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Android6.0 动态权限机制深入讲解

时间：2021-12-14 10:42:36|栏目：Android代码|点击： 次

前言

Android6.0以后引入了动态权限机制，一些系统权限的分配需要在app运行中进行分配，而不只是在AndroidManifest中指定。

本篇将针对动态权限的底层分配过程进行分析(基于Android-6.0.1)。

权限分配

我们先看一下请求分配权限的代码

//frameworks/support/v4/java/android/support/v4/app/ActivityCompat.java
public static void requestPermissions(final @NonNull Activity activity,
  final @NonNull String[] permissions, final int requestCode) {
 if (Build.VERSION.SDK_INT >= 23) {//对于Android M 以及以上的权限的分配
  ActivityCompatApi23.requestPermissions(activity, permissions, requestCode);
 } else if (activity instanceof OnRequestPermissionsResultCallback) {//Android M以下的权限分配
  Handler handler = new Handler(Looper.getMainLooper());
  handler.post(new Runnable() {
   @Override
   public void run() {
    //请求分配的权限结果，如分配就是PERMISSION_GRANTED
    final int[] grantResults = new int[permissions.length];
    
    PackageManager packageManager = activity.getPackageManager();
    String packageName = activity.getPackageName();

    final int permissionCount = permissions.length;
    //通过包管理的checkPermission来检验是否分配权限
    for (int i = 0; i < permissionCount; i++) {
     grantResults[i] = packageManager.checkPermission(
       permissions[i], packageName);
    }

    ((OnRequestPermissionsResultCallback) activity).onRequestPermissionsResult(
      requestCode, permissions, grantResults);
   }
  });
 }
}

requestPermissions对于Android M的前后版本都分别做了处理，Android M以上通过ActivityCompatApi23.requestPermissions进行权限的请求，而Android M以下通过PackageManager来检查Permission的分配情况。

//frameworks/support/v4/api23/android/support/v4/app/ActivityCompat23.java
class ActivityCompatApi23 {
 ...
 public static void requestPermissions(Activity activity, String[] permissions,
   int requestCode) {
  if (activity instanceof RequestPermissionsRequestCodeValidator) {
   ((RequestPermissionsRequestCodeValidator) activity)
     .validateRequestPermissionsRequestCode(requestCode);
  }
  //通过Android M的Activity处理
  activity.requestPermissions(permissions, requestCode);
 }
 ...
}

//frameworks/base/core/java/android/app/Activity.java
public final void requestPermissions(@NonNull String[] permissions, int requestCode) {
 if (mHasCurrentPermissionsRequest) {
  Log.w(TAG, "Can reqeust only one set of permissions at a time");
  // Dispatch the callback with empty arrays which means a cancellation.
  onRequestPermissionsResult(requestCode, new String[0], new int[0]);
  return;
 }
 //通过请求的权限构造Intent，弹出请求的窗口
 Intent intent = getPackageManager().buildRequestPermissionsIntent(permissions);
 startActivityForResult(REQUEST_PERMISSIONS_WHO_PREFIX, intent, requestCode, null);
 mHasCurrentPermissionsRequest = true;
}

ActivityCompat23将请求权限的任务交给Activity来完成，在Activity中，通过请求的permission来构造一个Intent随后启动Activity来弹出请求的界面。Intent的构造是通过PackageManager的buildRequestPermissionsIntent方法构造的。

public Intent buildRequestPermissionsIntent(@NonNull String[] permissions) {
 if (ArrayUtils.isEmpty(permissions)) {
  throw new NullPointerException("permission cannot be null or empty");
 }
 Intent intent = new Intent(ACTION_REQUEST_PERMISSIONS);
 intent.putExtra(EXTRA_REQUEST_PERMISSIONS_NAMES, permissions);
 intent.setPackage(getPermissionControllerPackageName());
 return intent;
}

Intent的action是ACTION_REQUEST_PERMISSIONS，它是这么定义的

public static final String ACTION_REQUEST_PERMISSIONS =
   "android.content.pm.action.REQUEST_PERMISSIONS";

随后一个参数就是具体请求的permission数组和一个权限分派控制的相关的包名。所以activity的请求窗口是通过隐式启动的。

/packages/apps/PackageInstaller/AndroidManifest.xml
<activity android:name=".permission.ui.GrantPermissionsActivity"
  android:configChanges="orientation|keyboardHidden|screenSize"
  android:excludeFromRecents="true"
  android:theme="@style/GrantPermissions">
 <intent-filter>
  <action android:name="android.content.pm.action.REQUEST_PERMISSIONS" />
  <category android:name="android.intent.category.DEFAULT" />
 </intent-filter>
</activity>

