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2020
10-08

androidQ sd卡权限使用详解

默认情况下，如果应用以 Android Q 为目标平台，则在访问外部存储设备中的文件时会进入过滤视图。应用可以使用 Context.getExternalFilesDir() 将专用于自己的文件存储在特定于自己的目录中。

1. 临时停用分区存储行为:

以 Android 9（API 级别 28）或更低版本为目标平台。
如果您以 Android Q 为目标平台，请在应用的清单文件中将 requestLegacyExternalStorage 的值设为 true。

<manifest ... >
 <!-- This attribute is "false" by default on apps targeting Android Q. -->
 <application android:requestLegacyExternalStorage="true" ... >
 ...
 </application>
</manifest>

2. 如何实现隔离存储:

2.1 ApplicationInfo新增PRIVATE_FLAG_REQUEST_LEGACY_EXTERNAL_STORAGE标记

PackageParser.java:

if (sa.getBoolean(
  R.styleable.AndroidManifestApplication_requestLegacyExternalStorage,
  owner.applicationInfo.targetSdkVersion < Build.VERSION_CODES.Q)) {
 ai.privateFlags |= ApplicationInfo.PRIVATE_FLAG_REQUEST_LEGACY_EXTERNAL_STORAGE;
}

ApplicationInfo.java:

public boolean hasRequestedLegacyExternalStorage() {
 return (privateFlags & PRIVATE_FLAG_REQUEST_LEGACY_EXTERNAL_STORAGE) != 0;
}

2.2 grantRuntimePermission()重新挂载视图

apk启动时默认挂载runtime/default视图，grantRuntimePermission()时如果是READ_EXTERNAL_STORAGE或WRITE_EXTERNAL_STORAGE，则会获取挂载模式重新挂载对应视图。

PermissionManagerService.java:

 private void grantRuntimePermission(String permName, String packageName, boolean overridePolicy,
   int callingUid, final int userId, PermissionCallback callback) {
  ......
  if (READ_EXTERNAL_STORAGE.equals(permName)
    || WRITE_EXTERNAL_STORAGE.equals(permName)) {
   final long token = Binder.clearCallingIdentity();
   try {
    if (mUserManagerInt.isUserInitialized(userId)) {
     StorageManagerInternal storageManagerInternal = LocalServices.getService(
       StorageManagerInternal.class);
     storageManagerInternal.onExternalStoragePolicyChanged(uid, packageName);
    }
   } finally {
    Binder.restoreCallingIdentity(token);
   }
  }

 }

获取挂载模式这块android10有修改，没有设置Legacy标志的话，总是获取default挂载模式，没有读写权限。
android 10会设置属性[persist.sys.isolated_storage]: [true]，因此走到if(ENABLE_ISOLATED_STORAGE)中的getMountMode()。

public static boolean hasIsolatedStorage() {
 //[persist.sys.isolated_storage]: [true]
 //[sys.isolated_storage_snapshot]: [true]
 return SystemProperties.getBoolean("sys.isolated_storage_snapshot",
   SystemProperties.getBoolean("persist.sys.isolated_storage", true));
}

private static final boolean ENABLE_ISOLATED_STORAGE = StorageManager.hasIsolatedStorage();

public int getExternalStorageMountMode(int uid, String packageName) {
+ //android 10新增逻辑
+ if (ENABLE_ISOLATED_STORAGE) {
+  return getMountMode(uid, packageName);
+ }
 ......
 int mountMode = Integer.MAX_VALUE;
 for (ExternalStorageMountPolicy policy : mPolicies) {
  final int policyMode = policy.getMountMode(uid, packageName);
  if (policyMode == Zygote.MOUNT_EXTERNAL_NONE) {
   return Zygote.MOUNT_EXTERNAL_NONE;
  }
  mountMode = Math.min(mountMode, policyMode);
 }
 if (mountMode == Integer.MAX_VALUE) {
  return Zygote.MOUNT_EXTERNAL_NONE;
 }
 return mountMode;
}

