feat(script): implement some of the non-ordinary internal methods of Location

<https://html.spec.whatwg.org/multipage/#the-location-interface>

 - `[[GetPrototypeOf]]`: not yet
 - `[[SetPrototypeOf]]`: not yet
 - `[[IsExtensible]]`: `proxyhandler::is_extensible`
 - `[[PreventExtensions]]`: `proxyhandler::prevent_extensions`
 - `[[GetOwnProperty]]`: `CGDOMJSProxyHandler_getOwnPropertyDescriptor` (updated)
 - `[[DefineOwnProperty]]`: `CGDOMJSProxyHandler_defineProperty` (updated)
 - `[[Get]]`: `CGDOMJSProxyHandler_get` (updated)
 - `[[Set]]`: not yet
 - `[[Delete]]`: `CGDOMJSProxyHandler_delete` (updated)
 - `[[OwnPropertyKeys]]`: `CGDOMJSProxyHandler_ownPropertyKeys` (updated)

components/script/dom/bindings/codegen/CodegenRust.py

@@ -1656,10 +1656,15 @@ class MethodDefiner(PropertyDefiner):
     """
     A class for defining methods on a prototype object.
     """
-    def __init__(self, descriptor, name, static, unforgeable):
+    def __init__(self, descriptor, name, static, unforgeable, crossorigin=False):
         assert not (static and unforgeable)
+        assert not (static and crossorigin)
+        assert not (unforgeable and crossorigin)
         PropertyDefiner.__init__(self, descriptor, name)
 
+        # TODO: Separate the `(static, unforgeable, crossorigin) = (False, False, True)` case
+        #       to a separate class or something.
+
         # FIXME https://bugzilla.mozilla.org/show_bug.cgi?id=772822
         #       We should be able to check for special operations without an
         #       identifier. For now we check if the name starts with __
@@ -1668,14 +1673,15 @@ class MethodDefiner(PropertyDefiner):
         if not descriptor.interface.isCallback() or static:
             methods = [m for m in descriptor.interface.members if
                        m.isMethod() and m.isStatic() == static
+                       and (bool(m.getExtendedAttribute("CrossOriginCallable")) or not crossorigin)
                        and not m.isIdentifierLess()
-                       and MemberIsUnforgeable(m, descriptor) == unforgeable]
+                       and (MemberIsUnforgeable(m, descriptor) == unforgeable or crossorigin)]
         else:
             methods = []
         self.regular = [{"name": m.identifier.name,
                          "methodInfo": not m.isStatic(),
                          "length": methodLength(m),
-                         "flags": "JSPROP_ENUMERATE",
+                         "flags": "JSPROP_READONLY" if crossorigin else "JSPROP_ENUMERATE",
                          "condition": PropertyDefiner.getControllingCondition(m, descriptor)}
                         for m in methods]
 
@@ -1693,6 +1699,7 @@ class MethodDefiner(PropertyDefiner):
         # neither.
         if (not static
             and not unforgeable
+            and not crossorigin
             and descriptor.supportsIndexedProperties()):  # noqa
             if hasIterator(methods, self.regular):  # noqa
                 raise TypeError("Cannot have indexed getter/attr on "
@@ -1709,7 +1716,7 @@ class MethodDefiner(PropertyDefiner):
 
         # Generate the keys/values/entries aliases for value iterables.
         maplikeOrSetlikeOrIterable = descriptor.interface.maplikeOrSetlikeOrIterable
-        if (not static and not unforgeable
+        if (not static and not unforgeable and not crossorigin
                 and maplikeOrSetlikeOrIterable
                 and maplikeOrSetlikeOrIterable.isIterable()
                 and maplikeOrSetlikeOrIterable.isValueIterator()):
@@ -1754,7 +1761,7 @@ class MethodDefiner(PropertyDefiner):
             })
 
         isUnforgeableInterface = bool(descriptor.interface.getExtendedAttribute("Unforgeable"))
-        if not static and unforgeable == isUnforgeableInterface:
+        if not static and unforgeable == isUnforgeableInterface and not crossorigin:
             stringifier = descriptor.operations['Stringifier']
             if stringifier:
                 self.regular.append({
@@ -1765,6 +1772,7 @@ class MethodDefiner(PropertyDefiner):
                     "condition": PropertyDefiner.getControllingCondition(stringifier, descriptor)
                 })
         self.unforgeable = unforgeable
+        self.crossorigin = crossorigin
 
     def generateArray(self, array, name):
         if len(array) == 0:
@@ -1796,36 +1804,59 @@ class MethodDefiner(PropertyDefiner):
                     jitinfo = "ptr::null()"
                     accessor = 'Some(%s)' % m.get("nativeName", m["name"])
             if m["name"].startswith("@@"):
+                assert not self.crossorigin
                 name = 'JSPropertySpec_Name { symbol_: SymbolCode::%s as usize + 1 }' % m["name"][2:]
             else:
                 name = ('JSPropertySpec_Name { string_: %s as *const u8 as *const libc::c_char }'
                         % str_to_const_array(m["name"]))
             return (name, accessor, jitinfo, m["length"], flags, selfHostedName)
 
