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。
...
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。
//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。
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