*/;
+            size = minimum + quint8(request.data()[minimum - 1]) /*byte count*/;
         break;
     case QModbusPdu::WriteMultipleRegisters:
     case QModbusPdu::ReadWriteMultipleRegisters:
         minimum -= 2; // first 2 payload payload bytes
         if (request.dataSize() >= minimum)
-            size = minimum + request.data()[minimum - 1] /*byte count*/;
+            size = minimum + quint8(request.data()[minimum - 1]) /*byte count*/;
         break;
     case QModbusPdu::ReadFileRecord:
     case QModbusPdu::WriteFileRecord:
         if (request.dataSize() >= 1)
-            size = 1 /*byte count*/ + request.data()[0] /*actual bytes*/;
+            size = 1 /*byte count*/ + quint8(request.data()[0]) /*actual bytes*/;
         break;
     case QModbusPdu::EncapsulatedInterfaceTransport: {
         if (request.dataSize() < minimum)
@@ -671,7 +671,7 @@ int QModbusResponse::calculateDataSize(const QModbusResponse &response)
     case QModbusResponse::ReadWriteMultipleRegisters:
     case QModbusResponse::ReportServerId:
         if (response.dataSize() >= 1)
-            size = 1 /*byte count*/ + response.data()[0] /*actual bytes*/;
+            size = 1 /*byte count*/ + quint8(response.data()[0]) /*actual bytes*/;
         break;
     case QModbusResponse::ReadFifoQueue: {
         if (response.dataSize() >= 2) {
diff --git a/tests/auto/qmodbuspdu/tst_qmodbuspdu.cpp b/tests/auto/qmodbuspdu/tst_qmodbuspdu.cpp
index 76bfea3..9c74a6c 100644
--- a/tests/auto/qmodbuspdu/tst_qmodbuspdu.cpp
+++ b/tests/auto/qmodbuspdu/tst_qmodbuspdu.cpp
@@ -803,6 +803,25 @@ private slots:
             quint8(0xff)); // dummy additional data
         QCOMPARE(QModbusResponse::calculateDataSize(rdi), 22);
     }
+
+    void testCalculateLongDataSize()
+    {
+        QByteArray longData = QByteArray(128, ' ');
+        longData[0] = longData.size();
+        const QModbusResponse response(QModbusResponse::ReadCoils, longData);
+        QCOMPARE(QModbusResponse::calculateDataSize(response), 1 + longData.size());
+
+        const QModbusRequest wfrRequest(QModbusPdu::WriteFileRecord, longData);
+        QCOMPARE(QModbusRequest::calculateDataSize(wfrRequest), 1 + longData.size());
+
+        longData = QByteArray(4 + 128, ' ');
+        longData[4] = 128;
+        const QModbusRequest wmcRequest(QModbusPdu::WriteMultipleCoils, longData);
+        QCOMPARE(QModbusRequest::calculateDataSize(wmcRequest), 1 + longData.size());
+
+        const QModbusRequest wmrRequest(QModbusPdu::WriteMultipleRegisters, longData);
+        QCOMPARE(QModbusRequest::calculateDataSize(wmrRequest), 1 + longData.size());
+    }
 };
 
 QTEST_MAIN(tst_QModbusPdu)