Android: open /dev/graphics/fb0 failed, despite app signature

Question

In my system application for the latest jeally bean ( 4.3 ), signed with the fitting platform certificate running as android.uid.system. In my manifest, I added the permissions READ_FRAME_BUFFER, ACCESS_SURFACE_FLINGER.

In my app, I'm able to access the SurfaceFlinger service via the ScreenShotClient ( c++ ). Accessing the /dev/graphics/fb0 device directly ( also using native code, c++ ) via open always failed with a permission error.

I wrote an little command line tool, which is able to open the /dev/graphics/fb0. For sure my command line tool is running as root user, and my app is running as system user.

ls -l -a /dev/graphics/fb0 crw-rw---- root graphics 29, 0 2013-09-30 07:31 fb0

For my understanding only the root user, or a member of the graphics group can access the fb0. For my understanding the READ_FRAME_BUFFER permission, is exactly for accessing /dev/graphics/fb? Can someone please give me a hint?

Answer 1

1. In /frameworks/base/core/res/AndroidManifest.xml, You can see what access rights apply to each of the system-defined permissions. ->In this case, you'll see that READ_FRAME_BUFFER is marked as android:protectionLevel="signature | system", and ACCESS_SURFACE_FLINGER is marked as android:protectionLevel="signature".
2. So logically in any system app, having these both permission you should not face any issue in accessing framebuffer and surface flinger. (Its also written in /frameworks/base/core/res/AndroidManifest.xml in comment where READ_FRAME_BUFFER permission is defined, that It is to get access to the frame buffer data)

So there might be something else creating the problem, You may want to post some error log messages etc.