Bug 1719070 - UA: Add support for a Nimbus experiment to override Firefox's User-Agent string. r=andreio,necko-reviewers,dragana

The ANDROID #ifdefs are necessary because Nimbus does not yet support Android in Gecko. The planned UA experiment won't include Android, so that's not a problem.

Differential Revision: https://phabricator.services.mozilla.com/D121112

browser/components/resistfingerprinting/test/browser/browser_navigator.js

@@ -398,4 +398,63 @@ add_task(async function runOverrideTest() {
   await testWorkerNavigator();
 
   await testUserAgentHeader();
+
+  // Pop general.appname.override etc
+  await SpecialPowers.popPrefEnv();
+
+  // Pop privacy.resistFingerprinting
+  await SpecialPowers.popPrefEnv();
 });
+
+// Only test the Firefox and Gecko experiment prefs on desktop.
+if (AppConstants.platform != "android") {
+  add_task(async function setupFirefoxVersionExperiment() {
+    // Mock Nimbus experiment settings
+    const { ExperimentFakes } = ChromeUtils.import(
+      "resource://testing-common/NimbusTestUtils.jsm"
+    );
+    let doExperimentCleanup = await ExperimentFakes.enrollWithFeatureConfig({
+      featureId: "firefox100",
+      value: { firefoxVersion: 100 },
+    });
+
+    let experimentOscpu;
+    switch (AppConstants.platform) {
+      case "win":
+        experimentOscpu =
+          cpuArch == "x86_64"
+            ? "Windows NT 10.0; Win64; x64"
+            : "Windows NT 10.0";
+        break;
+      case "macosx":
+        experimentOscpu = "Macintosh; Intel Mac OS X 10.15";
+        break;
+      default:
+        experimentOscpu = "X11; Linux x86_64";
+        break;
+    }
+
+    let experimentUserAgent = `Mozilla/5.0 (${experimentOscpu}; rv:100.0) Gecko/20100101 Firefox/100.0`;
+
+    expectedResults = {
+      testDesc: "FirefoxVersionExperimentTest",
+      appVersion: DEFAULT_APPVERSION[AppConstants.platform],
+      hardwareConcurrency: navigator.hardwareConcurrency,
+      oscpu: DEFAULT_OSCPU[AppConstants.platform],
+      platform: DEFAULT_PLATFORM[AppConstants.platform],
+      userAgentNavigator: experimentUserAgent,
+      userAgentHeader: experimentUserAgent,
+    };
+
+    await testNavigator();
+
+    // Skip worker tests due to intermittent bug 1713764. This is unlikely to be
+    // a scenario that affects users enrolled in our "Firefox 100" experiment.
+    // await testWorkerNavigator();
+
+    await testUserAgentHeader();
+
+    // Clear Nimbus experiment prefs and session data
+    await doExperimentCleanup();
+  });
+}

netwerk/protocol/http/nsHttpHandler.cpp

@@ -98,6 +98,10 @@
 #  include "nsCocoaFeatures.h"
 #endif
 
+#ifndef ANDROID
+#  include "mozilla/browser/NimbusFeatures.h"
+#endif  // ANDROID
+
 //-----------------------------------------------------------------------------
 #include "mozilla/net/HttpChannelChild.h"
 
@@ -124,6 +128,11 @@
 #define NS_HTTP_PROTOCOL_FLAGS \
   (URI_STD | ALLOWS_PROXY | ALLOWS_PROXY_HTTP | URI_LOADABLE_BY_ANYONE)
 
+#ifndef ANDROID
+#  define UA_EXPERIMENT_NAME "firefox100"_ns
+#  define UA_EXPERIMENT_VAR "firefoxVersion"_ns
+#endif  // ANDROID
+
 //-----------------------------------------------------------------------------
 
 using mozilla::dom::Promise;
@@ -132,6 +141,42 @@ namespace mozilla::net {
 
 LazyLogModule gHttpLog("nsHttp");
 
