How to know when WRITE_EXTERNAL_STORAGE is required - android

I am upgrading my Android app's Target SDK Version to 23 (Android M) which has the new Runtime permissions (https://developer.android.com/training/permissions/requesting.html). My app has declared WRITE_EXTERNAL_STORAGE in its manifest, which must now me requested at runtime. Of course, I'd like to only request it when necessary.
I have an abstraction for different storage types, one implements local storage through normal File I/O (not the Android Content Provider stuff). In this class, I get a file path (like /mnt/sdcard/... or /data/data/...) and before accessing that file (read and write mode), I want to check if I have to call RequestPermissions for WRITE_EXTERNAL_STORAGE.
So the question is: What's the safest and simplest way to determine whether a file can be read and written without that permission (e.g. because it's inside getExternalFilesDir()) or not?

You can see here that you only have to ask for WRITE_EXTERNAL_STORAGE when your application needs to write to external storage.
But:
Starting in API level 19, this permission is not required to
read/write files in your application-specific directories returned by
getExternalFilesDir(String) and getExternalCacheDir().
First of all I recommend you to avoid requesting permissions using Intents, because it's a best practice and improves a lot the user experience.
If you can't use an Intent to avoid writing with your app, and you know that some day the user will have to write externally, I think the better would be to ask for the permissions the first time the user takes the "write in external storage" path.
As far as I know, in API23 you only have to ask for permissions once, so I think the easyest way would be to ask for the permissions at the first time that the user needs the functionality, I think that then when he'll execute it again, permissions would remain accepted. You can check your granted permissions with the procedure shown here.
Here you can read:
The user is prompted to give permission once, either at run time or at
install time (depending on the user's Android version). After that,
your app can perform the operation without requiring additional
interaction from the user. However, if the user doesn't grant the
permission (or revokes it later on), your app becomes unable to
perform the operation at all.

Related

Android permissions related issue

I have an Android app that requires permission to read/write from/to external storage. When the app starts, it prompts the user to allow permission for external storage (and a bunch of other services - location, camera, etc).
Here's how we do it:
ActivityCompat.requestPermissions(this, new String[]{WRITE_EXTERNAL_STORAGE, CAMERA,ACCESS_FINE_LOCATION,INTERNET,WRITE_SETTINGS,ACCESS_WIFI_STATE,READ_PHONE_STATE}, PERMISSION_REQUEST_CODE)
Now, there is also an authentication service for which we need to read from external storage. We call this authentication service during startup.
Now, problem is: When you run the app for the 1st time after install, this call to authentication service may happen often before the permission to external storage is given. Because the authentication service has no permission to external storage, it fails authentication.
Workaround we do now: manually enable permission after install and then run the app. Not a long term solution though.
I did some reading and it seems in older API's we can force the permissions to be done during apk install itself. I am on API 29. Is it possible here to prompt the user to give permissions during install itself?
I also understand that this is considered a critical service and we can't bypass permissions. Rather we cannot force permissions to be default.
What I've done for now:
Initialized the authentication service later in the code so it runs only after permissions are done.
Is there a better solution available? Like some way to either force permissions for external storage by default or some other way? From what I have read, it doesnt look possible for the latest Android version. However, if there is a way to do this, pls let me know.
Thanks,
Anand
In order to know if the user has allowed or not the application with your requested permissions, you should override onRequestPermissionsResult(requestCode: Int, permissions: Array<out String>, grantResults: IntArray).
This way, you can do something like this :
if (requestCode == PERMISSION_REQUEST_CODE) {
for (elem in grantResults) {
if (elem != PackageManager.PERMISSION_GRANTED)
// Abort your feature here
}
// Do your feature here
}
If every permission has been passed you can then do what you want.
The Android Developer Documentation has a nice tutorial, more complete than I've done above, but you get the idea.
Not possible to force permission at installation time, you would need to target API level < 23 but now you need to use 28/29 to create/update apps on PlayStore. You could consider to use an alternative storage location like getCacheDir() moving files where you need later when your app has permission or just getExternalFilesDir() path where you can read/write without additional permissions.
Is it possible here to prompt the user to give permissions during install itself?
From official doc:
If the device is running Android 6.0 (API level 23) or higher, and the app's targetSdkVersion is 23 or higher, the user isn't notified of any app permissions at install time.
Also:
If the device is running Android 5.1.1 (API level 22) or lower, or the app's targetSdkVersion is 22 or lower while running on any version of Android, the system automatically asks the user to grant all dangerous permissions for your app at install-time (see figure 2).
In any case pay attention to this point:
Beginning with Android 6.0 (API level 23), users can revoke permissions from any app at any time, even if the app targets a lower API level.
In other word it means that today you have to check for and request permissions in your code.
Initialized the authentication service later in the code so it runs only after permissions are done.
You can use the ContextCompat.checkSelfPermission also in a Service.
When you start the service you can check if the permission is granted and avoid to fail.
You can use ActivityCompat.requestPermissions only in an Activity, but you can use the callback onRequestPermissionsResult to start your service.
Is there a better solution available?
You can't force the permission but for example your Service can check for permissions it needs and if it hasn't been granted yet, you can create a friendly notification to inform user.
Critical permissions like read/write external storage, camera, SMS, contacts, and a few others cannot be forcefully asked for prior to installation. So, they've to be asked for at runtime. The best practice is to ask for them just when they're actually needed instead of asking for them all at a time in the beginning.
If your service uses one of those permissions, you've to check for that permission every time before starting the service and then start the service only if the permission is granted. Otherwise, ask for permission, override onActivityResult and then start service only if the permission is granted.
Also, keep in mind to handle the scenario when the user marks the checkbox "Don't ask again" and denies permission. In that case, the user won't see the prompt again. So, you'll have to open permission settings using intent.

