Running a command as a shell user from an Android app

Issue

I am trying to execute an fopen() function on a file that is given permissions only to "shell" from a native (C++) application that is triggered from a service on my Android application. When I run the native code as a PIE from the shell, I am able to open the file for reading, but if I try from the Android application, it fails to open the file as the Android application is run in a different user space and so I am not able to open the file. My question is, is it possible to run the command as a "shell" user or a child of "shell" from the Android application. I want to be able to do this without rooting the device so su is out of question.

Solution

You can't change the user ID of your app without a rooted device. If you could, the security model wouldn't be very useful. If your app needs access to the file, you will need to grant appropriate permissions.

The other common workaround is to have a service, running as the "shell" user, whose job is to open the file and hand back a file descriptor. The tricky part is that you need a way to launch that service as the "shell" user, which brings us back to needing "su".

FWIW, the situation is the same whether you're coding in Java or C++.

Answered By - fadden