Fix clobbering of overridden canvas methods

doc/tests.md

@@ -50,9 +50,11 @@ the code below. This should take several minutes.
 $ BROWSER=chrome pytest -v
 ```
 
-macOS users may need to provide the full path to the browser application folder. For example, to run tests on macOS in Firefox:
+macOS users may need to provide the full path to the browser application folder. For example, to run tests on macOS:
 ```bash
 $ BROWSER=/Applications/Firefox.app/Contents/MacOS/firefox-bin pytest -v
+# or
+$ BROWSER=/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome pytest -v
 ```
 
 For more information, see our Travis CI [setup](/scripts/setup_travis.sh) and

src/js/contentscripts/fingerprinting.js

@@ -204,14 +204,19 @@ function getFpPageScript() {
       );
 
       item.obj[item.propName] = (function (orig) {
+        // set to true after the first write, if the method is not
+        // restorable. Happens if another library also overwrites
+        // this method.
+        var skip_monitoring = false;
 
         function wrapped() {
           var args = arguments;
 
           if (is_canvas_write) {
             // to avoid false positives,
-            // bail if the text being written is too short
-            if (!args[0] || args[0].length < 5) {
+            // bail if the text being written is too short,
+            // of if we've already sent a monitoring payload
+            if (skip_monitoring || !args[0] || args[0].length < 5) {
               return orig.apply(this, args);
             }
           }
@@ -237,7 +242,13 @@ function getFpPageScript() {
             // optimization: one canvas write is enough,
             // restore original write method
             // to this CanvasRenderingContext2D object instance
-            this[item.propName] = orig;
+            // Careful! Only restorable if we haven't already been replaced
+            // by another lib, such as the hidpi polyfill
+            if (this[item.propName] === wrapped) {
+              this[item.propName] = orig;
+            } else {
+              skip_monitoring = true;
+            }
           }
 
           return orig.apply(this, args);

tests/selenium/fingerprinting_test.py

@@ -36,6 +36,13 @@ def detected_tracking(self, domain, page_url):
     map[tracker_origin].indexOf(site_origin) != -1
 );""".format(domain, page_url))
 
+    def get_fillText_source(self):
+        return self.js("""
+            const canvas = document.getElementById("writetome");
+            const ctx = canvas.getContext("2d");
+            return ctx.fillText.toString();
+        """)
+
     # TODO can fail because our content script runs too late: https://crbug.com/478183
     @pbtest.repeat_if_failed(3)
     def test_canvas_fingerprinting_detection(self):
@@ -69,6 +76,24 @@ def test_canvas_fingerprinting_detection(self):
             "Canvas fingerprinting resource was detected as a fingerprinter."
         )
 
+    # Privacy Badger overrides a few functions on canvas contexts to check for fingerprinting.
+    # In previous versions, it would restore the native function after a single call. Unfortunately,
+    # this would wipe out polyfills that had also overridden the same functions, such as the very
+    # popular "hidpi-canvas-polyfill".
+    def test_canvas_polyfill_clobbering(self):
+        FIXTURE_URL = (
+            "https://efforg.github.io/privacybadger-test-fixtures/html/"
+            "fingerprinting_canvas_hidpi.html"
+        )
+
+        # visit the page
+        self.load_url(FIXTURE_URL)
+
+        # check that we did not restore the native function (should be hipdi polyfill)
+        self.assertNotIn("[native code]", self.get_fillText_source(),
+            "Canvas context fillText is not native version (polyfill has been retained)."
+        )
+
 
 if __name__ == "__main__":
     unittest.main()

tests/selenium/pbtest.py

@@ -44,7 +44,7 @@ def unix_which(command, silent=False):
 
 def get_browser_type(string):
     for t in BROWSER_TYPES:
-        if t in string:
+        if t in string.lower():
             return t
     raise ValueError("couldn't get browser type from %s" % string)