Options for dealing with Android 6.0's new permissions requirements from a service or model component?

I'm looking into porting some existing code to take Android M's new way of dealing with permissions into consideration. However the permission API needs to have an activity associated with it (for example the requestPermissions() method's first parameter is an activity).
So how should a service that needs to check if a permissions has been granted and request for permissions use this new API if the service doesn't have an activity?
Is it possible for the service to create a dummy invisible activity just for use with the permissions API? (if its possible I don't like the thought of doing that anyway though).
Or suppose its not a service but a model class that needs to perform a permissions check, in MVC a model shouldn't have any knowledge of the Vs and Cs and yet now either it has to in order to know which Activity to use with the permission API. Or potentially lots of code might have to migrate from model code into Activity code.
Any thoughts on how to migrate non activity based code that needs to check/prompt for permissions over to Android 6.0?
Update: I left out an important piece of information - this is code that is pre-installed (our company provides code that device manufacture's place in rom) and often may be run at device boot time and run in the background. Therefore the usual situation of a user being prompted for permission when they launch the app or later (and there therefore being an activity at that point) does not necessarily apply.
So how should a service that needs to check if a permissions has been granted and request for permissions use this new API if the service doesn't have an activity?
There is almost always an activity, except for pre-installed apps and plugins for other apps. Otherwise, your service is unlikely to ever run, as nothing will have used an explicit Intent to start up one of your app's components, so it will remain in the stopped state.
For the ~99.9% of Android apps that have an activity already, if the permissions are needed for the whole operation of the app, request them on first run. As Snild Dolkow notes, if the user later revokes the permission through Settings, you can detect that without an activity, then use other UI options (e.g., Notification, app widget) to let the user know that operation is suspended until they grant you the permissions again, which they would then do through your activity.
Is it possible for the service to create a dummy invisible activity just for use with the permissions API?
Presumably you can have a Theme.NoDisplay activity use requestPermissions(). However, from the user's standpoint, it will not make much sense, unless there's some alternative UI (app widget?) that they are interacting with. Popping up a permission dialog out of nowhere is unlikely to make you popular.
UPDATE 2019-06-15: Note that Android Q bans services popping up activities frmo the background. Please use a notification instead.
in MVC a model shouldn't have any knowledge of the Vs and Cs and yet now either it has to in order to know which Activity to use with the permission API
Do not touch the models until you have requested the permission, and gracefully fail if the permission is revoked. You already have to gracefully fail in other circumstances (out of disk space, no Internet connection, etc.), so a revoked permission should be handled in much the same way.
using this new 6.0 API seems like an recipe for bad design and tight coupling
You are welcome to your opinion. Based on what I have read, the Android engineers believe that asking the user for permissions is part of the user experience and is best handled at the UI layer as a result.
Again: the vast majority of Android apps will not have a problem with this, as they have a user interface. Apps that do not have a user interface and need dangerous permissions are in for some amount of rework.
this is code that is pre-installed (our company provides code that device manufacture's place in rom) and often may be run at device boot time
First, please understand that this is so far from normal that you can't even see normal from where you are due to the curvature of the Earth. :-) You can't really complain that Google did not optimize this particular scenario.
As I understand it, even system apps should be asking for runtime permissions. The Camera app did, for example, on the 6.0 preview. That being said, there's gotta be some database on the device somewhere that is tracking what has been granted, and presumably there is some way to pre-populate it. However, the user could still revoke it from Settings, presumably. But, the manufacturer could pull some stunts (e.g., messing with the Settings app) to possibly even preclude that scenario. I'd be looking in the same area as "how do I get it so my app cannot be force-stopped?" that device manufacturers can do.
Your alternatives would be to get rid of the dangerous permissions or to migrate your app off the SDK and into a standard Linux binary that would be run as part of the boot process and be put into a Linux user group that has access to the stuff that you need.
Ask for it when the user enables whatever feature your service provides. They'll be in one of your activities at the time. Yes, it means that your activities need knowledge of what permissions your services will require.
The service can always check for the permission by itself, though, since checkSelfPermission() is available in all Context instances. So you don't need an activity for that.
I guess an alternative would be to have your service post a notification saying "feature X requires you to approve more permissions". Actually, that may be a good idea regardless, in case the user goes into settings and revokes any permissions after the fact. That notification would then take the user to some activity with an "enable feature X" button/checkbox -- ask for the permission when that is selected.
You can send a notification. Look this library to manage the permissions: permission library

