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>

2 files changed, 12 insertions, 12 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;
         }
diff --git a/src/qml/compiler/qv4instr_moth_p.h b/src/qml/compiler/qv4instr_moth_p.h
index a9de71e7ad..789f875ba7 100644
--- a/src/qml/compiler/qv4instr_moth_p.h
+++ b/src/qml/compiler/qv4instr_moth_p.h
@@ -366,9 +366,9 @@ QT_BEGIN_NAMESPACE
         MOTH_ADJUST_CODE(int, nargs); \
         MOTH_DECODE_ARGS(name, int, nargs, __VA_ARGS__) \
         goto op_main_##name; \
-    op_char_##name: \
-        MOTH_ADJUST_CODE(char, nargs); \
-        MOTH_DECODE_ARGS(name, char, nargs, __VA_ARGS__) \
+    op_byte_##name: \
+        MOTH_ADJUST_CODE(qint8, nargs); \
+        MOTH_DECODE_ARGS(name, qint8, nargs, __VA_ARGS__) \
     op_main_##name: \
         ; \
 
@@ -380,10 +380,10 @@ QT_BEGIN_NAMESPACE
         MOTH_ADJUST_CODE(int, nargs); \
         MOTH_DECODE_ARGS(name, int, nargs, __VA_ARGS__) \
         goto op_main_##name; \
-    op_char_##name: \
+    op_byte_##name: \
         base_ptr = code; \
-        MOTH_ADJUST_CODE(char, nargs); \
-        MOTH_DECODE_ARGS(name, char, nargs, __VA_ARGS__) \
+        MOTH_ADJUST_CODE(qint8, nargs); \
+        MOTH_DECODE_ARGS(name, qint8, nargs, __VA_ARGS__) \
     op_main_##name: \
         ; \
 
@@ -408,7 +408,7 @@ QT_BEGIN_NAMESPACE
 #define COLLECT_LABELS(instr) \
     INSTR_##instr(GET_LABEL)
 #define GET_LABEL_INSTRUCTION(name, ...) \
-    &&op_char_##name,
+    &&op_byte_##name,
 #define COLLECT_LABELS_WIDE(instr) \
     INSTR_##instr(GET_LABEL_WIDE)
 #define GET_LABEL_WIDE_INSTRUCTION(name, ...) \
@@ -428,7 +428,7 @@ QT_BEGIN_NAMESPACE
 #define MOTH_INSTR_CASE_AND_JUMP(instr) \
     INSTR_##instr(GET_CASE_AND_JUMP)
 #define GET_CASE_AND_JUMP_INSTRUCTION(name, ...) \
-    case static_cast<uchar>(Instr::Type::name): goto op_char_##name;
+    case static_cast<uchar>(Instr::Type::name): goto op_byte_##name;
 #define MOTH_INSTR_CASE_AND_JUMP_WIDE(instr) \
     INSTR_##instr(GET_CASE_AND_JUMP_WIDE)
 #define GET_CASE_AND_JUMP_WIDE_INSTRUCTION(name, ...) \