Android-specific changes to linux

Android-specific changes to linux

1. Why changes needed?

   • Need finer control over which devices are suspended

2. Wakelocks added to kernel

   • Kernel set to suspend as soon and as often as possible
     • If app inactive, will get suspended
   • Music player gets wake lock, if it is still playing while the screen is turned off
   • Drivers need to actively prevent system from suspending device

3. Binder

   • Android uses remote procedure consoles frequently
     • Binder is RPC mechanism
   • Facilitates app communication
     • Permissions - which app can access calendar/contacts
     • Efficiency
   • Swapping memory between processes needs to be efficient - normally requires two context switches
     • Binder allows copies directly from user space of writer to user space of reader
   • Uses thread pool

4. Shared memory

   • Android kernel has new mechanism
     • Provides reference counting so that the kernel can reclaim resources

5. Alarm timers

   • Normal linux kernel timers not sufficient for advanced power management
   • Alarm timers wake up particular process even if system is suspended
     • Application will grab wake lock once woken up

6. Low memory killer

   • Introduces separate process killers

7. Security Enhancements

   • Each app has separate user id
   • All data exchange between apps forced to go via binder
     • Fine grain access control
   • Selinux kernel extensions allow user to specify which file can be accessed by which process

8. Network Security

   • Linux kernel allows any process to open sock and initiate network communication
     • Android introduced network permissions, filter by groupID
       • Permissions need to access, IP, BlueTooth, or raw sockets

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