[WebAssembly] Fix for use of uninitialized member in WasmObjectWriter…

….cpp

Currently, section indices may be passed uninitialized by value if
writing the section fails. Removes section indices form class
initialization and returns them from the write{Code,Data}Section
function calls instead.

Patch by Gui Andrade!

Differential Revision: https://reviews.llvm.org/D81702

llvm/lib/MC/WasmObjectWriter.cpp

@@ -224,11 +224,8 @@ class WasmObjectWriter : public MCObjectWriter {
 
   // Relocations for fixing up references in the code section.
   std::vector<WasmRelocationEntry> CodeRelocations;
-  uint32_t CodeSectionIndex;
-
   // Relocations for fixing up references in the data section.
   std::vector<WasmRelocationEntry> DataRelocations;
-  uint32_t DataSectionIndex;
 
   // Index values to use for fixing up call_indirect type indices.
   // Maps function symbols to the index of the type of the function
@@ -335,9 +332,9 @@ class WasmObjectWriter : public MCObjectWriter {
   void writeExportSection(ArrayRef<wasm::WasmExport> Exports);
   void writeElemSection(ArrayRef<uint32_t> TableElems);
   void writeDataCountSection();
-  void writeCodeSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
-                        ArrayRef<WasmFunction> Functions);
-  void writeDataSection(const MCAsmLayout &Layout);
+  uint32_t writeCodeSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
+                            ArrayRef<WasmFunction> Functions);
+  uint32_t writeDataSection(const MCAsmLayout &Layout);
   void writeEventSection(ArrayRef<wasm::WasmEventType> Events);
   void writeGlobalSection(ArrayRef<wasm::WasmGlobal> Globals);
   void writeRelocSection(uint32_t SectionIndex, StringRef Name,
@@ -894,15 +891,14 @@ void WasmObjectWriter::writeDataCountSection() {
   endSection(Section);
 }
 
-void WasmObjectWriter::writeCodeSection(const MCAssembler &Asm,
-                                        const MCAsmLayout &Layout,
-                                        ArrayRef<WasmFunction> Functions) {
+uint32_t WasmObjectWriter::writeCodeSection(const MCAssembler &Asm,
+                                            const MCAsmLayout &Layout,
+                                            ArrayRef<WasmFunction> Functions) {
   if (Functions.empty())
-    return;
+    return 0;
 
   SectionBookkeeping Section;
   startSection(Section, wasm::WASM_SEC_CODE);
-  CodeSectionIndex = Section.Index;
 
   encodeULEB128(Functions.size(), W.OS);
 
@@ -922,15 +918,15 @@ void WasmObjectWriter::writeCodeSection(const MCAssembler &Asm,
   applyRelocations(CodeRelocations, Section.ContentsOffset, Layout);
 
   endSection(Section);
+  return Section.Index;
 }
 
-void WasmObjectWriter::writeDataSection(const MCAsmLayout &Layout) {
+uint32_t WasmObjectWriter::writeDataSection(const MCAsmLayout &Layout) {
   if (DataSegments.empty())
-    return;
+    return 0;
 
   SectionBookkeeping Section;
   startSection(Section, wasm::WASM_SEC_DATA);
-  DataSectionIndex = Section.Index;
 
   encodeULEB128(DataSegments.size(), W.OS); // count
 
@@ -952,6 +948,7 @@ void WasmObjectWriter::writeDataSection(const MCAsmLayout &Layout) {
   applyRelocations(DataRelocations, Section.ContentsOffset, Layout);
 
   endSection(Section);
+  return Section.Index;
 }
 
 void WasmObjectWriter::writeRelocSection(
@@ -1698,8 +1695,8 @@ uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
   writeExportSection(Exports);
   writeElemSection(TableElems);
   writeDataCountSection();
-  writeCodeSection(Asm, Layout, Functions);
-  writeDataSection(Layout);
+  uint32_t CodeSectionIndex = writeCodeSection(Asm, Layout, Functions);
+  uint32_t DataSectionIndex = writeDataSection(Layout);
   for (auto &CustomSection : CustomSections)
     writeCustomSection(CustomSection, Asm, Layout);
   writeLinkingMetaDataSection(SymbolInfos, InitFuncs, Comdats);