正常模式下hasLegacy=false，走到if判断的DEFAULT分支；legacy模式hasLegacy=true，与之前保持一致，有write权限就走到WRITE模式分支。

private int getMountModeInternal(int uid, String packageName) {
 try {
  ......
  final boolean hasRead = StorageManager.checkPermissionAndCheckOp(mContext, false, 0,
    uid, packageName, READ_EXTERNAL_STORAGE, OP_READ_EXTERNAL_STORAGE);
  final boolean hasWrite = StorageManager.checkPermissionAndCheckOp(mContext, false, 0,
    uid, packageName, WRITE_EXTERNAL_STORAGE, OP_WRITE_EXTERNAL_STORAGE);
  ......
  final boolean hasLegacy = mIAppOpsService.checkOperation(OP_LEGACY_STORAGE,
    uid, packageName) == MODE_ALLOWED;
  if (hasLegacy && hasWrite) {
   return Zygote.MOUNT_EXTERNAL_WRITE;
  } else if (hasLegacy && hasRead) {
   return Zygote.MOUNT_EXTERNAL_READ;
  } else {
   return Zygote.MOUNT_EXTERNAL_DEFAULT;
  }
 } catch (RemoteException e) {
  // Should not happen
 }
 return Zygote.MOUNT_EXTERNAL_NONE;
}

2.3 Legacy Storage属性对权限的影响

安装apk时，就会根据requestLegacyExternalStorage属性来对ops state进行设置，修改OP_LEGACY_STORAGE的默认状态。

<manifest ... >
 <application android:requestLegacyExternalStorage="true" ... >
 </application>
</manifest>

 
//Q 正常模式
LEGACY_STORAGE: mode=ignore
//Q legacy模式
LEGACY_STORAGE: mode=allow

PermissionPolicyService启动时首先进行权限变化监听：

public void onStart() {
 permManagerInternal.addOnRuntimePermissionStateChangedListener(
 this::synchronizePackagePermissionsAndAppOpsAsyncForUser);
}

private void synchronizePackagePermissionsAndAppOpsAsyncForUser(@NonNull String packageName,
  @UserIdInt int changedUserId) {
 if (isStarted(changedUserId)) {
  synchronized (mLock) {
   if (mIsPackageSyncsScheduled.add(new Pair<>(packageName, changedUserId))) {
    FgThread.getHandler().sendMessage(PooledLambda.obtainMessage(
      PermissionPolicyService
        ::synchronizePackagePermissionsAndAppOpsForUser,
      this, packageName, changedUserId));
   }
   ......
  }
 }
}

APK安装时，会根据requestLegacyExternalStorage属性来通知storage权限变化，调用关系如下：

//调用关系：
1.PackageManagerService.java:
 installPackagesLI()
  commitPackagesLocked()
   updateSettingsLI()
    updateSettingsInternalLI()
     2.PermissionManagerService.java:
      mPermissionManager.updatePermissions()
       restorePermissionState()

//关键代码：
private void restorePermissionState(@NonNull PackageParser.Package pkg, boolean replace,
  @Nullable String packageOfInterest, @Nullable PermissionCallback callback) {
......
  //判断requestLegacyExternalStorage属性
  updatedUserIds = checkIfLegacyStorageOpsNeedToBeUpdated(pkg, replace, updatedUserIds);
......
 for (int userId : updatedUserIds) {
  notifyRuntimePermissionStateChanged(pkg.packageName, userId);
 }
}

最终调用到PermissionPolicyService的监听函数synchronizePackagePermissionsAndAppOpsForUser()，进行默认权限获取和设置。

当apk安装时，声明了requestLegacyExternalStorage="true"属性，并且声明了READ_EXTERNAL_STORAGE、WRITE_EXTERNAL_STORAGE，那么addOpIfRestricted()就会将LEGACY_STORAGE设置为allow模式。

//调用关系：
synchronizePackagePermissionsAndAppOpsForUser()：
 synchroniser.addPackage()
  addOpIfRestricted()//LEGACY_STORAGE加入到mOpsToAllow
 synchroniser.syncPackages()
  setUidModeAllowed()
   setUidMode()//设置LEGACY_STORAGE为allow

//关键代码：
private void addOpIfRestricted(@NonNull PermissionInfo permissionInfo,
  @NonNull PackageInfo pkg) {
......
  //forPermission()会根据requestLegacyExternalStorage的值进行返回
  final SoftRestrictedPermissionPolicy policy =
    SoftRestrictedPermissionPolicy.forPermission(mContext, pkg.applicationInfo,
      mContext.getUser(), permission);

