[android-kernel] Global Android file access controller / file filter

['Fabian B' via Android Linux Kernel Development]

I would like to implement a global file access controller. Each file call
(for example open an image by an app) should be checked by a
policy-controller. Example: Pictures, which are made from 09pm to 08am
should not be displayed to the current user - not important if the picture
is used in a gallery app or as an in-game resource of a mobile game.



I know, there is the UserManager api in the framework code on the top of
the architecture stack. But I like to share pictures between users, if the
current read-condition allows it. The context switch should be go "on the
fly", without changing the user.



At first, I thought I have to edit/extend the Android framework source code
(or the Java api source code) like the FileInputStream or Java File api.
But now I know that there are some NDK libraries, which have their own file
access by the bionic library
<https://android.googlesource.com/platform/bionic/>.



My question is now: Is it possible and enough to edit the bionic library
(for example the function fopen(3) ) or do I have to edit the read syscall
on kernel level to reach a global access controller? Or is the right way to
have a look in the vfs? I also read something about AppArmor
<https://wiki.ubuntu.com/AppArmor>, a LSM for MAC - but is this a possible
solution?



The big important point is to get the dynamic policies from a self-written
system app. This could also be a challenge.



I hope that some people here have some tips for me where I have to look for
it. Also ideas or hints for possibilities are welcome!