1. Understanding Android’s
System
• Mir Saman Tajbakhsh
• https://mstajbakhsh.ir

2. Headlines
 Android’s System Image
 System Modification
 System and Platform Apps
 SELinux and MAC Permissions
 System Rebuild
 OEM and Boot

3. Android System Image
 All Android systems has a system.img which is the main OS including apps and configs
 The system image is mountable since it is in ext4 file system usually.
 But the image should be converted to raw image before mounting.
 The Android’s Sparge image can be converted to Raw image using this tool:
 https://github.com/anestisb/android-simg2img

4. System Modification
 First of all, we should mount the raw image.
 sudo mount –t ext4 –o loop system.img.raw MOUNT_POINT
 Then all the system.img is modifiable.
 One interesting folder is system📂which contains all system apps and private apps

5. System and Platform Apps
 System apps are applications that placed in /system/ folder.
 They can not be removed without root, but can be disabled.
 If you’re root, you can move apps to /system folder, so change your app from third party app to
system app.
 If the application has same signature as the system, therefore you’re platform app and then you
have a lots of abilities.

6. SELinux and MAC Permissions
 All file and folders have their special SELinux permission.
 The permission should be applied on your apps.
 After applying the SELinux permission, you can renew the system signature by creating new
signature.
 The new signature is placed in MAC.

7. System Rebuild
 After changing the system, you can rebuild the system
 After that, you can make the system.img for AP.tar

8. OEM and Boot
 For flashing the system, you should firstly unlock the phone’s boot loader.
 You can search the Internet for this matter.
 After that, you can flash the custom AP.tar with custom vbmeta for ignoring boot signature of the
system.img.

9. Resources
1. Gist by tonio-nucci
https://gist.github.com/tonio-nucci/8ff2894c42c6320a50dc4b70b32a716a
2. https://github.com/mirsamantajbakhsh/AndroidSystemModification
3. https://youtu.be/yZgYIaSm2ZE
4. https://mstajbakhsh.ir/android-system-image-modification