Add native JNI to root service to example

example/.gitignore

@@ -1 +1,2 @@
 /build
+/.cxx

example/build.gradle.kts

@@ -23,6 +23,11 @@ android {
             )
         }
     }
+    externalNativeBuild {
+        cmake {
+            path = file("src/main/cpp/CMakeLists.txt")
+        }
+    }
 }
 
 dependencies {

example/src/main/aidl/com/topjohnwu/libsuexample/ITestService.aidl

@@ -6,4 +6,5 @@ package com.topjohnwu.libsuexample;
 interface ITestService {
     int getPid();
     int getUid();
+    String readPartitions();
 }

example/src/main/cpp/CMakeLists.txt

@@ -0,0 +1,14 @@
+# Sets the minimum version of CMake required to build your native library.
+# This ensures that a certain set of CMake features is available to
+# your build.
+
+cmake_minimum_required(VERSION 3.4.1)
+
+# Specifies a library name, specifies whether the library is STATIC or
+# SHARED, and provides relative paths to the source code. You can
+# define multiple libraries by adding multiple add_library() commands,
+# and CMake builds them for you. When you build your app, Gradle
+# automatically packages shared libraries with your APK.
+
+add_library(native-lib SHARED
+            test.cpp)

example/src/main/cpp/test.cpp

@@ -0,0 +1,26 @@
+//
+// Created by John Wu on 7/26/20.
+//
+#include <jni.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <string>
+
+extern "C" JNIEXPORT JNICALL
+jint Java_com_topjohnwu_libsuexample_MainActivity_00024ExampleService_nativeGetUid(
+		JNIEnv *env, jobject instance) {
+	return getuid();
+}
+
+extern "C" JNIEXPORT JNICALL
+jstring Java_com_topjohnwu_libsuexample_MainActivity_00024ExampleService_nativeReadFile(
+		JNIEnv *env, jobject instance, jstring name) {
+	const char *path = env->GetStringUTFChars(name, nullptr);
+	int fd = open(path, O_RDONLY);
+	env->ReleaseStringUTFChars(name, path);
+	char buf[4096];
+	buf[read(fd, buf, sizeof(buf) - 1)] = 0;
+	return env->NewStringUTF(buf);
+}

example/src/main/java/com/topjohnwu/libsuexample/MainActivity.java

@@ -71,6 +71,15 @@ public class MainActivity extends Activity {
     // Demonstrate RootService
     static class ExampleService extends RootService {
 
+        static {
+            // Only load in root process
+            if (Process.myUid() == 0)
+                System.loadLibrary("native-lib");
+        }
+
+        native int nativeGetUid();
+        native String nativeReadFile(String file);
+
         @Override
         public IBinder onBind(@NonNull Intent intent) {
             return new ITestService.Stub() {
@@ -81,7 +90,12 @@ public class MainActivity extends Activity {
 
                 @Override
                 public int getUid() {
-                    return Process.myUid();
+                    return nativeGetUid();
+                }
+
+                @Override
+                public String readPartitions() {
+                    return nativeReadFile("/proc/partitions");
                 }
             };
         }
@@ -109,6 +123,7 @@ public class MainActivity extends Activity {
         try {
             consoleList.add("Remote PID: " + testIPC.getPid());
             consoleList.add("Remote UID: " + testIPC.getUid());
+            consoleList.add("/proc/partitions:\n" + testIPC.readPartitions());
         } catch (RemoteException e) {
             Log.e(TAG, "Remote error", e);
         }