从intent-fliter可以看到，这个GrantPermissionsActivity就是我们进行权限分配的弹出窗口。GrantPermissionsActivity它的布局文件定义在packages/apps/PackageInstaller/res/layout/grant_permissions.xml，从GrantPermissionsActivity的实现来看它就是一个长的像Dialog的activity，这里我们重点关注在该Activity中对权限的允许和拒绝的处理。

//packages/apps/PackageInstaller/src/com/android/packageinstaller/permission/ui/GrantPermissionsDefaultViewHandler.java
public void onClick(View view) {
 switch (view.getId()) {
  case R.id.permission_allow_button://允许
   if (mResultListener != null) {
    view.clearAccessibilityFocus();
    mResultListener.onPermissionGrantResult(mGroupName, true, false);
   }
   break;
  case R.id.permission_deny_button://拒绝
   mAllowButton.setEnabled(true);
   if (mResultListener != null) {
    view.clearAccessibilityFocus();
    mResultListener.onPermissionGrantResult(mGroupName, false,
      mDoNotAskCheckbox.isChecked());
   }
   break;
  case R.id.do_not_ask_checkbox://不再询问
   mAllowButton.setEnabled(!mDoNotAskCheckbox.isChecked());
   break;
 }
}

这里是通过GrantPermissionsDefaultViewHandler来控制GrantPermissionsActivity的ui视图，按钮的点击事件是通过GrantPermissionsViewHandler.ResultListener接口来处理的，GrantPermissionsActivity实现了该接口。

@Override
public void onPermissionGrantResult(String name, boolean granted, boolean doNotAskAgain) {
 if (isObscuredTouch()) {
  showOverlayDialog();
  finish();
  return;
 }
 GroupState groupState = mRequestGrantPermissionGroups.get(name);
 if (groupState.mGroup != null) {
  if (granted) {
   groupState.mGroup.grantRuntimePermissions(doNotAskAgain);//权限组内部的权限分配
   groupState.mState = GroupState.STATE_ALLOWED;//重置权限组的状态
  } else {
   groupState.mGroup.revokeRuntimePermissions(doNotAskAgain);
   groupState.mState = GroupState.STATE_DENIED;
  }
  updateGrantResults(groupState.mGroup);
 }
 //下一个组权限的授权
 if (!showNextPermissionGroupGrantRequest()) {
  setResultAndFinish();
 }
}

onPermissionGrantResult的三个参数分别是name代表了权限组的名字，granted表示是否进行权限分配，doNotAskAgain代表是否询问权限。内部的mRequestGrantPermissionGroups是一个LinkedHashMap<String, GroupState>，它的key是权限组名，值为GroupState，它代表了待授权的权限组Map。需要注意的是权限和权限组的概念是不同的，一个权限所属一个权限组，要给权限组可以对应多个权限。而我们传递给GrantPermissionsActivity的是权限数组(注意并不是权限组)，在GrantPermissionsActivity创建的时候，会将我们请求的权限分别匹配到其对应的权限组中，这会重新计算权限组的状态。这个方法对name对应的权限组进行授权或者拒绝，然后处理下一个权限组。

//packages/apps/PackageInstaller/src/com/android/packageinstaller/permission/ui/GrantPermissionsActivity.java
public class GrantPermissionsActivity extends OverlayTouchActivity
  implements GrantPermissionsViewHandler.ResultListener {
 private String[] mRequestedPermissions;//请求的权限数组
 private int[] mGrantResults;//权限分配的结果数组
 //请求的权限数组对应的权限组Map
 private LinkedHashMap<String, GroupState> mRequestGrantPermissionGroups = new LinkedHashMap<>();
 ...
 @Override
 public void onCreate(Bundle icicle) {
  ...
  //加载应用权限组
  mAppPermissions = new AppPermissions(this, callingPackageInfo, null, false,
   new Runnable() {
    @Override
    public void run() {
     setResultAndFinish();
    }
   });
  //遍历权限组
  for (AppPermissionGroup group : mAppPermissions.getPermissionGroups()) {
   boolean groupHasRequestedPermission = false;
   for (String requestedPermission : mRequestedPermissions) {
    //如果请求的权限在该组内则标记groupHasRequestedPermission为true
    if (group.hasPermission(requestedPermission)) {
     groupHasRequestedPermission = true;
     break;
    }
   }
   if (!groupHasRequestedPermission) {
    continue;
   }
   // We allow the user to choose only non-fixed permissions. A permission
   // is fixed either by device policy or the user denying with prejudice.
   if (!group.isUserFixed() && !group.isPolicyFixed()) {
    switch (permissionPolicy) {
     case DevicePolicyManager.PERMISSION_POLICY_AUTO_GRANT: {
      if (!group.areRuntimePermissionsGranted()) {
       group.grantRuntimePermissions(false);
      }
      group.setPolicyFixed();
     } break;