-        return self.generateGuardedArray(
-            array, name,
+        specTemplate = (
             '    JSFunctionSpec {\n'
             '        name: %s,\n'
             '        call: JSNativeWrapper { op: %s, info: %s },\n'
             '        nargs: %s,\n'
             '        flags: (%s) as u16,\n'
             '        selfHostedName: %s\n'
-            '    }',
+            '    }')
+        specTerminator = (
             '    JSFunctionSpec {\n'
             '        name: JSPropertySpec_Name { string_: ptr::null() },\n'
             '        call: JSNativeWrapper { op: None, info: ptr::null() },\n'
             '        nargs: 0,\n'
             '        flags: 0,\n'
             '        selfHostedName: ptr::null()\n'
-            '    }',
-            'JSFunctionSpec',
-            condition, specData)
+            '    }')
+
+        if self.crossorigin:
+            groups = groupby(array, lambda m: condition(m, self.descriptor))
+            assert len(list(groups)) == 1  # can't handle mixed condition
+            elems = [specTemplate % specData(m) for m in array]
+            return dedent(
+                """
+                const %s: &[JSFunctionSpec] = &[
+                %s,
+                %s,
+                ];
+                """) % (name, ',\n'.join(elems), specTerminator)
+        else:
+            return self.generateGuardedArray(
+                array, name,
+                specTemplate, specTerminator,
+                'JSFunctionSpec',
+                condition, specData)
 
 
 class AttrDefiner(PropertyDefiner):
-    def __init__(self, descriptor, name, static, unforgeable):
+    def __init__(self, descriptor, name, static, unforgeable, crossorigin=False):
         assert not (static and unforgeable)
+        assert not (static and crossorigin)
+        assert not (unforgeable and crossorigin)
         PropertyDefiner.__init__(self, descriptor, name)
+
+        # TODO: Separate the `(static, unforgeable, crossorigin) = (False, False, True)` case
+        #       to a separate class or something.
+
         self.name = name
         self.descriptor = descriptor
         self.regular = [
@@ -1837,12 +1868,16 @@ class AttrDefiner(PropertyDefiner):
             }
             for m in descriptor.interface.members if
             m.isAttr() and m.isStatic() == static
-            and MemberIsUnforgeable(m, descriptor) == unforgeable
+            and (MemberIsUnforgeable(m, descriptor) == unforgeable or crossorigin)
+            and (not crossorigin
+                 or m.getExtendedAttribute("CrossOriginReadable")
+                 or m.getExtendedAttribute("CrossOriginWritable"))
         ]
         self.static = static
         self.unforgeable = unforgeable
+        self.crossorigin = crossorigin
 
-        if not static and not unforgeable and not (
+        if not static and not unforgeable and not crossorigin and not (
                 descriptor.interface.isNamespace() or descriptor.interface.isCallback()
         ):
             self.regular.append({
@@ -1858,6 +1893,10 @@ class AttrDefiner(PropertyDefiner):
 
         def getter(attr):
             attr = attr['attr']
+
+            if self.crossorigin and not attr.getExtendedAttribute("CrossOriginReadable"):
+                return "JSNativeWrapper { op: None, info: 0 as *const JSJitInfo }"
+
             if self.static:
                 accessor = 'get_' + self.descriptor.internalNameFor(attr.identifier.name)
                 jitinfo = "0 as *const JSJitInfo"
@@ -1874,8 +1913,10 @@ class AttrDefiner(PropertyDefiner):
 
         def setter(attr):
             attr = attr['attr']
-            if (attr.readonly and not attr.getExtendedAttribute("PutForwards")
-                    and not attr.getExtendedAttribute("Replaceable")):
+
+            if ((attr.readonly and not attr.getExtendedAttribute("PutForwards")
+                    and not attr.getExtendedAttribute("Replaceable"))
+                    or (self.crossorigin and not attr.getExtendedAttribute("CrossOriginReadable"))):
                 return "JSNativeWrapper { op: None, info: 0 as *const JSJitInfo }"
 
             if self.static:
@@ -1941,12 +1982,24 @@ class AttrDefiner(PropertyDefiner):
                 }
 """
 
-        return self.generateGuardedArray(
-            array, name,
-            template,
-            '    JSPropertySpec::ZERO',
-            'JSPropertySpec',
-            condition, specData)
+        if self.crossorigin:
+            groups = groupby(array, lambda m: condition(m, self.descriptor))
+            assert len(list(groups)) == 1  # can't handle mixed condition
+            elems = [template(m) % specData(m) for m in array]
+            return dedent(
+                """
+                const %s: &[JSPropertySpec] = &[
+                %s,
+                    JSPropertySpec::ZERO,
+                ];
+                """) % (name, ',\n'.join(elems))
+        else:
+            return self.generateGuardedArray(
+                array, name,
+                template,
+                '    JSPropertySpec::ZERO',
+                'JSPropertySpec',
+                condition, specData)
 
 
 class ConstDefiner(PropertyDefiner):
@@ -3071,6 +3124,25 @@ class PropertyArrays():
         return define
 
