This Post shows you how to root GALAXY S2 GT-I9100 on 4.1.2

This rooting process is pretty safe to do, does not erase anything on your phone, it just installs a custom kernel (which comes with a custom recovery) and we root the phone using SU binary and Superuser app.  That's it!

#Step 1. Turn your Galaxy S2 GT-i9100 off.  Then hold down Volume Down, Center Home, and Power buttons together for about 3 seconds until you see the warning screen.

#Step 2. Hit Volume Up (you will be in ODIN Download mode) then connect a micro-USB cable from your Galaxy S2 to your computer.

#Step 3. Run ODIN3v1.85.exe by double-clicking on it.  If you don't see a yellow highlighted box with a random COM number

#Step 5. press on PDA and Choose the Jeboo kernel .tar file you downloaded earlier.

#Step 6. Hit the "Start" button and this will start flashing the kernel.  Once it says "PASS!" your Galaxy S2 GT-i9100 should reboot.

#Step 7. Once rebooted, download Superuser zip file straight to your phone or you can also copy it over from your computer.

#Step 8. Turn your phone again then hold down Volume Up, Center Home, and Power buttons together for about 20 seconds until your phone reboots into CWM Recovery.

#Step 9. Choose "Install Zip".

#Step 10. Choose "Choose zip from Internal sdcard" or "Choose zip from sdcard". (Depending on where the Superuser zip file is)

#Step 11. Find the Superuser zip file, it's most likely in your download folder.

#Step 12. Installing this file installs su binary and Superuser app to your Galaxy S2 GT-i9100, giving you full root access.

Reboot and you should find yourself with a SuperSU super user app.

YOUR DONE !!!!!!

JEEBO KERNEL http://downloadandroidrom.com/file/GalaxyS2/kernels/JB/jeboo_kernel_i9100_v1-2a.tar

CWM SUPERSU  http://downloadandroidrom.com/file/tools/SuperSU/CWM-SuperSU-v0.99.zip

ODIN    http://stockroms.net/file/Tools/ODIN/Odin3v185.zip