     case DevicePolicyManager.PERMISSION_POLICY_AUTO_DENY: {
      if (group.areRuntimePermissionsGranted()) {
       group.revokeRuntimePermissions(false);
      }
      group.setPolicyFixed();
     } break;

     default: {
      //权限组是否已经分配了Runtime Permission,如果没有，则添加到mRequestGrantPermissionGroups中
      if (!group.areRuntimePermissionsGranted()) {
       mRequestGrantPermissionGroups.put(group.getName(),
         new GroupState(group));
      } else {
       group.grantRuntimePermissions(false);
       updateGrantResults(group);
      }
     } break;
    }
   } else {
    // if the permission is fixed, ensure that we return the right request result
    updateGrantResults(group);
   }
  }
  ...
  if (!showNextPermissionGroupGrantRequest()) {
   setResultAndFinish();
  }
 }
}

在GrantPermissionsActivity的onCreate方法中，根据请求的权限计算所属权限组的状态，首先创建AppPermissions对象，这时会去加载应用的权限组。同时遍历用于请求的权限数组并找到其对应的权限组，同时判断该权限组是否已经分配了动态权限，如果未授权则添加到待授权的权限组Map中。到这里我们还未看到真正的授权过程，在前面onPermissionGrantResult方法中，授权是通过GroupState中的成员mGroup的grantRuntimePermissions方法进一步进行权限分配的。而GroupState的定义如下

private static final class GroupState {
 static final int STATE_UNKNOWN = 0;
 static final int STATE_ALLOWED = 1;
 static final int STATE_DENIED = 2;

 final AppPermissionGroup mGroup;
 int mState = STATE_UNKNOWN;

 GroupState(AppPermissionGroup group) {
  mGroup = group;
 }
}

GroupState有三个状态STATE_UNKNOWN，STATE_ALLOWED，STATE_DENIED，它内部的mGroup实际上是个AppPermissionGroup，这些AppPermissionGroup是在AppPermissions加载的。

//packages/apps/PackageInstaller/src/com/android/packageinstaller/permission/model/AppPermissionGroup.java
public boolean grantRuntimePermissions(boolean fixedByTheUser) {
 final boolean isSharedUser = mPackageInfo.sharedUserId != null;
 final int uid = mPackageInfo.applicationInfo.uid;

 // We toggle permissions only to apps that support runtime
 // permissions, otherwise we toggle the app op corresponding
 // to the permission if the permission is granted to the app.
 //遍历权限组对应的权限
 for (Permission permission : mPermissions.values()) {
  if (mAppSupportsRuntimePermissions) {//支持动态权限分配
   // Do not touch permissions fixed by the system.
   if (permission.isSystemFixed()) {//系统权限则返回
    return false;
   }

   // Ensure the permission app op enabled before the permission grant.
   //打开permssion可以被grant的选项
   if (permission.hasAppOp() && !permission.isAppOpAllowed()) {
    permission.setAppOpAllowed(true);
    mAppOps.setUidMode(permission.getAppOp(), uid, AppOpsManager.MODE_ALLOWED);
   }

   // Grant the permission if needed.
   //进行动态分配，通过PMS完成
   if (!permission.isGranted()) {
    permission.setGranted(true);
    mPackageManager.grantRuntimePermission(mPackageInfo.packageName,
      permission.getName(), mUserHandle);
   }

   // Update the permission flags.
   if (!fixedByTheUser) {
    // Now the apps can ask for the permission as the user
    // no longer has it fixed in a denied state.
    if (permission.isUserFixed() || permission.isUserSet()) {
     permission.setUserFixed(false);
     permission.setUserSet(true);
     mPackageManager.updatePermissionFlags(permission.getName(),
       mPackageInfo.packageName,
       PackageManager.FLAG_PERMISSION_USER_FIXED
         | PackageManager.FLAG_PERMISSION_USER_SET,
       0, mUserHandle);
    }
   }
  } else {//Adnroid M以下的版本权限分配
   .... 
  }
 }
 return true;
}