  final int op = policy.resolveAppOp();
  if (op != OP_NONE) {
   switch (policy.getDesiredOpMode()) {
    case MODE_DEFAULT:
     mOpsToDefault.add(new OpToChange(uid, pkg.packageName, op));
     break;
    case MODE_ALLOWED:
     //在声明READ_EXTERNAL_STORAGE权限下，会将LEGACY_STORAGE加入到mOpsToAllow
     if (policy.shouldSetAppOpIfNotDefault()) {
      mOpsToAllow.add(new OpToChange(uid, pkg.packageName, op));
     } else {
      mOpsToAllowIfDefault.add(
        new OpToChange(uid, pkg.packageName, op));
     }
     break;
......
}

public static @NonNull SoftRestrictedPermissionPolicy forPermission(@NonNull Context context,
  @Nullable ApplicationInfo appInfo, @Nullable UserHandle user,
  @NonNull String permission) {
 switch (permission) {
  case READ_EXTERNAL_STORAGE: {
   if (appInfo != null) {
    boolean hasAnyRequestedLegacyExternalStorage =
      appInfo.hasRequestedLegacyExternalStorage();

    hasRequestedLegacyExternalStorage = hasAnyRequestedLegacyExternalStorage;
   }

   return new SoftRestrictedPermissionPolicy() {
    @Override
    public int getDesiredOpMode() {
     if (applyRestriction) {
      return MODE_DEFAULT;
     } else if (hasRequestedLegacyExternalStorage) {
      //声明了requestLegacyExternalStorage就返回allow
      return MODE_ALLOWED;
     } else {
      return MODE_IGNORED;
     }
    }

    @Override
    public boolean shouldSetAppOpIfNotDefault() {
     return getDesiredOpMode() != MODE_IGNORED;
    }
   };
  }

3. sdcard路径权限说明:

  • rwx:421，umask默认为八进制022(----w--w-)
  • /mnt/runtime/default的gid为1015，也就是sdcard_rw；mask 为6，八进制006，group sdcard_rw可读写，也就是other没有rw权限
  • /mnt/runtime/read的gid为9997，也就是everybody；mask 为23，八进制027，group everybody可读、不可写，other没有读写执行权限
  • /mnt/runtime/write的gid为9997，也就是everybody；mask 为7，八进制007，group everybody可读写，other没有读写可执行权限
  • /data/media on /mnt/runtime/default/emulated type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=1015,multiuser,mask=6,derive_gid,default_normal)

/data/media on /mnt/runtime/read/emulated type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=9997,multiuser,mask=23,derive_gid,default_normal)
/data/media on /mnt/runtime/write/emulated type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=9997,multiuser,mask=7,derive_gid,default_normal)

/mnt/runtime/default:

drwxrwx--x 3 root sdcard_rw 4096 2018-12-18 03:41 Android
drwxrwx--x 3 root sdcard_rw 4096 2018-12-18 06:11 DCIM

/mnt/runtime/read:

drwxr-x--- 3 root everybody 4096 2018-12-18 03:41 Android
drwxr-x--- 3 root everybody 4096 2018-12-18 06:11 DCIM

/mnt/runtime/write:

drwxrwx--- 3 root everybody 4096 2018-12-18 03:41 Android
drwxrwx--- 3 root everybody 4096 2018-12-18 06:11 DCIM

/sdcard/Android/data:

drwxrwx--- 4 u0_a64 everybody 4096 2018-12-18 06:11 com.android.camera2
drwxrwx--- 3 u0_a15 everybody 4096 2018-12-18 03:41 com.google.android.gms
drwxrwx--- 4 u0_a84 everybody 4096 2018-12-18 03:41 com.google.android.youtube

4. sdcard文件存储示例:

4.1 getExternalFilesDir()随卸载而删除

///storage/emulated/0/Android/data/com.xx.xx/files
File file = File(context.getExternalFilesDir(null), "test.txt");

4.2 媒体文件
媒体文件使用MediaStore操作，卸载后不会删除。
访问其他应用生成的照片、视频、音频，需要READ_EXTERNAL_STORAGE权限。

4.3 存储访问框架(SAF)
访问其他应用创建的文件，例如"Download"目录，必须使用存储访问框架，用户通过框架选择特定文件。

4.4 照片中的位置信息
需要ACCESS_MEDIA_LOCATION权限，才能获取元数据中的位置信息。

<permission android:name="android.permission.ACCESS_MEDIA_LOCATION"
 android:permissionGroup="android.permission-group.UNDEFINED"
 android:label="@string/permlab_mediaLocation"
 android:description="@string/permdesc_mediaLocation"
 android:protectionLevel="dangerous" />

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