 
+class CGCrossOriginProperties(CGThing):
+    def __init__(self, descriptor):
+        self.methods = MethodDefiner(descriptor, "CrossOriginMethods", static=False,
+                                     unforgeable=False, crossorigin=True)
+        self.attributes = AttrDefiner(descriptor, "CrossOriginAttributes", static=False,
+                                      unforgeable=False, crossorigin=True)
+
+    def define(self):
+        return str(self.methods) + str(self.attributes) + dedent(
+            """
+            const CROSS_ORIGIN_PROPERTIES: proxyhandler::CrossOriginProperties =
+                proxyhandler::CrossOriginProperties {
+                    attributes: sCrossOriginAttributes,
+                    methods: sCrossOriginMethods,
+                };
+            """
+        )
+
+
 class CGCollectJSONAttributesMethod(CGAbstractMethod):
     """
     Generate the CollectJSONAttributes method for an interface descriptor
@@ -3451,11 +3523,12 @@ class CGDefineProxyHandler(CGAbstractMethod):
 
     def definition_body(self):
         customDefineProperty = 'proxyhandler::define_property'
-        if self.descriptor.operations['IndexedSetter'] or self.descriptor.operations['NamedSetter']:
+        if self.descriptor.isMaybeCrossOriginObject() or self.descriptor.operations['IndexedSetter'] or \
+           self.descriptor.operations['NamedSetter']:
             customDefineProperty = 'defineProperty'
 
         customDelete = 'proxyhandler::delete'
-        if self.descriptor.operations['NamedDeleter']:
+        if self.descriptor.isMaybeCrossOriginObject() or self.descriptor.operations['NamedDeleter']:
             customDelete = 'delete'
 
         getOwnEnumerablePropertyKeys = "own_property_keys"
@@ -5353,6 +5426,26 @@ class CGDOMJSProxyHandler_getOwnPropertyDescriptor(CGAbstractExternMethod):
         indexedGetter = self.descriptor.operations['IndexedGetter']
 
         get = "let cx = SafeJSContext::from_ptr(cx);\n"
+
+        if self.descriptor.isMaybeCrossOriginObject():
+            get += dedent(
+                """
+                if !proxyhandler::is_platform_object_same_origin(cx, proxy) {
+                    if !proxyhandler::cross_origin_get_own_property_helper(
+                        cx, proxy, &CROSS_ORIGIN_PROPERTIES, id, desc
+                    ) {
+                        return false;
+                    }
+                    if desc.obj.is_null() {
+                        return proxyhandler::cross_origin_property_fallback(cx, proxy, id, desc);
+                    }
+                    return true;
+                }
+
+                // Safe to enter the Realm of proxy now.
+                let _ac = JSAutoRealm::new(*cx, proxy.get());
+                """)
+
         if indexedGetter:
             get += "let index = get_array_index_from_id(*cx, Handle::from_raw(id));\n"
 
@@ -5450,6 +5543,17 @@ class CGDOMJSProxyHandler_defineProperty(CGAbstractExternMethod):
     def getBody(self):
         set = "let cx = SafeJSContext::from_ptr(cx);\n"
 
+        if self.descriptor.isMaybeCrossOriginObject():
+            set += dedent(
+                """
+                if !proxyhandler::is_platform_object_same_origin(cx, proxy) {
+                    return proxyhandler::report_cross_origin_denial(cx, id, "define");
+                }
+
+                // Safe to enter the Realm of proxy now.
+                let _ac = JSAutoRealm::new(*cx, proxy.get());
+                """)
+
         indexedSetter = self.descriptor.operations['IndexedSetter']
         if indexedSetter:
             set += ("let index = get_array_index_from_id(*cx, Handle::from_raw(id));\n"
@@ -5497,6 +5601,18 @@ class CGDOMJSProxyHandler_delete(CGAbstractExternMethod):
 
     def getBody(self):
         set = "let cx = SafeJSContext::from_ptr(cx);\n"
+
+        if self.descriptor.isMaybeCrossOriginObject():
+            set += dedent(
+                """
+                if !proxyhandler::is_platform_object_same_origin(cx, proxy) {
+                    return proxyhandler::report_cross_origin_denial(cx, id, "delete");
+                }
+
+                // Safe to enter the Realm of proxy now.
+                let _ac = JSAutoRealm::new(*cx, proxy.get());
+                """)
+
         if self.descriptor.operations['NamedDeleter']:
             if self.descriptor.hasUnforgeableMembers:
                 raise TypeError("Can't handle a deleter on an interface that has "
@@ -5524,6 +5640,17 @@ class CGDOMJSProxyHandler_ownPropertyKeys(CGAbstractExternMethod):
             let unwrapped_proxy = UnwrapProxy(proxy);
             """)
 
