lib: harden webidl dictionary member reads

Member descriptors are plain object literals that spell out only the
members they need, so createDictionaryConverter() reading the optional
validator, defaultValue and required members off them resolves through
%Object.prototype%.

Copy each descriptor once at construction time with every key present.
They keep an ordinary prototype because a null-prototype object literal
lands in V8 dictionary mode, and dictionaries with no defaults and no
required members now skip steps 4.1.5 and 4.1.6.

Signed-off-by: Filip Skokan <panva.ip@gmail.com>
PR-URL: #65115
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Aviv Keller <me@aviv.sh>

Author: panva

lib/internal/webidl.js

@@ -20,6 +20,7 @@ const {
   NumberIsNaN,
   NumberMAX_SAFE_INTEGER,
   NumberMIN_SAFE_INTEGER,
+  ObjectPrototypeHasOwnProperty,
   ObjectPrototypeIsPrototypeOf,
   SafeArrayIterator,
   SafeSet,
@@ -699,16 +700,47 @@ function createDictionaryConverter(
   const dictionaries = ArrayIsArray(members[0]) ? members : [members];
   const sortedDictionaries = [];
 
+  function ownMember(member, key) {
+    return ObjectPrototypeHasOwnProperty(member, key) ? member[key] : undefined;
+  }
+
+  // Dictionaries with no defaults and no required members skip steps
+  // 4.1.5/4.1.6 entirely, keeping the absent-member path free.
+  let anyMissingMemberHandling = false;
+
   // Web IDL dictionary conversion steps 3-4 process inherited dictionaries
   // from least-derived to most-derived and sort only within each dictionary.
   // Callers with inheritance pass one member array per dictionary level.
   for (let i = 0; i < dictionaries.length; i++) {
-    ArrayPrototypePush(
-      sortedDictionaries,
-      ArrayPrototypeToSorted(dictionaries[i], compareMembers),
+    const sortedMembers = ArrayPrototypeToSorted(
+      dictionaries[i],
+      compareMembers,
     );
+    // Definition sites spell out only the members they need, so reading the
+    // optional ones below would resolve through %Object.prototype%.
+    // Re-materialize each descriptor once with every key present, copied from
+    // own properties only. The ordinary prototype is deliberate: nothing
+    // consults it now, and detaching it measurably slows these reads down.
+    for (let j = 0; j < sortedMembers.length; j++) {
+      const member = sortedMembers[j];
+      const defaultValue = ownMember(member, 'defaultValue');
+      const required = ownMember(member, 'required');
+      if (typeof defaultValue === 'function' || required) {
+        anyMissingMemberHandling = true;
+      }
+      sortedMembers[j] = {
+        key: ownMember(member, 'key'),
+        converter: ownMember(member, 'converter'),
+        defaultValue,
+        required,
+        validator: ownMember(member, 'validator'),
+      };
+    }
+    ArrayPrototypePush(sortedDictionaries, sortedMembers);
   }
 
+  const hasMissingMemberHandling = anyMissingMemberHandling;
+
   return function(jsDict, options = kEmptyObject) {
     // Step 1: reject non-object, non-null, non-undefined values.
     if (jsDict != null && type(jsDict) !== 'Object') {
@@ -747,14 +779,16 @@ function createDictionaryConverter(
           member.validator?.(idlMemberValue, jsDict);
           // Step 4.1.4.2: set idlDict[key] to the IDL value.
           idlDict[key] = idlMemberValue;
-        } else if (typeof member.defaultValue === 'function') {
-          // Step 4.1.5: store the member default value.
-          idlDict[key] = member.defaultValue();
-        } else if (member.required) {
-          // Step 4.1.6: required missing members throw.
-          throw makeException(
-            missingDictionaryMemberMessage(dictionaryName, key),
-            makeOptions(options, options.context, 'ERR_MISSING_OPTION'));
+        } else if (hasMissingMemberHandling) {
+          if (typeof member.defaultValue === 'function') {
+            // Step 4.1.5: store the member default value.
+            idlDict[key] = member.defaultValue();
+          } else if (member.required) {
+            // Step 4.1.6: required missing members throw.
+            throw makeException(
+              missingDictionaryMemberMessage(dictionaryName, key),
+              makeOptions(options, options.context, 'ERR_MISSING_OPTION'));
+          }
         }
       }
     }