Handle different variants of AndroidRuntime::start()

In Android 5.1.1_r19, Google introduced an additonal parameter for this
method. We need to detect which one the ROM uses and call it.

This assumes that the mangled name is consistent across all ROMs and
that the object instance is implicitely handled as first parameter
to the method. On Android 5.0, we assume that this change is not
merged. If required, the dynamic way could simply be enabled for this
version as well.

Fixes rovo89#71.

app_main2.cpp

@@ -23,6 +23,7 @@
 #include <sys/prctl.h>
 
 #include "xposed.h"
+#include <dlfcn.h>
 
 static bool isXposedLoaded = false;
 
@@ -193,6 +194,36 @@ static const char ABI_LIST_PROPERTY[] = "ro.product.cpu.abilist32";
 static const char ZYGOTE_NICE_NAME[] = "zygote";
 #endif
 
+static void runtimeStart(AppRuntime& runtime, const char *classname, const Vector<String8>& options, bool zygote)
+{
+#if PLATFORM_SDK_VERSION == 21
+  runtime.start(classname, options);
+#elif PLATFORM_SDK_VERSION >= 23
+  runtime.start(classname, options, zygote);
+#else
+  // try newer variant (5.1.1_r19 and later) first
+  void (*ptr1)(AppRuntime&, const char*, const Vector<String8>&, bool);
+  *(void **) (&ptr1) = dlsym(RTLD_DEFAULT, "_ZN7android14AndroidRuntime5startEPKcRKNS_6VectorINS_7String8EEEb");
+
+  if (ptr1 != NULL) {
+    ptr1(runtime, classname, options, zygote);
+    return;
+  }
+
+  // fall back to older variant
+  void (*ptr2)(AppRuntime&, const char*, const Vector<String8>&);
+  *(void **) (&ptr2) = dlsym(RTLD_DEFAULT, "_ZN7android14AndroidRuntime5startEPKcRKNS_6VectorINS_7String8EEE");
+
+  if (ptr2 != NULL) {
+    ptr2(runtime, classname, options);
+    return;
+  }
+
+  // should not happen
+  LOG_ALWAYS_FATAL("app_process: could not locate AndroidRuntime::start() method.");
+#endif
+}
+
 int main(int argc, char* const argv[])
 {
     if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
@@ -318,9 +349,9 @@ int main(int argc, char* const argv[])
 
     isXposedLoaded = xposed::initialize(zygote, startSystemServer, className, argc, argv);
     if (zygote) {
-        runtime.start(isXposedLoaded ? XPOSED_CLASS_DOTS_ZYGOTE : "com.android.internal.os.ZygoteInit", args);
+        runtimeStart(runtime, isXposedLoaded ? XPOSED_CLASS_DOTS_ZYGOTE : "com.android.internal.os.ZygoteInit", args, zygote);
     } else if (className) {
-        runtime.start(isXposedLoaded ? XPOSED_CLASS_DOTS_TOOLS : "com.android.internal.os.RuntimeInit", args);
+        runtimeStart(runtime, isXposedLoaded ? XPOSED_CLASS_DOTS_TOOLS : "com.android.internal.os.RuntimeInit", args, zygote);
     } else {
         fprintf(stderr, "Error: no class name or --zygote supplied.\n");
         app_usage();