[wasm] Mutable globals cannot be exported

R=titzer@chromium.org, rossberg@chromium.org
TEST=unittests/WasmModuleVerifyTest.ExportMutableGlobal

Review-Url: https://codereview.chromium.org/2481263003
Cr-Commit-Position: refs/heads/master@{#40838}

src/wasm/module-decoder.cc

@@ -478,7 +478,12 @@ class ModuleDecoder : public Decoder {
           case kExternalGlobal: {
             WasmGlobal* global = nullptr;
             exp->index = consume_global_index(module, &global);
-            if (global) global->exported = true;
+            if (global) {
+              if (global->mutability) {
+                error("mutable globals cannot be exported");
+              }
+              global->exported = true;
+            }
             break;
           }
           default:

test/unittests/wasm/module-decoder-unittest.cc

@@ -248,6 +248,47 @@ TEST_F(WasmModuleVerifyTest, ZeroGlobals) {
   if (result.val) delete result.val;
 }
 
+TEST_F(WasmModuleVerifyTest, ExportMutableGlobal) {
+  {
+    static const byte data[] = {
+        SECTION(Global, 6),  // --
+        1,
+        kLocalI32,                  // local type
+        0,                          // immutable
+        WASM_INIT_EXPR_I32V_1(13),  // init
+        SECTION(Export, 8),         // --
+        1,                          // Export count
+        4,                          // name length
+        'n',                        // --
+        'a',                        // --
+        'm',                        // --
+        'e',                        // --
+        kExternalGlobal,            // global
+        0,                          // global index
+    };
+    EXPECT_VERIFIES(data);
+  }
+  {
+    static const byte data[] = {
+        SECTION(Global, 6),         // --
+        1,                          // --
+        kLocalI32,                  // local type
+        1,                          // mutable
+        WASM_INIT_EXPR_I32V_1(13),  // init
+        SECTION(Export, 8),         // --
+        1,                          // Export count
+        4,                          // name length
+        'n',                        // --
+        'a',                        // --
+        'm',                        // --
+        'e',                        // --
+        kExternalGlobal,            // global
+        0,                          // global index
+    };
+    EXPECT_FAILURE(data);
+  }
+}
+
 static void AppendUint32v(std::vector<byte>& buffer, uint32_t val) {
   while (true) {
     uint32_t next = val >> 7;