android Q sdcard權限詳解

默認情況下，如果應用以 Android Q 爲目標平臺，則在訪問外部存儲設備中的文件時會進入過濾視圖。應用可以使用 Context.getExternalFilesDir() 將專用於自己的文件存儲在特定於自己的目錄中。

1. 臨時停用分區存儲行爲:

• 以 Android 9（API 級別 28）或更低版本爲目標平臺。
• 如果您以 Android Q 爲目標平臺，請在應用的清單文件中將 requestLegacyExternalStorage 的值設爲 true。

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

2. 如何實現隔離存儲:

2.1 ApplicationInfo新增PRIVATE_FLAG_REQUEST_LEGACY_EXTERNAL_STORAGE標記

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()重新掛載視圖
apk啓動時默認掛載runtime/default視圖，grantRuntimePermission()時如果是READ_EXTERNAL_STORAGE或WRITE_EXTERNAL_STORAGE，則會獲取掛載模式重新掛載對應視圖。

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);
            }
        }

    }

獲取掛載模式這塊android10有修改，沒有設置Legacy標誌的話，總是獲取default掛載模式，沒有讀寫權限。
android 10會設置屬性[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新增邏輯
+    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判斷的DEFAULT分支；legacy模式hasLegacy=true，與之前保持一致，有write權限就走到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屬性對權限的影響
安裝apk時，就會根據requestLegacyExternalStorage屬性來對ops state進行設置，修改OP_LEGACY_STORAGE的默認狀態。

<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啓動時首先進行權限變化監聽：

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安裝時，會根據requestLegacyExternalStorage屬性來通知storage權限變化，調用關係如下：

//調用關係：
1.PackageManagerService.java:
    installPackagesLI()
        commitPackagesLocked()
            updateSettingsLI()
                updateSettingsInternalLI()
                    2.PermissionManagerService.java:
                        mPermissionManager.updatePermissions()
                            restorePermissionState()

//關鍵代碼：
private void restorePermissionState(@NonNull PackageParser.Package pkg, boolean replace,
        @Nullable String packageOfInterest, @Nullable PermissionCallback callback) {
......
        //判斷requestLegacyExternalStorage屬性
        updatedUserIds = checkIfLegacyStorageOpsNeedToBeUpdated(pkg, replace, updatedUserIds);
......
    for (int userId : updatedUserIds) {
        notifyRuntimePermissionStateChanged(pkg.packageName, userId);
    }
}

最終調用到PermissionPolicyService的監聽函數synchronizePackagePermissionsAndAppOpsForUser()，進行默認權限獲取和設置。
當apk安裝時，聲明瞭requestLegacyExternalStorage="true"屬性，並且聲明瞭READ_EXTERNAL_STORAGE、WRITE_EXTERNAL_STORAGE，那麼addOpIfRestricted()就會將LEGACY_STORAGE設置爲allow模式。

//調用關係：
synchronizePackagePermissionsAndAppOpsForUser()：
    synchroniser.addPackage()
        addOpIfRestricted()//LEGACY_STORAGE加入到mOpsToAllow
    synchroniser.syncPackages()
        setUidModeAllowed()
            setUidMode()//設置LEGACY_STORAGE爲allow

//關鍵代碼：
private void addOpIfRestricted(@NonNull PermissionInfo permissionInfo,
        @NonNull PackageInfo pkg) {
......
        //forPermission()會根據requestLegacyExternalStorage的值進行返回
        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:
                    //在聲明READ_EXTERNAL_STORAGE權限下，會將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) {
                        //聲明瞭requestLegacyExternalStorage就返回allow
                        return MODE_ALLOWED;
                    } else {
                        return MODE_IGNORED;
                    }
                }

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

3. sdcard路徑權限說明:

• rwx:421，umask默認爲八進制022(----w--w-)
• /mnt/runtime/default的gid爲1015，也就是sdcard_rw；mask 爲6，八進制006，group sdcard_rw可讀寫，也就是other沒有rw權限
• /mnt/runtime/read的gid爲9997，也就是everybody；mask 爲23，八進制027，group everybody可讀、不可寫，other沒有讀寫執行權限
• /mnt/runtime/write的gid爲9997，也就是everybody；mask 爲7，八進制007，group everybody可讀寫，other沒有讀寫可執行權限

/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文件存儲示例:

4.1 getExternalFilesDir()隨卸載而刪除

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

4.2 媒體文件
媒體文件使用MediaStore操作，卸載後不會刪除。
訪問其他應用生成的照片、視頻、音頻，需要READ_EXTERNAL_STORAGE權限。

4.3 存儲訪問框架(SAF)
訪問其他應用創建的文件，例如"Download"目錄，必須使用存儲訪問框架，用戶通過框架選擇特定文件。

4.4 照片中的位置信息
需要ACCESS_MEDIA_LOCATION權限，才能獲取元數據中的位置信息。

<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" />