Fix decoding of bytecode instructions on ARM generated on x86-64

Due to the difference of sign of the char type, byte code is encoded
differently on x86-64 than if it was on ARM and it also is decoded
differently. The problem at hand here was that negative jumps were
encoded as two-byte instructions (opcode and negative offset as byte) on
x86-64 when qmlcachegen is run. At run-time the negative offset was read
into a char type and consequently interpreted as a positive jump,
leading to crashes.

The explicit use of qint8 as signed byte type in encoding/decoding
sensitive parts avoids the decoding issue and should also result in
consistent encoding.

The added auto-test is (among other configurations) run in the CI under
qemu, which means the x86-64 host-built qmlcachegen will generate byte
code and the tst_qmlcachegen ARM binary will run under qemu and
reproduce this scenario.

Task-number: QTBUG-65521
Change-Id: I615628f53475dad38a41095c6e7ffea0c34d58ac
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 4 insertions, 4 deletions

diff --git a/src/qml/compiler/qv4bytecodegenerator.cpp b/src/qml/compiler/qv4bytecodegenerator.cpp
index 05bbf25292..03105d2b71 100644
--- a/src/qml/compiler/qv4bytecodegenerator.cpp
+++ b/src/qml/compiler/qv4bytecodegenerator.cpp
@@ -82,7 +82,7 @@ void BytecodeGenerator::packInstruction(I &i)
         Wide
     } width = Normal;
     for (int n = 0; n < nMembers; ++n) {
-        if (width == Normal && (static_cast<char>(instructionsAsInts[n]) != instructionsAsInts[n]))
+        if (width == Normal && (static_cast<qint8>(instructionsAsInts[n]) != instructionsAsInts[n]))
             width = Wide;
     }
     char *code = i.packed;
@@ -91,7 +91,7 @@ void BytecodeGenerator::packInstruction(I &i)
         *reinterpret_cast<uchar *>(code) = type;
         ++code;
         for (int n = 0; n < nMembers; ++n) {
-            char v = static_cast<char>(instructionsAsInts[n]);
+            qint8 v = static_cast<qint8>(instructionsAsInts[n]);
             memcpy(code, &v, 1);
             code += 1;
         }
@@ -113,7 +113,7 @@ void BytecodeGenerator::adjustJumpOffsets()
             continue;
         Q_ASSERT(i.linkedLabel != -1 && labels.at(i.linkedLabel) != -1);
         const auto &linkedInstruction = instructions.at(labels.at(i.linkedLabel));
-        char *c = i.packed + i.offsetForJump;
+        qint8 *c = reinterpret_cast<qint8*>(i.packed + i.offsetForJump);
         int jumpOffset = linkedInstruction.position - (i.position + i.size);
 //        qDebug() << "adjusting jump offset for instruction" << index << i.position << i.size << "offsetForJump" << i.offsetForJump << "target"
 //                 << labels.at(i.linkedLabel) << linkedInstruction.position << "jumpOffset" << jumpOffset;
@@ -123,7 +123,7 @@ void BytecodeGenerator::adjustJumpOffsets()
             memcpy(c, &jumpOffset, sizeof(int));
         } else {
             Q_ASSERT(i.offsetForJump == i.size - 1);
-            char o = jumpOffset;
+            qint8 o = jumpOffset;
             Q_ASSERT(o == jumpOffset);
             *c = o;
         }