权限的分配最终是通过PMS的grantRuntimePermission方法来完成的。

//frameworks/base/services/core/java/com/android/server/pm/PackageManagerService.java
@Override
public void grantRuntimePermission(String packageName, String name, final int userId) {
 ...
 mContext.enforceCallingOrSelfPermission(
   android.Manifest.permission.GRANT_RUNTIME_PERMISSIONS,
   "grantRuntimePermission");
 
 enforceCrossUserPermission(Binder.getCallingUid(), userId, true, false,
   "grantRuntimePermission");

 final int uid;
 final SettingBase sb;
  
 synchronized (mPackages) {
  //取到Package对象
  final PackageParser.Package pkg = mPackages.get(packageName);
  if (pkg == null) {
   throw new IllegalArgumentException("Unknown package: " + packageName);
  }
  //取到全局设置中的权限信息 
  final BasePermission bp = mSettings.mPermissions.get(name);
  if (bp == null) {
   throw new IllegalArgumentException("Unknown permission: " + name);
  }

  enforceDeclaredAsUsedAndRuntimeOrDevelopmentPermission(pkg, bp);

  uid = UserHandle.getUid(userId, pkg.applicationInfo.uid);
  sb = (SettingBase) pkg.mExtras;//从pkg中取到应用的设置信息SettingBase
  if (sb == null) {
   throw new IllegalArgumentException("Unknown package: " + packageName);
  }
  //取到权限状态 
  final PermissionsState permissionsState = sb.getPermissionsState();
  
  final int flags = permissionsState.getPermissionFlags(name, userId);
  if ((flags & PackageManager.FLAG_PERMISSION_SYSTEM_FIXED) != 0) {
   throw new SecurityException("Cannot grant system fixed permission: "
     + name + " for package: " + packageName);
  }

  if (bp.isDevelopment()) {
   // Development permissions must be handled specially, since they are not
   // normal runtime permissions. For now they apply to all users.
   if (permissionsState.grantInstallPermission(bp) !=
     PermissionsState.PERMISSION_OPERATION_FAILURE) {
    scheduleWriteSettingsLocked();
   }
   return;
  }
  //通过PermissionsState进行动态权限的分配
  final int result = permissionsState.grantRuntimePermission(bp, userId);
  ....
 }
 .....
}

在PMS的grantRuntimePermission方法中首先根据包名取到应用安装时的Package对象，这个Package对象中包含了应用的一些设置信息，通过这个设置信息可以取到当前应用的PermissionState，它维护了当前应用的权限授予情况。同时根据参数name,也就是权限名获取全新的配置信息BasePermission对象，它时从mSettings中取到的，mSettings是PMS的全局设置，它在PMS启动的时候初始化，里面包含了平台支持的所有权限。最后权限的分配进一步通过PermissionState来完成

//frameworks/base/services/core/java/com/android/server/pm/PermissionsState.java
//动态权限的分配
public int grantRuntimePermission(BasePermission permission, int userId) {
 enforceValidUserId(userId);
 if (userId == UserHandle.USER_ALL) {
  return PERMISSION_OPERATION_FAILURE;
 }
 return grantPermission(permission, userId);
}

private int grantPermission(BasePermission permission, int userId) {
 if (hasPermission(permission.name, userId)) {
  return PERMISSION_OPERATION_FAILURE;
 }
 //计算用户组id
 final boolean hasGids = !ArrayUtils.isEmpty(permission.computeGids(userId));
 final int[] oldGids = hasGids ? computeGids(userId) : NO_GIDS;
 //将权限包装成PermissionData添加到应用的权限列表中
 PermissionData permissionData = ensurePermissionData(permission);
 //授予权限，修改PermissionState的mGranted属性
 if (!permissionData.grant(userId)) {
  return PERMISSION_OPERATION_FAILURE;
 }

 if (hasGids) {
  final int[] newGids = computeGids(userId);//重新计算用户的权限组id
  //权限组id是否发生变化
  if (oldGids.length != newGids.length) {
   return PERMISSION_OPERATION_SUCCESS_GIDS_CHANGED;
  }
 }
 return PERMISSION_OPERATION_SUCCESS;
}

在grantPermission方法中首先会计算当前用户进程当前拥有的组id，然后再通过ensurePermissionData将权限添加到应用的PermissionData列表中，这里返回一个PermissionData，通过该对象的grant方法进行最终的分配，事实上它其实是修改内部PermissionState成员的mGranted状态为true。最后会对用户的组id进行重新计算，如果发生变化则返回PERMISSION_OPERATION_SUCCESS_GIDS_CHANGED，否则返回PERMISSION_OPERATION_SUCCESS

//保证权限被添加到用户列表中
private PermissionData ensurePermissionData(BasePermission permission) {
 if (mPermissions == null) {
  mPermissions = new ArrayMap<>();
 }
 PermissionData permissionData = mPermissions.get(permission.name);
 if (permissionData == null) {
  permissionData = new PermissionData(permission);
  mPermissions.put(permission.name, permissionData);
 }
 return permissionData;
}

