Fix address overflow bug in wasm2c

This only occurs when the immediate offset is small (`int` sized). The
stack offset is `u32` and the immediate is an `int`, so the usual
arithmetic conversions converts the result to a `u32`, which wraps the
address before checking for overflow.

There are already spec tests for overflow, but these use an offset of
`4294967295`, which is `long` (at least on LP64 systems). This means
that the sum's type is `u32 + long` which is `long`. This is why the
tests pass. I've added additional tests for these cases here:
WebAssembly/spec#1188

This fixes issue #1400.

src/c-writer.cc

@@ -2080,10 +2080,10 @@ void CWriter::Write(const LoadExpr& expr) {
 
   Type result_type = expr.opcode.GetResultType();
   Write(StackVar(0, result_type), " = ", func, "(", ExternalPtr(memory->name),
-        ", (u64)(", StackVar(0));
+        ", (u64)(", StackVar(0), ")");
   if (expr.offset != 0)
-    Write(" + ", expr.offset);
-  Write("));", Newline());
+    Write(" + ", expr.offset, "u");
+  Write(");", Newline());
   DropTypes(1);
   PushType(result_type);
 }
@@ -2108,10 +2108,10 @@ void CWriter::Write(const StoreExpr& expr) {
   assert(module_->memories.size() == 1);
   Memory* memory = module_->memories[0];
 
-  Write(func, "(", ExternalPtr(memory->name), ", (u64)(", StackVar(1));
+  Write(func, "(", ExternalPtr(memory->name), ", (u64)(", StackVar(1), ")");
   if (expr.offset != 0)
     Write(" + ", expr.offset);
-  Write("), ", StackVar(0), ");", Newline());
+  Write(", ", StackVar(0), ");", Newline());
   DropTypes(2);
 }
 

test/wasm2c/address-overflow.txt

@@ -0,0 +1,12 @@
+;;; TOOL: run-spec-wasm2c
+(module
+  (memory 1)
+  (func (export "test") (param i32)
+    local.get 0
+    i32.load8_u offset=1
+    drop)
+)
+(assert_trap (invoke "test" (i32.const -1)) "out of bounds memory access")
+(;; STDOUT ;;;
+1/1 tests passed.
+;;; STDOUT ;;)