Android Marshmallow, "dangerous" protection level and system components/apps

I'm developing an application that going to be pr-installed (as a system app) on the firmware.
from the documentation so far about the relation between system apps, new permissions model, and the protection levels - I don't understand exactly when system app needs (if at all) to request user permission.
My problems starts when I try to use the WRITE_EXTERNAL_STORAGE permission. from the documentation I can see that it marked as "dangerous" permission.
- does "dangerous" permissions grant automatically to system apps?
when I use WRITE_EXTERNAL_STORAGE permission (as a system app) I'm getting security exception, and I don't know if it's mean that even tough my app installed as a system app - "dangerous" permissions must be requested by the user..
another point to mention:
to check the app behavior as a system app, I'm installing my application APK on the sys-priv directory (the device is rooted) of a nexus 5 running SDK preview 3. this is when I'm getting the security exception when attep to use methods requires the external storage permission..
After a lot of digging and debugging, I finally found some clue of granting runtime permission on marshmallow for system app, with a lot of inspirations in this stackoverflow ticket.
The key logic is in DefaultPermissionGrantPolicy. After systemReady, PackageManagerService checks if this user's default runtime permissions are not set yet(i.e. this is a new user), if so, PackageManagerService calls DefaultPermissionGrantPolicy.grantDefaultPermissions() to check/grant permissions:
public void grantDefaultPermissions(int userId) {
grantPermissionsToSysComponentsAndPrivApps(userId);
grantDefaultSystemHandlerPermissions(userId);
}
There are two cases that your built-in app may be automatically granted with runtime permission.
A> grantPermissionsToSysComponentsAndPrivApps -> will grant runtime permission with FLAG_PERMISSION_SYSTEM_FIXED and FLAG_PERMISSION_GRANTED_BY_DEFAULT.
if your system app has uid<10000, you will be granted with permissions for your user group.
if your system app fits all below conditions, it will be granted the permissions.
is a privilegedApp (under /system/priv-app/)
is persistent (android:persistent="true")
signed with platform signature.
B> grantDefaultSystemHandlerPermissions -> will grant runtime permission with FLAG_PERMISSION_GRANTED_BY_DEFAULT .
If your app is considered as a "default platform handler app", (i.e. your app is "expected to work out-of-the-box", like camera, dialer, SMS, calendar .etc, you can read more in method grantDefaultSystemHandlerPermissions()).
Other than that, your system application needs to ask user for granting dangerous permission, as long as it has targetSdk set to 23.
Quoting the release notes for the 2nd M preview:
Apps included in the system image are no longer granted dangerous permissions automatically. All apps should check for and request permissions at runtime.
That fits with what I recall seeing when I first used the stock Camera app on a Nexus 5 with the final(?) 6.0 preview firmware — it too asked for the runtime permission.
So, AFAIK, system apps have to ask for runtime permissions, as do non-system apps.

how are android security permissions checked at run-time?