//根据用户权限列表计算用户的gid
public int[] computeGids(int userId) {
 enforceValidUserId(userId);

 int[] gids = mGlobalGids;

 if (mPermissions != null) {
  final int permissionCount = mPermissions.size();
  for (int i = 0; i < permissionCount; i++) {
   String permission = mPermissions.keyAt(i);
   if (!hasPermission(permission, userId)) {
    continue;
   }
   PermissionData permissionData = mPermissions.valueAt(i);
   //取到权限对应的组id数组，可见权限可以通过多个gid描述
   final int[] permGids = permissionData.computeGids(userId);
   if (permGids != NO_GIDS) {
    //将权限对应的组id添加到用户的组id数组中
    gids = appendInts(gids, permGids);
   }
  }
 }

 return gids;
}

ensurePermissionData方法确保将权限对应的PermissionData添加到PermissonsState的权限列表中，后续通过computeGids计算用户userId对应的组id，并将其添加到用户的组id数组mGlobalGids中。其中内置权限的gid映射是定义在/etc/permission/platform.xml

<permissions>
 ???
 <permission name="android.permission.READ_EXTERNAL_STORAGE" >
  <group gid="sdcard_r" />
 </permission>

 <permission name="android.permission.WRITE_EXTERNAL_STORAGE" >
  <group gid="sdcard_r" />
  <group gid="sdcard_rw" />
 </permission>
 
 <permission name="android.permission.INTERNET" >
  <group gid="inet" />
 </permission>
 ???
</permissions>

至此，我们明白了权限的本质实际上就是一组gid，这组gid对应的是一些整型，这些映射关系存放在system/core/include/private/android_filesystem_config.h中，其中的定义如下

#define AID_NET_BT_ADMIN 3001 /* bluetooth: create any socket */
#define AID_NET_BT  3002 /* bluetooth: create sco, rfcomm or l2cap sockets */
#define AID_INET   3003 /* can create AF_INET and AF_INET6 sockets */
#define AID_SDCARD_RW  1015 /* external storage write access */

static const struct android_id_info android_ids[] = {
 ...
 { "bluetooth",  AID_BLUETOOTH, },
 { "sdcard_rw",  AID_SDCARD_RW, },
 { "net_bt_admin", AID_NET_BT_ADMIN, },
 { "net_bt",  AID_NET_BT, },
 { "inet",   AID_INET, },
 ...
}

通过将权限映射成一组gid，然后作为补充gid赋值给用户进程，也就是权限分配的本质。

//PermisssionsState.PermissionData
public boolean grant(int userId) {
 if (!isCompatibleUserId(userId)) {
  return false;
 }

 if (isGranted(userId)) {
  return false;
 }

 PermissionState userState = mUserStates.get(userId);
 if (userState == null) {
  userState = new PermissionState(mPerm.name);
  mUserStates.put(userId, userState);
 }
 //分配权限置true
 userState.mGranted = true;

 return true;
}

通过PermissionData的grant方法，为对应的用户创建PermissionState，并将mGranted置为true表示分配了该权限给
该用户。

当然权限分配完成后，下次不需要再次分配，当我们重新启动手机后，并需要再次对权限进行分配，这是因为PMS为所有的package记录了权限分配的情况，在Android6.0之前，package所有的权限信息都是存放在data/system/packages.xml配置文件中,在应用中启动时候读取该配置就可以直到权限分配了哪些权限。但在Android6.0后，运行时权限放在了data/system/users/0/runtime-permissions.xml中，而普通权限保持不变依然存放在packages.xml中，而且默认granted就是true。那么在分配完成权限后需要将权限的分配信息持久化到该文件中。

