Extract files from stock firmware images
when I still hadn't the device, I wanted to know exactly what's included in stock ROMs to have a better idea of what to expect. I hence downloaded a stock firmare and the stock system.img (see below for the steps).
Ok, so what? Well, when KK was released I decided to do the same (I was still waiting for the device), but I couldn't. Unlike before, I didn't find a single system.img, but multiple files (3 to be exact, maybe it's too big to be flashed at once with fastboot, I don't know, I'm new to this) and couldn't understand how the original image was splitted to generate those files.
Did anyone see something similar already and sucesfully merged splitted filesystems?
I know I could simply ask for a system dump (or wait for KK), but now I'm curious to know on how to do this. I tried few things but I couldn't find any way to do it. Maybe I could see how fastboot treat these files, but I wonder if anyone already knows the answer.
Anyway, here the steps to mount the system.img of our stock JB firmwares. Maybe there's an easier way, I honestly don't know. As far as I know, converting the sparge image should be enough, but I had to do more:
#Convert sparse image with simg2img
simg2img system.img system.img.raw.tmp
#UTF8 may slow down grep, switch to C
#Look for the ext4 magic and calculate its position
magic=`grep -aobP -m1 '\x53\xEF' system.img.raw.tmp | head -1 | cut -d":" -f1`
#Remove extra header with dd
dd if=system.img.raw.tmp of=system.img.raw ibs=$offset skip=1
#Remove temp file
Now you can mount system.img.raw
as a normal ext4 filesystem.