This question has been asked before at How does Android enforce permissions?. While the discussions there are good, the question is still not fully answered.
In the development environment, exceptions are thrown when the app tries to do something that requires permissions not declared in AndroidManifest.xml. So how does the run-time system implement the run-time checking?
I guess it's most likely done in the core framework, which may or may not need support from native code. But I don't know what source code files in AOSP are relevant to this.
Android uses a lot of the standard Linux(-kernel?) mechanisms especially when it comes to hardware restrictions.
Every app gets assigned a new unique (Linux-)user id and whenever the app process is created the system creates it with that user id. The id will never change unless you remove the app. That means for accessing the lower system levels your app will appear as a certain user and every (Linux-)permission system that works with users will also apply to your app.
If you request WRITE_EXTERNAL_STORAGE in the manifest your app will also become member of the (Linux-)group (called sdcard_rw) that has permissions to write to that storage. Permissions on the filesystem are enforced to only allow writing to the system user (=owner) and the sdcard_rw group, anyone else (=other) may only read. See also Is Google blocking apps writing to SD cards
By doing that Android has to do pretty much nothing except for setting the correct UID/GIDs of the processes it spawns once the app starts and the rest is handled at lower levels. Apps that are not member of a certain group simply don't get access to certain hardware.
List of permission <> group mappings: platform.xml
There are also some (Android software) restrictions that are based on either the signature of your app and / or simply by looking up the permissions your app requested: e.g. ContextImpl#checkPermission() - but those permissions have to be checked at every entrypoint to code that allows restricted actions.
From time to time people discover ways to e.g. turn on GPS programmatically because a check like that is missing somewhere.
With regard to your second paragraph, "exceptions" are runtime faults. Permissions are not enforced at build time, only at run time.
Accessing hardware, low level operating system resources, and system files generally requires the app userid to be a member of an appropriate group which it may be assigned by the package manager as a result of having a corresponding android permission. (Familiar examples of that would be network sockets, and the sdcard write which zapl mentioned, but also system-only things like talking directly to the GSM modem or reading the raw touchscreen coordinates).
For the majority of android operations that are done by way of calling library functions which are stubs for interprocess communication to services running in a different process, the platform code running in the more privileged process on the receiving end of the ipc request checks with the package manager to find out if the calling application has the necessary android permission.
Many special permissions are only available to apps signed with the system signature - even if another app claims those in its manifest, they will not be applied by the package manager.

intervening Android Install-time Permission granting mechanism

I'm new in Android. I have an Idea to enrich user's knowledge whilst installing a desired application.
the idea is developing an application that can analyze .apk file of the application to check if it's over-privileged or not. and inform the user if this application which he's trying to install is over-privileged or not.
but since there's already a mechanism from Android which asks user's consent to grant whatever permission the application requests, I'm not sure if my application can somehow intervene this mechanism, postpone it, pause it or it can not.
I'm not sure if my application can somehow intervene this mechanism, postpone it, pause it
None of these are possible, sorry. You are welcome to create your own custom firmware that has this feature, but you cannot create this capability via an SDK application, for obvious security reasons.
I am not far from where you are ~ the entire mechanization you seek is based on an xml file in the "root" of the installation - it is called AndroidManifest.xml = all permission based issues should begin original first efforts on that file:
The AndroidManifest.xml File
Every application must have an AndroidManifest.xml file (with precisely that name) in its root directory. The manifest presents essential information about the application to the Android system, information the system must have before it can run any of the application's code. Among other things, the manifest does the following: .....
the "app-store" web based distribution system is supposed to pick that up and not only make some decisions on what to present to the user but as well differentiate to some extent what to do in the matter but as I just got a Droid-X emulator available in my installation I can tell you for a fact that "versioning" is subject to oversimplification as we cannot rely on users being tech-geeks

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