//packages.xml
<package 
 name="com.feelschaotic.demo" 
 codePath="/data/app/com.feelschaotic.demo-Gi5ksdF6mUDLakfOugCcwQ==" 
 nativeLibraryPath="/data/app/com.feelschaotic.demo-Gi5ksdF6mUDLakfOugCcwQ==/lib" 
 primaryCpuAbi="x86" 
 publicFlags="945307462" 
 privateFlags="0" 
 ft="16348dc3870" 
 it="16343f1d6aa" 
 ut="16348dc4c4d" 
 version="8220" 
 userId="10102">
  <sigs count="1">
   <cert index="20" key="..." />
  </sigs>
  <perms>
   <!-- 此处普通权限的 granted 全都默认是 true，且不可改变 granted 值-->
   <item name="android.permission.CHANGE_NETWORK_STATE" granted="true" flags="0" />
   <item name="android.permission.INTERNET" granted="true" flags="0" />
   <item name="android.permission.CHANGE_WIFI_STATE" granted="true" flags="0" />
   <item name="android.permission.ACCESS_NETWORK_STATE" granted="true" flags="0" />
  </perms>
  <proper-signing-keyset identifier="48" />
 </package>
<pkg name="com.feelschaotic.demo">
 <!-- 该demo我们故意拒绝了定位权限，可以看到：ACCESS_FINE_LOCATION 和 ACCESS_COARSE_LOCATION 的 granted 为 false -->
 <item name="android.permission.ACCESS_FINE_LOCATION" granted="false" flags="1" />
 <item name="android.permission.READ_EXTERNAL_STORAGE" granted="true" flags="0" />
 <item name="android.permission.ACCESS_COARSE_LOCATION" granted="false" flags="1" />
 <item name="android.permission.READ_PHONE_STATE" granted="true" flags="0" />
 <item name="android.permission.WRITE_EXTERNAL_STORAGE" granted="true" flags="0" />
 ...
</pkg>

在PMS的grantRuntimePermission分配完运行时权限后，最后会调用writeRuntimePermissionsForUserLPr将权限信息持久化到配置文件runtime-permissions.xml中,我们看看这个过程

public void writeRuntimePermissionsForUserLPr(int userId, boolean sync) {
 if (sync) {
  mRuntimePermissionsPersistence.writePermissionsForUserSyncLPr(userId);
 } else {
  mRuntimePermissionsPersistence.writePermissionsForUserAsyncLPr(userId);
 }
}

无论时同步方式还是异步方式的持久化，最后都会调用下面的方法进行

//写入权限到配置文件
private void writePermissionsSync(int userId) {
 //要写入的文件/data/system/users/0/runtime-permissions.xml
 AtomicFile destination = new AtomicFile(getUserRuntimePermissionsFile(userId));

 ArrayMap<String, List<PermissionState>> permissionsForPackage = new ArrayMap<>();
 ArrayMap<String, List<PermissionState>> permissionsForSharedUser = new ArrayMap<>();

 synchronized (mLock) {
  mWriteScheduled.delete(userId);
  //对所有的package进行处理
  final int packageCount = mPackages.size();
  for (int i = 0; i < packageCount; i++) {
   String packageName = mPackages.keyAt(i);
   //取到PackageSetting
   PackageSetting packageSetting = mPackages.valueAt(i);
   if (packageSetting.sharedUser == null) {//没有sharedUser的情况
    //取到PermissionsState，这个对象描述了包的权限信息
    PermissionsState permissionsState = packageSetting.getPermissionsState();
    List<PermissionState> permissionsStates = permissionsState
      .getRuntimePermissionStates(userId);//获取全新分配列表
    if (!permissionsStates.isEmpty()) {
     //存放在permissionsForPackage这个Map中，以包名为键
     permissionsForPackage.put(packageName, permissionsStates);
    }
   }
  }
  //有shareUser的情况
  final int sharedUserCount = mSharedUsers.size();
  for (int i = 0; i < sharedUserCount; i++) {
   String sharedUserName = mSharedUsers.keyAt(i);
   SharedUserSetting sharedUser = mSharedUsers.valueAt(i);
   PermissionsState permissionsState = sharedUser.getPermissionsState();
   List<PermissionState> permissionsStates = permissionsState
     .getRuntimePermissionStates(userId);
   if (!permissionsStates.isEmpty()) {
    permissionsForSharedUser.put(sharedUserName, permissionsStates);
   }
  }
 }
 //写配置
 FileOutputStream out = null;
 try {
  //取到输出流
  out = destination.startWrite();

  XmlSerializer serializer = Xml.newSerializer();
  serializer.setOutput(out, StandardCharsets.UTF_8.name());
  serializer.setFeature(
    "http://xmlpull.org/v1/doc/features.html#indent-output", true);
  serializer.startDocument(null, true);
  serializer.startTag(null, TAG_RUNTIME_PERMISSIONS);

