How can I call a system service from a vendor service?

[Harikrishnan S]

Hi,
I am trying to get a system service which is written in java, from a hal service written in cpp.
I wrote a simple client application to get the hal service and  call its methods.
The method called from hal service looks like this is:

Return<int32_t> Test::getMyService(int32_t valueIn) {
   android::sp<android::IServiceManager> sm = android::defaultServiceManager();
   static sp<IBinder>   mBinder;

   //getting the system service written in java
   mBinder = sm->getService(String16("com.example.MyService"));
   if (mBinder == NULL)
   {
       ALOGE("MyService is not found.");
   }
   return int32_t {};
}

The hal service is run using '/dev/hwbinder' and uses a hidl interface.

I am getting the following error on logcat:

hw-Parcel: **** enforceInterface() expected 'android.hidl.manager@1.0::IServiceManager' but read ''
ServiceManager: Waiting for service 'com.example.MyService' on '/dev/hwbinder'...


Is it possible to call a system service from a hal/hidl service like this way?
What is the issue here?

[Steven Moreland]

Hello,

We can't use libbinder to get a system service like this, also, libbinder won't work on /dev/hwbinder (only libhwbinder works on /dev/hwbinder). This is because libbinder/libhwbinder use different ways of communication. Interfaces used between the system.img (platform) and vendor.img (hardware-specific) pieces of Android need to be stable.

There are two main options:
- use a HIDL service here (either a callback in the existing interface or a new interface)
- in Android 11 (aosp/master only right now): use a '@VintfStability' AIDL interface and use libbinder_ndk from the vendor side.

[Harikrishnan S]

Hi Steven,

Thanks for your valuable reply!. First option I was able to try out. I created a HIDL service and used callback from system service.

I dint understood the second option. I know Vinf is vendor interface. But how can an AIDL interface be used to communicate between system and vendor service? Can you elaborate a little more on this.

[Steven Moreland]

Here's an example which works in Android 11:
https://r.android.com/q/topic:staidlvib

[Yue Xu]

Hi Steven. I created a stable AIDL service in vendor (with  '@VintfStability' ). And when I tried to connect to the service from an app, I got this error message in logcat:

`BpBinder: Cannot do a user transaction on a system stability binder in a vendor stability context.`

Could you shed some light on this? Thanks.

[Steven Moreland]

This CL adds an additional log here that will tell you the exact type which is having a problem:
https://android-review.googlesource.com/c/platform/frameworks/native/+/1582874

The error does sound like an interface is missing @VintfStability.

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[Yue Xu]

Steven, I compiled the stable AIDLs into java stub code to use in my app now, but the stub code seems to access hidden Android method: markVintfStability().

`AndroidRuntime: Caused by: java.lang.NoSuchMethodError: No virtual method markVintfStability()V in class XXX`

If I can't access this method, how to use stable AIDL in my app?

Thanks.

'Steven Moreland' via android-platform <android-...@googlegroups.com> 于2021年3月12日周五 上午11:26写道：

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[Yue Xu]

Thanks Steven. Yes, due to the build system we are using, I needed to copy over the AIDLS into my app, and the copied AIDLS aren't stable.

'Steven Moreland' via android-platform <android-...@googlegroups.com> 于2021年3月12日周五 上午11:26写道：

This CL adds an additional log here that will tell you the exact type which is having a problem:

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[Steven Moreland]

AIDL interfaces aren't a supported way to extend platform APIs to an app. Please see these links, respectively, for adding Java and C APIs:
https://developer.android.com/guide/topics/manifest/uses-library-element
https://source.android.com/devices/tech/config/namespaces_libraries

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[Jerrylee1 Peters]

I don't think this is the same subject cause I'm a bit confused about computers but am interested at the same time. As silly as it seems I've been looking for a way to look at my cheating girlfriends facebook messages and there's no way I can ever get ahold of her phone let alone download anything on it that spy apps require you to do. 

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