+        if self.descriptor.isMaybeCrossOriginObject():
+            body += dedent(
+                """
+                if !proxyhandler::is_platform_object_same_origin(cx, proxy) {
+                    return proxyhandler::cross_origin_own_property_keys(cx, proxy, &CROSS_ORIGIN_PROPERTIES, props);
+                }
+
+                // Safe to enter the Realm of proxy now.
+                let _ac = JSAutoRealm::new(*cx, proxy.get());
+                """)
+
         if self.descriptor.operations['IndexedGetter']:
             body += dedent(
                 """
@@ -5583,6 +5710,18 @@ class CGDOMJSProxyHandler_getOwnEnumerablePropertyKeys(CGAbstractExternMethod):
             let unwrapped_proxy = UnwrapProxy(proxy);
             """)
 
+        if self.descriptor.isMaybeCrossOriginObject():
+            body += dedent(
+                """
+                if !proxyhandler::is_platform_object_same_origin(cx, proxy) {
+                    // There are no enumerable cross-origin props, so we're done.
+                    return true;
+                }
+
+                // Safe to enter the Realm of proxy now.
+                let _ac = JSAutoRealm::new(*cx, proxy.get());
+                """)
+
         if self.descriptor.operations['IndexedGetter']:
             body += dedent(
                 """
@@ -5621,6 +5760,18 @@ class CGDOMJSProxyHandler_hasOwn(CGAbstractExternMethod):
     def getBody(self):
         indexedGetter = self.descriptor.operations['IndexedGetter']
         indexed = "let cx = SafeJSContext::from_ptr(cx);\n"
+
+        if self.descriptor.isMaybeCrossOriginObject():
+            indexed += dedent(
+                """
+                if !proxyhandler::is_platform_object_same_origin(cx, proxy) {
+                    return proxyhandler::cross_origin_has_own(cx, proxy, &CROSS_ORIGIN_PROPERTIES, id, bp);
+                }
+
+                // Safe to enter the Realm of proxy now.
+                let _ac = JSAutoRealm::new(*cx, proxy.get());
+                """)
+
         if indexedGetter:
             indexed += ("let index = get_array_index_from_id(*cx, Handle::from_raw(id));\n"
                         + "if let Some(index) = index {\n"
@@ -5682,6 +5833,18 @@ class CGDOMJSProxyHandler_get(CGAbstractExternMethod):
 
     # https://heycam.github.io/webidl/#LegacyPlatformObjectGetOwnProperty
     def getBody(self):
+        if self.descriptor.isMaybeCrossOriginObject():
+            maybeCrossOriginGet = dedent(
+                """
+                if !proxyhandler::is_platform_object_same_origin(cx, proxy) {
+                    return proxyhandler::cross_origin_get(cx, proxy, receiver, id, vp);
+                }
+
+                // Safe to enter the Realm of proxy now.
+                let _ac = JSAutoRealm::new(*cx, proxy.get());
+                """)
+        else:
+            maybeCrossOriginGet = ""
         getFromExpando = """\
 rooted!(in(*cx) let mut expando = ptr::null_mut::<JSObject>());
 get_expando_object(proxy, expando.handle_mut());
@@ -5737,6 +5900,9 @@ if !expando.is_null() {
 //MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy),
 //"Should not have a XrayWrapper here");
 let cx = SafeJSContext::from_ptr(cx);
+
+%s
+
 let proxy_lt = Handle::from_raw(proxy);
 let vp_lt = MutableHandle::from_raw(vp);
 let id_lt = Handle::from_raw(id);
@@ -5753,7 +5919,7 @@ if found {
 }
 %s
 vp.set(UndefinedValue());
-return true;""" % (getIndexedOrExpando, getNamed)
+return true;""" % (maybeCrossOriginGet, getIndexedOrExpando, getNamed)
 
     def definition_body(self):
         return CGGeneric(self.getBody())
@@ -6392,6 +6558,9 @@ class CGDescriptor(CGThing):
         if descriptor.proxy:
             cgThings.append(CGDefineProxyHandler(descriptor))
 
+        if descriptor.isMaybeCrossOriginObject():
+            cgThings.append(CGCrossOriginProperties(descriptor))
+
         properties = PropertyArrays(descriptor)
 
         if defaultToJSONMethod:
@@ -6410,15 +6579,24 @@ class CGDescriptor(CGThing):
                 cgThings.append(CGDOMJSProxyHandler_get(descriptor))
                 cgThings.append(CGDOMJSProxyHandler_hasOwn(descriptor))
 
