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 +486,72 @@
     {
       event.preventDefault();
     }
   },
   true);
 }
 
 exports["prevent-inline-scripts"] = preventInlineScripts;
+
+/**
+ * Generates a random alphanumeric ID consisting of 6 base-36 digits
+ * 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.configurable)
+    return false;
+
+  Object.defineProperty(object, property, descriptor);
+  return true;
+}
+
+/**
+ * Patches a property on the window object to abort execution when the
+ * property is read.
+ * No error will be printed in the console.

[Manish Jethani, 2018/10/31 19:54:49]

Nit: Again, is the "No error will be printed" part really relevant to a filter
list author or anyone working on this code? If you think so, that's fine, but in
that case we could keep the tense consistent (i.e. "No error is printed to the
console").

[Manish Jethani, 2018/10/31 19:54:49]

Nit: Even though it doesn't matter to the generated documentation (since
whitespace is not relevant for HTML), I leave a blank line between paragraphs
here. We could do the same in this case (i.e. a blank line before and after "No
error ...")

[hub, 2018/10/31 20:44:16]

Done.

[hub, 2018/10/31 20:44:17]

Done.

+ * The idea originates from

[Manish Jethani, 2018/10/31 19:54:49]

Nit: Period at the end of the sentence.

[hub, 2018/10/31 20:44:16]

Done.

+ * {@link https://github.com/uBlockOrigin/uAssets/blob/80b195436f8f8d78ba713237bfc268ecfc9d9d2b/filters/resources.txt#L1703 uBlock Origin}
+ *
+ * @param {string} property The name of the property.
+ */
+function abortOnPropertyRead(property)
+{
+  if (!property)
+    return;
+
+  let rid = randomId();
+
+  function abort()
+  {
+    throw new ReferenceError(rid);
+  }
+
+  if (wrapPropertyAccess(window, property, {get: abort, set() {}}))
+  {
+    let {onerror} = window;

[Manish Jethani, 2018/10/31 19:38:16]

Can we move this line outside the if block? In theory somebody could want to
wrap "window.onerror" and then this would bomb.

[hub, 2018/10/31 20:44:16]

It can be moved.

But it would still bomb if it was wrapped, on the line below.

[Manish Jethani, 2018/10/31 20:52:22]

Alright, fair enough.

+    window.onerror = (message, ...rest) =>
+    {
+      if (typeof message == "string" && message.includes(rid))
+        return true;
+      if (typeof onerror == "function")
+        return (() => {}).call.call(onerror, this, message, ...rest);
+    };
+  }
+}
+
+exports["abort-on-property-read"] = makeInjector(abortOnPropertyRead,
+                                                 wrapPropertyAccess,
+                                                 randomId);