Trying to flash a dd image I took from the system partition

Long-time UNIX guy here, but relatively new to the world of Android.

I have recently purchased an Asus MemoPAD (ME103K) ; I then became root, and took a `dd` image of the read-only `system` partition to the external SD card:

Code:

---

    $ su
    # dd if=/dev/block/platform/msm_sdcc.1/by-name/system \
            of=/storage/MicroSD/system.img bs=1M
    # ls -l /storage/MicroSD/system.img
    -rw-r--r-- 1 root root 2147483648 Sep 27 13:15 system.img

---

The size (exactly 2GiB) was a bit suspicious - could it be that this was because of the FAT32 partition on the SD card?

No, it was not - `tune2fs -l` revealed that this was indeed, a valid EXT4 image, exactly sized at 2GiB, which passed `fsck -f` with no errors at all.
And `fastboot` (from the linux machine attached to the tablet) concurred, after an `adb reboot bootloader`:

Code:

---

    linuxbox# fastboot getvar all
    (bootloader)  version-bootloader: 3.03
    (bootloader)  version-hardware: rev_c
    (bootloader)  variant: LEOPARDCAT 16G
    (bootloader)  version-baseband: H00_0.16.F_0521
    (bootloader)  serialno: 0a3dXXXX
    ...
    (bootloader)  partition-type:system: ext4
    (bootloader)  partition-size:system: 0x0000000080000000

---

That size, is indeed 2GB:

Code:

---

    linuxbox# python2 -c 'print 0x0000000080000000'
    2147483648

---

So, all is good - I have a backup of the image. Now to test restoring it.

I try to flash the system.img back to the tablet - to make sure I can recover from anything, the sort of bullet-proof backup we do in the Unix world (*e.g. restore contents of a drive via `dd if=backup.image of=/dev/sdXXX`*).

Everything related to `adb` and `fastboot` work flawlessly - so I try...

Code:

---

    linux_box# fastboot devices
    0a3dXXXX    fastboot

    linux_box# mount /dev/sdcard /mnt/sdcard
    linux_box# cp /mnt/sdcard/system.img .
    linux_box# fastboot flash system system.img
    error: cannot load 'system.img'

---

Hmm. I download and build the `android-tools-5.1.1` of my distribution from sources, adding debug information - and step in the debugger, to see this failure:

Code:

---

    # gdb --args fastboot flash system system.img
    ...

---

(sorry, can't paste links yet - see this image: http i stack imgur com 9bIEM.png

Interesting - even though I am in a 64bit machine, apparently there are issues that turn the file size "negative" (in a 32bit world, the file size of my image, 2^31, is indeed considered negative - to be exact, `-2147483648`.

OK, fine - how do they flash large image files in Android?

Googling, searching - turns out they use this `make_ext4fs` tool, that creates flashable images. In fact it is part of what I just compiled, so I might as well use it:

Code:

---

    # mkdir /system
    # mount -o loop,ro system.img /system
    # ls -l /system
    total 208
    drwxr-xr-x 106 root root  8192 Sep 17 22:24 app
    drwxr-xr-x  3 root 2000  8192 Sep 26 21:08 bin
    -rw-r--r--  1 root root  6847 Sep 12 16:59 build.prop
    drwxr-xr-x  19 root root  4096 Sep 26 21:08 etc
    drwxr-xr-x  2 root root  4096 Aug 11 22:27 fonts
    drwxr-xr-x  4 root root  4096 Sep 12 16:56 framework
    drwxr-xr-x  10 root root  16384 Sep 12 16:59 lib
    drwxr-xr-x  2 root root  4096 Jan  1  1970 lost+found
    drwxr-xr-x  3 root root  4096 Aug 11 22:18 media
    drwxr-xr-x  59 root root  4096 Aug 11 22:29 priv-app
    -rw-r--r--  1 root root 126951 Aug  1  2008 recovery-from-boot.p
    drwxr-xr-x  3 root root  4096 Aug 11 21:02 scripts
    drwxr-xr-x  3 root root  4096 Aug 11 21:02 tts
    drwxr-xr-x  11 root root  4096 Sep 26 21:08 usr
    drwxr-xr-x  8 root 2000  4096 Aug 11 22:29 vendor
    drwxr-xr-x  2 root 2000  4096 Sep 26 21:09 xbin
       
    # ../extras/source/extras/ext4_utils/make_ext4fs \
          -l 2048M new_system.img /system
    Creating filesystem with parameters:
        Size: 2147483648
        Block size: 4096
        Blocks per group: 32768
        Inodes per group: 8192
        Inode size: 256
        Journal blocks: 8192
        Label:
        Blocks: 524288
        Block groups: 16
        Reserved block group size: 127
    Created filesystem with 2666/131072 inodes and 375014/524288 blocks

---

Cool - so I can apparently build system images from plain old folders. The sky will be my limit - I'll be able to add anything I want to this image.

Let's burn it...

Code:

---

    # fastboot flash system new_system.img
    erasing 'system'...
    OKAY [  0.064s]
    sending 'system' (2088960 KB)...
    ^C

---

I waited for 1h before hitting that Ctrl-C. And had to power-cycle the tablet, which booted back in fastboot mode.

This is not looking good.

What if I build a smaller image? Maybe the 2GB are somehow an issue, and this partition is not used to full capacity - it has free space:

Code:

---

    # ../extras/source/extras/ext4_utils/make_ext4fs \
          -l 1536M new_system.img /system
   
    #  ./fastboot flash system system.img
    erasing 'system'...
    OKAY [  0.065s]
    sending 'system' (1572864 KB)...
    OKAY [ 51.039s]
    writing 'system'...
    OKAY [235.080s]
    finished. total time: 286.183s

---

OK, this looks very promising (and only took 5 min). I guess I can now reboot back and everything should be normal, yes?

No :)

(sorry, can't paste links yet - see this image: http i stack imgur com U7wiX.png

I don't mind a temporarily bricked device, as long as I **do** get to control it in the end (machines that I am not a master of, are machines I don't care to operate ;)

Any ideas on what I did wrong and what I can do to fix this?

Thanks in advance.

P.S. I checked the Asus support page for my tablet - they only provide the sources for the kernel, and the Over-the-air .zip file. That in turn contains a file-system level backup from the root - i.e. the `system` folder exists in there as just a folder, not an image, not a `system.img` that I can flash - so that doesn't really help me.