Add some checking in EmscriptenGlueGenerator::generateStackInitialization (#1944)

We expect the stack pointer to be of a certain type.  This fixes
a segfault we are seeing when passed a binary which doesn't quite
meet our expectations.

1 files changed, 12 insertions, 4 deletions

diff --git a/src/wasm/wasm-emscripten.cpp b/src/wasm/wasm-emscripten.cpp
index a21c45476..ea24b945d 100644
--- a/src/wasm/wasm-emscripten.cpp
+++ b/src/wasm/wasm-emscripten.cpp
@@ -47,10 +47,15 @@ void addExportedFunction(Module& wasm, Function* function) {
 }
 
 Global* EmscriptenGlueGenerator::getStackPointerGlobal() {
-  // Assumption: first global is __stack_pointer
-  // TODO(sbc): Once mutable globals are a thing we shouldn't need this
-  // at all since we can simply export __stack_pointer.
-  return wasm.globals[0].get();
+  // Assumption: The first non-imported global is global is __stack_pointer
+  // TODO(sbc): Find a better way to discover the stack pointer.  Perhaps the
+  // linker could export it by name?
+  for (auto& g : wasm.globals) {
+    if (!g->imported()) {
+      return g.get();
+    }
+  }
+  Fatal() << "stack pointer global not found";
 }
 
 Expression* EmscriptenGlueGenerator::generateLoadStackPointer() {
@@ -157,6 +162,9 @@ Function* EmscriptenGlueGenerator::generateMemoryGrowthFunction() {
 
 void EmscriptenGlueGenerator::generateStackInitialization(Address addr) {
   auto* stackPointer = getStackPointerGlobal();
+  assert(!stackPointer->imported());
+  if (!stackPointer->init || !stackPointer->init->is<Const>())
+    Fatal() << "stack pointer global is not assignable";
   stackPointer->init->cast<Const>()->value = Literal(int32_t(addr));
 }