-                if descriptor.operations['IndexedSetter'] or descriptor.operations['NamedSetter']:
+                if descriptor.isMaybeCrossOriginObject() or descriptor.operations['IndexedSetter'] or \
+                   descriptor.operations['NamedSetter']:
                     cgThings.append(CGDOMJSProxyHandler_defineProperty(descriptor))
 
                 # We want to prevent indexed deleters from compiling at all.
                 assert not descriptor.operations['IndexedDeleter']
 
-                if descriptor.operations['NamedDeleter']:
+                if descriptor.isMaybeCrossOriginObject() or descriptor.operations['NamedDeleter']:
                     cgThings.append(CGDOMJSProxyHandler_delete(descriptor))
 
+                if descriptor.isMaybeCrossOriginObject():
+                    # TODO: CGDOMJSProxyHandler_getPrototype(descriptor),
+                    # TODO: CGDOMJSProxyHandler_getPrototypeIfOrdinary(descriptor),
+                    # TODO: CGDOMJSProxyHandler_setPrototype(descriptor),
+                    # TODO: CGDOMJSProxyHandler_setImmutablePrototype(descriptor),
+                    # TODO: CGDOMJSProxyHandler_set(descriptor),
+                    pass
+
                 # cgThings.append(CGDOMJSProxyHandler(descriptor))
                 # cgThings.append(CGIsMethod(descriptor))
                 pass

components/script/dom/bindings/codegen/Configuration.py

@@ -299,6 +299,9 @@ class Descriptor(DescriptorProvider):
                 if iface:
                     iface.setUserData('hasConcreteDescendant', True)
 
+            if self.isMaybeCrossOriginObject():
+                self.proxy = True
+
             if self.proxy:
                 iface = self.interface
                 while iface.parent:
@@ -404,6 +407,11 @@ class Descriptor(DescriptorProvider):
     def supportsIndexedProperties(self):
         return self.operations['IndexedGetter'] is not None
 
+    def isMaybeCrossOriginObject(self):
+        # If we're isGlobal and have cross-origin members, we're a Window, and
+        # that's not a cross-origin object.  The WindowProxy is.
+        return self.concrete and self.interface.hasCrossOriginMembers and not self.isGlobal()
+
     def hasDescendants(self):
         return (self.interface.getUserData("hasConcreteDescendant", False)
                 or self.interface.getUserData("hasProxyDescendant", False))

components/script/dom/bindings/proxyhandler.rs

@@ -7,24 +7,43 @@
 #![deny(missing_docs)]
 
 use crate::dom::bindings::conversions::is_dom_proxy;
+use crate::dom::bindings::error::{throw_dom_exception, Error};
+use crate::dom::bindings::principals::ServoJSPrincipalsRef;
 use crate::dom::bindings::utils::delete_property_by_id;
-use js::glue::GetProxyHandlerFamily;
+use crate::dom::globalscope::GlobalScope;
+use crate::realms::{AlreadyInRealm, InRealm};
+use crate::script_runtime::JSContext as SafeJSContext;
+use js::conversions::ToJSValConvertible;
+use js::glue::{
+    GetProxyHandler, GetProxyHandlerFamily, InvokeGetOwnPropertyDescriptor, RUST_SYMBOL_TO_JSID,
+};
 use js::glue::{GetProxyPrivate, SetProxyPrivate};
+use js::jsapi;
 use js::jsapi::GetStaticPrototype;
 use js::jsapi::Handle as RawHandle;
 use js::jsapi::HandleId as RawHandleId;
 use js::jsapi::HandleObject as RawHandleObject;
+use js::jsapi::HandleValue as RawHandleValue;
+use js::jsapi::JS_AtomizeAndPinString;
 use js::jsapi::JS_DefinePropertyById;
+use js::jsapi::JS_GetOwnPropertyDescriptorById;
+use js::jsapi::JS_IsExceptionPending;
+use js::jsapi::MutableHandle as RawMutableHandle;
+use js::jsapi::MutableHandleIdVector as RawMutableHandleIdVector;
 use js::jsapi::MutableHandleObject as RawMutableHandleObject;
+use js::jsapi::MutableHandleValue as RawMutableHandleValue;
 use js::jsapi::ObjectOpResult;
+use js::jsapi::{jsid, GetObjectRealmOrNull, GetRealmPrincipals, JSFunctionSpec, JSPropertySpec};
 use js::jsapi::{DOMProxyShadowsResult, JSContext, JSObject, PropertyDescriptor};
+use js::jsapi::{GetWellKnownSymbol, SymbolCode};
 use js::jsapi::{JSErrNum, SetDOMProxyInformation};
 use js::jsval::ObjectValue;
 use js::jsval::UndefinedValue;
 use js::rust::wrappers::JS_AlreadyHasOwnPropertyById;
 use js::rust::wrappers::JS_NewObjectWithGivenProto;
-use js::rust::{Handle, HandleObject, MutableHandle, MutableHandleObject};
-use std::ptr;
+use js::rust::wrappers::{AppendToIdVector, RUST_INTERNED_STRING_TO_JSID};
+use js::rust::{get_context_realm, Handle, HandleObject, MutableHandle, MutableHandleObject};
+use std::{ffi::CStr, ptr};
 
