Bug 1511763 - Part 2: Make GetSpoofedUserAgent() infallible. r=tjr

Differential Revision: https://phabricator.services.mozilla.com/D13658

--HG--
extra : rebase_source : 467a960f591f5e2043ca8fc504730ad38358dcde
extra : intermediate-source : 08d3f83d23c553e44ba7be1cbc3d93cd2b0b1216
extra : source : c20507956e062eecf23f0856c8b5c372621891cf

dom/base/Navigator.cpp

@@ -1688,10 +1688,7 @@ nsresult Navigator::GetUserAgent(nsPIDOMWindowInner* aWindow,
   // specific OS version, etc.
   if (!aIsCallerChrome && nsContentUtils::ShouldResistFingerprinting()) {
     nsAutoCString spoofedUA;
-    nsresult rv = nsRFPService::GetSpoofedUserAgent(spoofedUA, false);
-    if (NS_WARN_IF(NS_FAILED(rv))) {
-      return rv;
-    }
+    nsRFPService::GetSpoofedUserAgent(spoofedUA, false);
     CopyASCIItoUTF16(spoofedUA, aUserAgent);
     return NS_OK;
   }

netwerk/protocol/http/nsHttpHandler.cpp

@@ -497,13 +497,8 @@ nsresult nsHttpHandler::Init() {
     mAppVersion.AssignLiteral(MOZ_APP_UA_VERSION);
   }
 
-  // Generating the spoofed User Agent for fingerprinting resistance.
-  rv = nsRFPService::GetSpoofedUserAgent(mSpoofedUserAgent, true);
-  if (NS_FAILED(rv)) {
-    // Empty mSpoofedUserAgent to make sure the unsuccessful spoofed UA string
-    // will not be used anywhere.
-    mSpoofedUserAgent.Truncate();
-  }
+  // Generate the spoofed User Agent for fingerprinting resistance.
+  nsRFPService::GetSpoofedUserAgent(mSpoofedUserAgent, true);
 
   mSessionStartTime = NowInSeconds();
   mHandlerActive = true;

toolkit/components/resistfingerprinting/nsRFPService.cpp

@@ -652,30 +652,24 @@ uint32_t nsRFPService::GetSpoofedPresentedFrames(double aTime, uint32_t aWidth,
                       ((100 - boundedDroppedRatio) / 100.0));
 }
 
-/* static */
-nsresult nsRFPService::GetSpoofedUserAgent(nsACString& userAgent,
-                                           bool isForHTTPHeader) {
-  // This function generates the spoofed value of User Agent.
-  // We spoof the values of the platform and Firefox version, which could be
-  // used as fingerprinting sources to identify individuals.
-  // Reference of the format of User Agent:
-  // https://developer.mozilla.org/en-US/docs/Web/API/NavigatorID/userAgent
-  // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent
+static uint32_t GetSpoofedVersion() {
+  // If we can't get the current Firefox version, use a hard-coded ESR version.
+  const uint32_t kKnownEsrVersion = 60;
 
   nsresult rv;
   nsCOMPtr<nsIXULAppInfo> appInfo =
       do_GetService("@mozilla.org/xre/app-info;1", &rv);
-  NS_ENSURE_SUCCESS(rv, rv);
+  NS_ENSURE_SUCCESS(rv, kKnownEsrVersion);
 
   nsAutoCString appVersion;
   rv = appInfo->GetVersion(appVersion);
-  NS_ENSURE_SUCCESS(rv, rv);
+  NS_ENSURE_SUCCESS(rv, kKnownEsrVersion);
 
   // The browser version will be spoofed as the last ESR version.
   // By doing so, the anonymity group will cover more versions instead of one
   // version.
   uint32_t firefoxVersion = appVersion.ToInteger(&rv);
-  NS_ENSURE_SUCCESS(rv, rv);
+  NS_ENSURE_SUCCESS(rv, kKnownEsrVersion);
 
 #ifdef DEBUG
   // If we are running in Firefox ESR, determine whether the formula of ESR
@@ -690,13 +684,24 @@ nsresult nsRFPService::GetSpoofedUserAgent(nsACString& userAgent,
   // Starting with Firefox 52, a new ESR version will be released every
   // eight Firefox versions: 52, 60, 68, ...
   // We infer the last and closest ESR version based on this rule.
-  uint32_t spoofedVersion = firefoxVersion - ((firefoxVersion - 4) % 8);
+  return firefoxVersion - ((firefoxVersion - 4) % 8);
+}
+
+/* static */
+void nsRFPService::GetSpoofedUserAgent(nsACString& userAgent,
+                                       bool isForHTTPHeader) {
+  // This function generates the spoofed value of User Agent.
+  // We spoof the values of the platform and Firefox version, which could be
+  // used as fingerprinting sources to identify individuals.
+  // Reference of the format of User Agent:
+  // https://developer.mozilla.org/en-US/docs/Web/API/NavigatorID/userAgent
+  // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent
+
+  uint32_t spoofedVersion = GetSpoofedVersion();
   const char* spoofedOS = isForHTTPHeader ? SPOOFED_HTTP_UA_OS : SPOOFED_UA_OS;
   userAgent.Assign(nsPrintfCString(
       "Mozilla/5.0 (%s; rv:%d.0) Gecko/%s Firefox/%d.0", spoofedOS,
       spoofedVersion, LEGACY_UA_GECKO_TRAIL, spoofedVersion));
-
-  return rv;
 }
 
 static const char* gCallbackPrefs[] = {

toolkit/components/resistfingerprinting/nsRFPService.h

@@ -185,8 +185,7 @@ class nsRFPService final : public nsIObserver {
                                             uint32_t aHeight);
 
   // This method generates the spoofed value of User Agent.
-  static nsresult GetSpoofedUserAgent(nsACString& userAgent,
-                                      bool isForHTTPHeader);
+  static void GetSpoofedUserAgent(nsACString& userAgent, bool isForHTTPHeader);
 
   /**
    * This method for getting spoofed modifier states for the given keyboard