  String fingerprint = mFingerprints.get(userId);
  if (fingerprint != null) {
   serializer.attribute(null, ATTR_FINGERPRINT, fingerprint);
  }
  //先写当前package的permission
  final int packageCount = permissionsForPackage.size();
  for (int i = 0; i < packageCount; i++) {
   String packageName = permissionsForPackage.keyAt(i);
   List<PermissionState> permissionStates = permissionsForPackage.valueAt(i);
   serializer.startTag(null, TAG_PACKAGE);//package
   serializer.attribute(null, ATTR_NAME, packageName);
   writePermissions(serializer, permissionStates);
   serializer.endTag(null, TAG_PACKAGE);
  }
  //写其shareUser进程的permission
  final int sharedUserCount = permissionsForSharedUser.size();
  for (int i = 0; i < sharedUserCount; i++) {
   String packageName = permissionsForSharedUser.keyAt(i);
   List<PermissionState> permissionStates = permissionsForSharedUser.valueAt(i);
   serializer.startTag(null, TAG_SHARED_USER);
   serializer.attribute(null, ATTR_NAME, packageName);
   writePermissions(serializer, permissionStates);
   serializer.endTag(null, TAG_SHARED_USER);
  }

  serializer.endTag(null, TAG_RUNTIME_PERMISSIONS);
  serializer.endDocument();
  destination.finishWrite(out);

  if (Build.FINGERPRINT.equals(fingerprint)) {
   mDefaultPermissionsGranted.put(userId, true);
  }
 // Any error while writing is fatal.
 } catch (Throwable t) {
  Slog.wtf(PackageManagerService.TAG,
    "Failed to write settings, restoring backup", t);
  destination.failWrite(out);
 } finally {
  IoUtils.closeQuietly(out);
 }
}

writePermissionsSync写配置的过程很简单，先打开配置文件/data/system/users/0/runtime-permissions.xml，随后对PMS中的每个package和sharedUser分别将其对应的权限分配列表按照包名和shareUserName存放在permissionsForPackage和permissionsForSharedUser中，随后打开输出流分别将其对应的运行时权限分配情况写入文件。

private void writePermissions(XmlSerializer serializer,
    List<PermissionState> permissionStates) throws IOException {
  for (PermissionState permissionState : permissionStates) {
    serializer.startTag(null, TAG_ITEM);
    serializer.attribute(null, ATTR_NAME,permissionState.getName());
    serializer.attribute(null, ATTR_GRANTED,
        String.valueOf(permissionState.isGranted()));
    serializer.attribute(null, ATTR_FLAGS,
        Integer.toHexString(permissionState.getFlags()));
    serializer.endTag(null, TAG_ITEM);
  }
}

writePermissions负责写tag 为package下的一条权限分配信息，如

<item name="android.permission.WRITE_EXTERNAL_STORAGE" granted="true" flags="0" />

权限的检测

权限检测是通过Context的checkSelfPermission方法来进行的。我们看下它的实现

@Override
public int checkSelfPermission(String permission) {
  if (permission == null) {
    throw new IllegalArgumentException("permission is null");
  }

  return checkPermission(permission, Process.myPid(), Process.myUid());
}
@Override
public int checkPermission(String permission, int pid, int uid) {
  if (permission == null) {
    throw new IllegalArgumentException("permission is null");
  }

  try {
    return ActivityManagerNative.getDefault().checkPermission(
        permission, pid, uid);
  } catch (RemoteException e) {
    return PackageManager.PERMISSION_DENIED;
  }
}

最终还是通过AMS的checkPermission来进行权限检查。

//frameworks/base/core/java/android/app/ActivityManager.java
@Override
public int checkPermission(String permission, int pid, int uid) {
  if (permission == null) {
    return PackageManager.PERMISSION_DENIED;
  }
  return checkComponentPermission(permission, pid, uid, -1, true);
}
int checkComponentPermission(String permission, int pid, int uid,
    int owningUid, boolean exported) {
  if (pid == MY_PID) {
    return PackageManager.PERMISSION_GRANTED;
  }
  return ActivityManager.checkComponentPermission(permission, uid,
      owningUid, exported);
}
/** @hide */
public static int checkComponentPermission(String permission, int uid,
    int owningUid, boolean exported) {
  // Root, system server get to do everything.
  final int appId = UserHandle.getAppId(uid);
  if (appId == Process.ROOT_UID || appId == Process.SYSTEM_UID) {
    return PackageManager.PERMISSION_GRANTED;
  }
  // Isolated processes don't get any permissions.
  if (UserHandle.isIsolated(uid)) {
    return PackageManager.PERMISSION_DENIED;
  }
  // If there is a uid that owns whatever is being accessed, it has
  // blanket access to it regardless of the permissions it requires.
  if (owningUid >= 0 && UserHandle.isSameApp(uid, owningUid)) {
    return PackageManager.PERMISSION_GRANTED;
  }
  // If the target is not exported, then nobody else can get to it.
  if (!exported) {
    /*
    RuntimeException here = new RuntimeException("here");
    here.fillInStackTrace();
    Slog.w(TAG, "Permission denied: checkComponentPermission() owningUid=" + owningUid,
        here);
    */
    return PackageManager.PERMISSION_DENIED;
  }
  if (permission == null) {
    return PackageManager.PERMISSION_GRANTED;
  }
  //通过PMS进行check
  try {
    return AppGlobals.getPackageManager()
        .checkUidPermission(permission, uid);
  } catch (RemoteException e) {
    // Should never happen, but if it does... deny!
    Slog.e(TAG, "PackageManager is dead?!?", e);
  }
  return PackageManager.PERMISSION_DENIED;
}