 /// Determine if this id shadows any existing properties for this proxy.
 pub unsafe extern "C" fn shadow_check_callback(
@@ -177,3 +196,322 @@ pub fn fill_property_descriptor(
     desc.getter = None;
     desc.setter = None;
 }
+
+/// <https://html.spec.whatwg.org/multipage/#isplatformobjectsameorigin-(-o-)>
+pub unsafe fn is_platform_object_same_origin(cx: SafeJSContext, obj: RawHandleObject) -> bool {
+    let subject_realm = get_context_realm(*cx);
+    let obj_realm = GetObjectRealmOrNull(*obj);
+    assert!(!obj_realm.is_null());
+
+    let subject = ServoJSPrincipalsRef::from_raw_unchecked(GetRealmPrincipals(subject_realm));
+    let obj = ServoJSPrincipalsRef::from_raw_unchecked(GetRealmPrincipals(obj_realm));
+
+    let subject_origin = subject.origin();
+    let obj_origin = obj.origin();
+
+    subject_origin.same_origin_domain(&obj_origin)
+}
+
+/// Report a cross-origin denial for a property, Always returns `false`, so it
+/// can be used as `return report_cross_origin_denial(...);`.
+///
+/// What this function does corresponds to the operations in
+/// <https://html.spec.whatwg.org/multipage/#the-location-interface> denoted as
+/// "Throw a `SecurityError` DOMException".
+pub unsafe fn report_cross_origin_denial(
+    cx: SafeJSContext,
+    _id: RawHandleId,
+    _access: &str,
+) -> bool {
+    let in_realm_proof = AlreadyInRealm::assert_for_cx(cx);
+    if !JS_IsExceptionPending(*cx) {
+        let global = GlobalScope::from_context(*cx, InRealm::Already(&in_realm_proof));
+        // TODO: include `id` and `access` in the exception message
+        throw_dom_exception(cx, &*global, Error::Security);
+    }
+    false
+}
+
+/// Property and method specs that correspond to the elements of
+/// [`CrossOriginProperties(O)`].
+///
+/// [`CrossOriginProperties(O)`]: https://html.spec.whatwg.org/multipage/#crossoriginproperties-(-o-)
+pub struct CrossOriginProperties {
+    pub attributes: &'static [JSPropertySpec],
+    pub methods: &'static [JSFunctionSpec],
+}
+
+impl CrossOriginProperties {
+    /// Enumerate the property keys defined by `self`.
+    fn keys(&self) -> impl Iterator<Item = *const std::os::raw::c_char> + '_ {
+        // Safety: All cross-origin property keys are strings, not symbols
+        self.attributes
+            .iter()
+            .map(|spec| unsafe { spec.name.string_ })
+            .chain(self.methods.iter().map(|spec| unsafe { spec.name.string_ }))
+            .filter(|ptr| !ptr.is_null())
+    }
+}
+
+/// Implementation of [`CrossOriginOwnPropertyKeys`].
+///
+/// [`CrossOriginOwnPropertyKeys`]: https://html.spec.whatwg.org/multipage/#crossoriginownpropertykeys-(-o-)
+pub unsafe fn cross_origin_own_property_keys(
+    cx: SafeJSContext,
+    _proxy: RawHandleObject,
+    cross_origin_properties: &'static CrossOriginProperties,
+    props: RawMutableHandleIdVector,
+) -> bool {
+    for key in cross_origin_properties.keys() {
+        let jsstring = JS_AtomizeAndPinString(*cx, key);
+        rooted!(in(*cx) let rooted = jsstring);
+        rooted!(in(*cx) let mut rooted_jsid: jsid);
+        RUST_INTERNED_STRING_TO_JSID(*cx, rooted.handle().get(), rooted_jsid.handle_mut());
+        AppendToIdVector(props, rooted_jsid.handle());
+    }
+    true
+}
+
+/// Implementation of [`CrossOriginGet`].
+///
+/// [`CrossOriginGet`]: https://html.spec.whatwg.org/multipage/#crossoriginget-(-o,-p,-receiver-)
+pub unsafe fn cross_origin_get(
+    cx: SafeJSContext,
+    proxy: RawHandleObject,
+    receiver: RawHandleValue,
+    id: RawHandleId,
+    vp: RawMutableHandleValue,
+) -> bool {
+    // > 1. Let `desc` be `? O.[[GetOwnProperty]](P)`.
+    rooted!(in(*cx) let mut descriptor = PropertyDescriptor::default());
+    if !InvokeGetOwnPropertyDescriptor(
+        GetProxyHandler(*proxy),
+        *cx,
+        proxy,
+        id,
+        descriptor.handle_mut().into(),
+    ) {
+        return false;
+    }
+    //    let descriptor = descriptor.get();
+
+    // > 2. Assert: `desc` is not undefined.
