Default dump the bootloader on a i9300

i have a stock samsung galaxy S3 i9300 running android 4.1.2, i'm interested in obtaining a dump of the bootloader area and eventually of all the internal flash, taking a quick look in ADB it looks like it's based on EMMC.

Do you have any suggestion/guideline to help me accomplishing this?

i think "cleanest" way is by flashing a recovery via heimdall/odin with a custom recovery providing root adb access?

heimdall once had dumping capabilities but the devs decided to remove that, and in ODIN the dump feature seems to be grayed out...

thanks in advance for the help