[wasm][bigint] Allow only bigints as i64-global imports

The fuzzer found a crash when we want to execute the {valueOf} function
of an imported value for an i64-global. The problem is that we cannot
execute JavaScript at that moment (I did not check why, I guess we open
some scope at some point). I checked the WebAssembly spec now, and it
defines that only numbers are valid values for imported globals. I
adjust our bigint implementation accordingly with this CL, i.e. that
only bigint values are valid as imported i64-globalsl.
I also created github issues to discuss this problem.

R=jkummerow@chromium.org

Bug: chromium:1001804
Change-Id: I47f0b31fab53163346f341ad290fd3c58e7707bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1792167
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63621}

src/wasm/module-instantiate.cc

@@ -1185,13 +1185,8 @@ bool InstanceBuilder::ProcessImportedGlobal(Handle<WasmInstanceObject> instance,
     return true;
   }
 
-  if (enabled_.bigint && global.type == kWasmI64) {
-    Handle<BigInt> bigint;
-
-    if (!BigInt::FromObject(isolate_, value).ToHandle(&bigint)) {
-      return false;
-    }
-    WriteGlobalValue(global, bigint->AsInt64());
+  if (enabled_.bigint && global.type == kWasmI64 && value->IsBigInt()) {
+    WriteGlobalValue(global, BigInt::cast(*value).AsInt64());
     return true;
   }
 

test/mjsunit/wasm/bigint.js

@@ -26,30 +26,30 @@ load("test/mjsunit/wasm/wasm-module-builder.js");
   let builder = new WasmModuleBuilder();
 
   let a_global_index = builder
-    .addImportedGlobal("mod", "a", kWasmI64)
+    .addImportedGlobal("mod", "a", kWasmI64);
 
   let b_global_index = builder
     .addImportedGlobal("mod", "b", kWasmI64);
 
-  let c_global_index = builder
-    .addImportedGlobal("mod", "c", kWasmI64);
-
   builder
     .addExportOfKind('a', kExternalGlobal, a_global_index)
     .addExportOfKind('b', kExternalGlobal, b_global_index)
-    .addExportOfKind('c', kExternalGlobal, c_global_index);
 
   let module = builder.instantiate({
     mod: {
       a: 1n,
       b: 2n ** 63n,
-      c: "123",
     }
   });
 
   assertEquals(module.exports.a.value, 1n);
   assertEquals(module.exports.b.value, - (2n ** 63n));
-  assertEquals(module.exports.c.value, 123n);
+})();
+
+(function TestJSBigIntGlobalImportInvalidType() {
+  let builder = new WasmModuleBuilder();
+  builder.addImportedGlobal("mod", "a", kWasmI64);
+  assertThrows(() => builder.instantiate({mod: { a: {} } }), WebAssembly.LinkError);
 })();
 
 (function TestJSBigIntToWasmI64MutableGlobal() {