+    assert!(
+        !descriptor.obj.is_null(),
+        "Callees should throw in all cases when they are not finding \
+        a property decriptor"
+    );
+
+    // > 3. If `! IsDataDescriptor(desc)` is true, then return `desc.[[Value]]`.
+    if is_data_descriptor(&descriptor) {
+        vp.set(descriptor.value);
+        return true;
+    }
+
+    // > 4. Assert: `IsAccessorDescriptor(desc)` is `true`.
+    assert!(is_accessor_descriptor(&descriptor));
+
+    // > 5. Let `getter` be `desc.[[Get]]`.
+    // >
+    // > 6. If `getter` is `undefined`, then throw a `SecurityError`
+    // >    `DOMException`.
+    rooted!(in(*cx) let mut getter = ptr::null_mut::<JSObject>());
+    get_getter_object(&descriptor, getter.handle_mut().into());
+    if getter.get().is_null() {
+        return report_cross_origin_denial(cx, id, "get");
+    }
+
+    rooted!(in(*cx) let mut getter_jsval = UndefinedValue());
+    getter.get().to_jsval(*cx, getter_jsval.handle_mut());
+
+    // > 7. Return `? Call(getter, Receiver)`.
+    jsapi::Call(
+        *cx,
+        receiver,
+        getter_jsval.handle().into(),
+        &jsapi::HandleValueArray::new(),
+        vp,
+    )
+}
+
+unsafe fn get_getter_object(d: &PropertyDescriptor, out: RawMutableHandleObject) {
+    if (d.attrs & jsapi::JSPROP_GETTER as u32) != 0 {
+        out.set(std::mem::transmute(d.getter));
+    }
+}
+
+/// <https://tc39.es/ecma262/#sec-isaccessordescriptor>
+fn is_accessor_descriptor(d: &PropertyDescriptor) -> bool {
+    d.attrs & (jsapi::JSPROP_GETTER as u32 | jsapi::JSPROP_SETTER as u32) != 0
+}
+
+/// <https://tc39.es/ecma262/#sec-isdatadescriptor>
+fn is_data_descriptor(d: &PropertyDescriptor) -> bool {
+    let is_accessor = is_accessor_descriptor(d);
+    let is_generic = d.attrs &
+        (jsapi::JSPROP_GETTER as u32 |
+            jsapi::JSPROP_SETTER as u32 |
+            jsapi::JSPROP_IGNORE_READONLY |
+            jsapi::JSPROP_IGNORE_VALUE) ==
+        jsapi::JSPROP_IGNORE_READONLY | jsapi::JSPROP_IGNORE_VALUE;
+    !is_accessor && !is_generic
+}
+
+/// Evaluate `CrossOriginGetOwnPropertyHelper(proxy, id) != null`.
+pub unsafe fn cross_origin_has_own(
+    cx: SafeJSContext,
+    _proxy: RawHandleObject,
+    cross_origin_properties: &'static CrossOriginProperties,
+    id: RawHandleId,
+    bp: *mut bool,
+) -> bool {
+    // TODO: Once we have the slot for the holder, it'd be more efficient to
+    //       use `ensure_cross_origin_property_holder`.
+    *bp = if let Some(key) =
+        crate::dom::bindings::conversions::jsid_to_string(*cx, Handle::from_raw(id))
+    {
+        cross_origin_properties.keys().any(|defined_key| {
+            let defined_key = CStr::from_ptr(defined_key);
+            defined_key.to_bytes() == key.as_bytes()
+        })
+    } else {
+        false
+    };
+
+    true
+}
+
+/// Implementation of [`CrossOriginGetOwnPropertyHelper`].
+///
+/// `cx` and `obj` are expected to be different-Realm here. `obj` can be a
+/// `WindowProxy` or a `Location` or a `DissimilarOrigin*` proxy for one of
+/// those.
+///
+/// [`CrossOriginGetOwnPropertyHelper`]: https://html.spec.whatwg.org/multipage/#crossorigingetownpropertyhelper-(-o,-p-)
+pub unsafe fn cross_origin_get_own_property_helper(
+    cx: SafeJSContext,
+    proxy: RawHandleObject,
+    cross_origin_properties: &'static CrossOriginProperties,
+    id: RawHandleId,
+    mut desc: RawMutableHandle<PropertyDescriptor>,
+) -> bool {
+    rooted!(in(*cx) let mut holder = ptr::null_mut::<JSObject>());
+
+    ensure_cross_origin_property_holder(
+        cx,
+        proxy,
+        cross_origin_properties,
+        holder.handle_mut().into(),
+    );
+
+    if !JS_GetOwnPropertyDescriptorById(*cx, holder.handle().into(), id, desc) {
+        return false;
+    }
+
+    if !desc.obj.is_null() {
+        desc.obj = proxy.get();
+    }
+
+    true
+}
+
+/// Implementation of [`CrossOriginPropertyFallback`].
+///
+
+/// [`CrossOriginPropertyFallback`]: https://html.spec.whatwg.org/multipage/#crossoriginpropertyfallback-(-p-)