在AMS中的一系列调用中，最终的权限还是通过PMS的checkUidPermission来进行check的。

//PMS
@Override
public int checkUidPermission(String permName, int uid) {
  final int userId = UserHandle.getUserId(uid);

  if (!sUserManager.exists(userId)) {
    return PackageManager.PERMISSION_DENIED;
  }

  synchronized (mPackages) {
    Object obj = mSettings.getUserIdLPr(UserHandle.getAppId(uid));
    if (obj != null) {
      final SettingBase ps = (SettingBase) obj;
      final PermissionsState permissionsState = ps.getPermissionsState();
      //通过PermissionsState来检查
      if (permissionsState.hasPermission(permName, userId)) {
        return PackageManager.PERMISSION_GRANTED;
      }
      //定位权限的检测特殊处理
      // Special case: ACCESS_FINE_LOCATION permission includes ACCESS_COARSE_LOCATION
      if (Manifest.permission.ACCESS_COARSE_LOCATION.equals(permName) && permissionsState
          .hasPermission(Manifest.permission.ACCESS_FINE_LOCATION, userId)) {
        return PackageManager.PERMISSION_GRANTED;
      }
    } else {
      ArraySet<String> perms = mSystemPermissions.get(uid);
      if (perms != null) {
        if (perms.contains(permName)) {
          return PackageManager.PERMISSION_GRANTED;
        }
        if (Manifest.permission.ACCESS_COARSE_LOCATION.equals(permName) && perms
            .contains(Manifest.permission.ACCESS_FINE_LOCATION)) {
          return PackageManager.PERMISSION_GRANTED;
        }
      }
    }
  }

  return PackageManager.PERMISSION_DENIED;
}

checkUidPermission首先根据userId从PMS的配置对象中取到SettingBase,然后取到用户对应的PermissionsState，再通过permissionsState的hasPermission判断是否有该权限。

//检测权限
public boolean hasPermission(String name, int userId) {
  enforceValidUserId(userId);

  if (mPermissions == null) {
    return false;
  }
  //取到权限对应的PermissionData
  PermissionData permissionData = mPermissions.get(name);
  //通过isGranted来判断
  return permissionData != null && permissionData.isGranted(userId);
}

从PermissionsState的权限列表中取到PermissionData，通过PermissionData的PermissionState对象的mGranted成员就知道权限是否分配了。

总结

在Android6.0之前的版本中，应用在安装的时候会将manifest中request的权限(即通过申请的权限)添加到Package对象的PackageSetting中，PMS为每个安装的app创建一个Package对象，这个是在安装过程中创建的，同时在安装过程中也会为每个app创建一个PackageSetting对象，并将其保存在Package对象的mExtra中，在PackageSetting内部保存了应用的签名信息和授予的权限列表，实际上PackageSetting本身就是继承自GrantedPermissions类，这个类从名字看就知道它负责已授权的permission。应用中授权的权限在安装完成后会将应用的信息(包括了权限，签名和应用的基本信息等)写入到pacakge.xml文件中，这样下次系统启动就可以通过读取该文件获取应用的授权信息。

在Aandroid6.0之后，google为了防止应用滥用权限对权限的授予进行了收缩，将危险的权限授予过程交给用户来决定，为了适应这样的变化，必须要将安装权限和运行时权限进行区分处理，安装权限保持原有的逻辑不变，对于动态权限的分配必然要对PackageSetting进行一个大手术，在Android6.0中PackageSetting不再继承自GrantedPermissions，而是继承自于SettingBase，它的内部也比以前复杂了一些，简单来说它内部维护了一个PermissionsState，它负责管理应用的权限，因此它内部存放着应用的授权的权限列表(实际上是一个ArrayMap<String, PermissionData>)，以及权限组对应的gids，此时的权限不再是仅仅是一个String，而是一个PermissionData，而PermissionData内部持有PermissionState即permission的状态，可以看到最终我们还是通过改变PermissionData的PermissionState来达到动态授权的目的。另外授予的动态权限最终会保存在runtime-permission.xml中。

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本文标题：Android6.0 动态权限机制深入讲解

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