DroidGuard: Make Build.getSerial() less flaky

Author: mar-v-in

play-services-droidguard/core/src/main/java/com/google/android/gms/droidguard/DroidGuardChimeraService.java

@@ -22,6 +22,7 @@
 import org.microg.gms.droidguard.PingData;
 import org.microg.gms.droidguard.Request;
 import org.microg.gms.droidguard.core.HardwareAttestationBlockingProvider;
+import org.microg.gms.droidguard.core.SerialUnflaky;
 
 import java.util.Collections;
 import java.util.concurrent.Executor;
@@ -115,6 +116,7 @@ public final GuardCallback b(String packageName) {
     public final IBinder onBind(Intent intent) {
         if (intent != null && intent.getAction() != null && intent.getAction().equals("com.google.android.gms.droidguard.service.START")) {
             HardwareAttestationBlockingProvider.ensureEnabled(DroidGuardPreferences.isHardwareAttestationBlocked(this));
+            SerialUnflaky.INSTANCE.fetch();
             return new DroidGuardServiceBroker(this);
         }
         return null;
@@ -129,6 +131,7 @@ public void onCreate() {
         this.c = new Object();
         this.d = new ThreadPoolExecutor(1, 1, 0, TimeUnit.NANOSECONDS, new LinkedBlockingQueue<>(1), new ThreadPoolExecutor.DiscardPolicy());
         HardwareAttestationBlockingProvider.ensureEnabled(DroidGuardPreferences.isHardwareAttestationBlocked(this));
+        SerialUnflaky.INSTANCE.fetch();
         super.onCreate();
     }
 

play-services-droidguard/core/src/main/kotlin/org/microg/gms/droidguard/core/SerialUnflaky.kt

@@ -0,0 +1,27 @@
+package org.microg.gms.droidguard.core
+
+import android.annotation.SuppressLint
+import android.os.Build.VERSION.SDK_INT
+import android.util.Log
+
+/**
+ * DroidGuard may invoke Build.getSerial(), which will throw an exception, as, in contrast to original GMS, it doesn't
+ * have the permission to do so.
+ *
+ * We found that on some systems, the behavior of Build.getSerial() is flaky, as it would only throw an exception on
+ * the first attempt invoking it, but would return "unknown" afterward (which is intended for apps with
+ * target SDK <= 28). DroidGuard doesn't like those flaky results, so to make them consistent, we just invoke
+ * Build.getSerial() here once.
+ */
+object SerialUnflaky {
+    @SuppressLint("MissingPermission")
+    fun fetch() {
+        if (SDK_INT >= 26) {
+            val res1 = runCatching { android.os.Build.getSerial() }.fold({ it }, { it.javaClass.name })
+            val res2 = runCatching { android.os.Build.getSerial() }.fold({ it }, { it.javaClass.name })
+            if (res1 != res2) {
+                Log.w("SerialUnflaky", "Build.getSerial() was flaky. res1=$res1, res2=$res2")
+            }
+        }
+    }
+}