+pub unsafe fn cross_origin_property_fallback(
+    cx: SafeJSContext,
+    proxy: RawHandleObject,
+    id: RawHandleId,
+    mut desc: RawMutableHandle<PropertyDescriptor>,
+) -> bool {
+    assert!(desc.obj.is_null(), "why are we being called?");
+
+    // > 1. If P is `then`, `@@toStringTag`, `@@hasInstance`, or
+    // >    `@@isConcatSpreadable`, then return `PropertyDescriptor{ [[Value]]:
+    // >    undefined, [[Writable]]: false, [[Enumerable]]: false,
+    // >    [[Configurable]]: true }`.
+    if is_cross_origin_allowlisted_prop(cx, id) {
+        *desc = PropertyDescriptor {
+            getter: None,
+            setter: None,
+            value: UndefinedValue(),
+            attrs: jsapi::JSPROP_READONLY as u32,
+            obj: proxy.get(),
+        };
+        return true;
+    }
+
+    // > 2. Throw a `SecurityError` `DOMException`.
+    report_cross_origin_denial(cx, id, "access")
+}
+
+unsafe fn is_cross_origin_allowlisted_prop(cx: SafeJSContext, id: RawHandleId) -> bool {
+    const ALLOWLISTED_SYMBOL_CODES: &[SymbolCode] = &[
+        SymbolCode::toStringTag,
+        SymbolCode::hasInstance,
+        SymbolCode::isConcatSpreadable,
+    ];
+
+    crate::dom::bindings::conversions::jsid_to_string(*cx, Handle::from_raw(id))
+        .filter(|st| st == "then")
+        .is_some() ||
+        {
+            rooted!(in(*cx) let mut allowed_id: jsid);
+            ALLOWLISTED_SYMBOL_CODES.iter().any(|&allowed_code| {
+                RUST_SYMBOL_TO_JSID(
+                    GetWellKnownSymbol(*cx, allowed_code),
+                    allowed_id.handle_mut().into(),
+                );
+                // `jsid`s containing `JS::Symbol *` can be compared by
+                // referential equality
+                allowed_id.get().asBits == id.asBits
+            })
+        }
+}
+
+/// Get the holder for cross-origin properties for the current global of the
+/// `JSContext`, creating one and storing it in a slot of the proxy object if it
+/// doesn't exist yet.
+///
+/// This essentially creates a cache of [`CrossOriginGetOwnPropertyHelper`]'s
+/// results for all property keys.
+///
+/// [`CrossOriginGetOwnPropertyHelper`]: https://html.spec.whatwg.org/multipage/#crossorigingetownpropertyhelper-(-o,-p-)
+unsafe fn ensure_cross_origin_property_holder(
+    cx: SafeJSContext,
+    _proxy: RawHandleObject,
+    cross_origin_properties: &'static CrossOriginProperties,
+    out_holder: RawMutableHandleObject,
+) -> bool {
+    // TODO: We don't have the slot to store the holder yet. For now,
+    //       the holder is constructed every time this function is called,
+    //       which is not only inefficient but also deviates from the
+    //       specification in a subtle yet observable way.
+
+    // Create a holder for the current Realm
+    out_holder.set(jsapi::JS_NewObjectWithGivenProto(
+        *cx,
+        ptr::null_mut(),
+        RawHandleObject::null(),
+    ));
+
+    if out_holder.get().is_null() ||
+        !jsapi::JS_DefineProperties(
+            *cx,
+            out_holder.handle(),
+            cross_origin_properties.attributes.as_ptr(),
+        ) ||
+        !jsapi::JS_DefineFunctions(
+            *cx,
+            out_holder.handle(),
+            cross_origin_properties.methods.as_ptr(),
+        )
+    {
+        return false;
+    }
+
+    // TODO: Store the holder in the slot that we don't have yet.
+
+    true
+}

components/script/dom/windowproxy.rs

@@ -1084,6 +1084,9 @@ pub fn new_window_proxy_handler() -> WindowProxyHandler {
 // These traps often throw security errors, and only pass on calls to methods
 // defined in the DissimilarOriginWindow IDL.
 
+// TODO: reuse the infrastructure in `proxyhandler.rs`. For starters, the calls
+//       to this function should be replaced with those to
+//       `report_cross_origin_denial`.
 #[allow(unsafe_code)]
 unsafe fn throw_security_error(cx: *mut JSContext, realm: InRealm) -> bool {
     if !JS_IsExceptionPending(cx) {