+#ifndef ANDROID
+static void ExperimentUserAgentUpdated(const char* /* aNimbusPref */,
+                                       void* aUserData) {
+  MOZ_ASSERT(aUserData != nullptr);
+  nsACString* aExperimentUserAgent = static_cast<nsACString*>(aUserData);
+
+  // Is this user enrolled in the Firefox 100 experiment?
+  int firefoxVersion =
+      NimbusFeatures::GetInt(UA_EXPERIMENT_NAME, UA_EXPERIMENT_VAR, 0);
+  if (firefoxVersion <= 0) {
+    aExperimentUserAgent->SetIsVoid(true);
+    return;
+  }
+
+  const char uaFormat[] =
+#  ifdef XP_WIN
+#    ifdef HAVE_64BIT_BUILD
+      "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:%d.0) Gecko/20100101 "
+      "Firefox/%d.0"
+#    else
+      "Mozilla/5.0 (Windows NT 10.0; rv:%d.0) Gecko/20100101 Firefox/%d.0"
+#    endif
+#  elif defined(XP_MACOSX)
+      "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:%d.0) Gecko/20100101 "
+      "Firefox/%d.0"
+#  else
+      // Linux, Android, FreeBSD, etc
+      "Mozilla/5.0 (X11; Linux x86_64; rv:%d.0) Gecko/20100101 Firefox/%d.0"
+#  endif
+      ;
+
+  aExperimentUserAgent->Truncate();
+  aExperimentUserAgent->AppendPrintf(uaFormat, firefoxVersion, firefoxVersion);
+}
+#endif  // ANDROID
+
 #ifdef ANDROID
 static nsCString GetDeviceModelId() {
   // Assumed to be running on the main thread
@@ -233,6 +278,10 @@ nsHttpHandler::nsHttpHandler()
 
   mUserAgentOverride.SetIsVoid(true);
 
+#ifndef ANDROID
+  mExperimentUserAgent.SetIsVoid(true);
+#endif  // ANDROID
+
   MOZ_ASSERT(!gHttpHandler, "HTTP handler already created!");
 
   nsCOMPtr<nsIXULRuntime> runtime = do_GetService("@mozilla.org/xre/runtime;1");
@@ -338,6 +387,13 @@ nsresult nsHttpHandler::Init() {
   Preferences::RegisterPrefixCallbacks(nsHttpHandler::PrefsChanged,
                                        gCallbackPrefs, this);
   PrefsChanged(nullptr);
+
+#ifndef ANDROID
+  // monitor Firefox Version Experiment enrollment
+  NimbusFeatures::OnUpdate(UA_EXPERIMENT_NAME, UA_EXPERIMENT_VAR,
+                           ExperimentUserAgentUpdated, &mExperimentUserAgent);
+#endif  // ANDROID
+
   Telemetry::ScalarSet(Telemetry::ScalarID::NETWORKING_HTTP3_ENABLED,
                        mHttp3Enabled);
 
@@ -699,6 +755,14 @@ const nsCString& nsHttpHandler::UserAgent() {
     return mUserAgentOverride;
   }
 
+#ifndef ANDROID
+  if (!mExperimentUserAgent.IsVoid()) {
+    LOG(("using Firefox 100 Experiment User-Agent : %s\n",
+         mExperimentUserAgent.get()));
+    return mExperimentUserAgent;
+  }
+#endif  // ANDROID
+
   if (mUserAgentIsDirty) {
     BuildUserAgent();
     mUserAgentIsDirty = false;

netwerk/protocol/http/nsHttpHandler.h

@@ -647,6 +647,11 @@ class nsHttpHandler final : public nsIHttpProtocolHandler,
   nsCString mUserAgent;
   nsCString mSpoofedUserAgent;
   nsCString mUserAgentOverride;
+
+#ifndef ANDROID
+  nsCString mExperimentUserAgent;
+#endif  // ANDROID
+
   bool mUserAgentIsDirty{true};  // true if mUserAgent should be rebuilt
   bool mAcceptLanguagesIsDirty{true};
 

toolkit/components/nimbus/FeatureManifest.js

@@ -84,6 +84,16 @@ const FeatureManifest = {
       },
     },
   },
+  firefox100: {
+    description: "Firefox User-Agent version",
+    isEarlyStartup: true,
+    variables: {
+      firefoxVersion: {
+        type: "int",
+        description: "Firefox version to spoof (or `0` to use default version)",
+      },
+    },
+  },
   newtab: {
     description: "The about:newtab page",
     isEarlyStartup: true,