Issue 6969 - Implement abort-on-property-read snippet

lib/content/snippets.js

Index: lib/content/snippets.js
===================================================================
--- a/lib/content/snippets.js
+++ b/lib/content/snippets.js
@@ -486,8 +486,73 @@
     {
       event.preventDefault();
     }
   },
   true);
 }
 
 exports["prevent-inline-scripts"] = preventInlineScripts;
+
+/**
+ * Generates a random alphanumeric ID consisting of 6 chars base-36 digits

[Manish Jethani, 2018/10/25 16:12:16]

s/chars//

[hub, 2018/10/25 20:26:10]

Done.

+ * from the range 100000..zzzzzz (both inclusive).
+ *
+ * @returns {string} The random ID.
+ */
+function randomId()
+{
+  // 2176782336 is 36^6 which mean 6 chars [a-z0-9]
+  // 60466176 is 36^5
+  // 2176782336 - 60466176 = 2116316160. This ensure to always have 6
+  // chars even if Math.random() returns its minimum value 0.0
+  return Math.floor(Math.random() * 2116316160 + 60466176).toString(36);
+}
+
+function wrapPropertyAccess(object, property, descriptor)
+{
+  let currentDescriptor = Object.getOwnPropertyDescriptor(object, property);
+  if (!currentDescriptor)
+    currentDescriptor = {};
+  if (typeof descriptor.get != "undefined")
+    currentDescriptor.get = descriptor.get;
+  if (typeof descriptor.set != "undefined")
+    currentDescriptor.set = descriptor.set;
+  Object.defineProperty(object, property, currentDescriptor);
+}
+
+/**
+ * Will patch a property on the window object to abort when read.
+ * It will intercept the onerror callback and block it if tagged.
+ * The idea originates from
+ * https://github.com/uBlockOrigin/uAssets/blob/80b195436f8f8d78ba713237bfc268ecfc9d9d2b/filters/resources.txt#L1703
+ *
+ * @todo handle properties of properties.
+ *
+ * @param {string} property the name of the property.
+ */
+function abortOnPropertyRead(property)
+{
+  if (!property)
+    return;
+
+  let rid = randomId();
+
+  function abort()
+  {
+    throw new ReferenceError(rid);
+  }
+
+  let {onerror} = window;
+  window.onerror = (message, ...rest) =>
+  {
+    if (typeof message == "string" && message.includes(rid))
+      return true;
+    if (typeof onerror == "function")
+      return onerror(message, ...rest);
+  };
+
+  wrapPropertyAccess(window, property, {get: abort, set() {}});
+}
+
+exports["abort-on-property-read"] = makeInjector(abortOnPropertyRead,
+                                                 wrapPropertyAccess,
+                                                 randomId);