Long-time UNIX guy here, but relatively new to the world of Android. Read on. **EPISODE 1: A New Backup (I hoped)** 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: $ 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`: 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: 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... linux_box# fastboot devices 0a3dXXXX fastboot linux_box# mount /dev/sdcard /mnt/sdcardfor a New Backupfor a New Backup 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: # gdb --args fastboot flash system system.img ... [![Failure due to negative size!][1]][1] 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: # 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... # 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: # ../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 :-) [![enter image description here][2]][2] 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. **EPISODE 2: Attack of the Custom Boots** In the absense of any sort of `recovery.img` from Asus (why would a manufacturer bother to publish a fastboot-flashable `recovery.img`? Why indeed...) and a similar absence on recovery images from the CWM and TWRP sites... I am left to battle all alone. Thankfully, the Over-the-air update file from Asus includes inside it... # unzip -l /opt/vendor/Asus/firmware/UL-K01E-WW-12.16.1.12-user.zip |\ grep boot.img$ 7368704 2011-03-22 11:21 boot.img ...my tablet's boot image. Now maybe - just maybe - I can do something with this. $ mkdir rootfs $ cd rootfs $ abootimg -x /path/to/boot.img $ ls -l bootimg.cfg initrd.img zImage Expanding the ramdisk... $ mkdir initrd $ cd initrd $ gzip -cd ../initrd.img | cpio -ivd ... $ vi default.prop First, let's set up to be root on the booted kernel: ro.secure=0 ro.debuggable=1 ro.adb.secure=0 androidboot.selinux=disabled I also copied the `/system/bin/sh` into `/sbin/sh` - from the over-the-air Asus .zip file. I did the same for [busybox](https://github.com/Gnurou/busybox-android). And repacked the boot.img... $ find . | cpio --create --format='newc' | gzip -9 > ../initrd.custom.gz $ cd .. $ abootimg --create ../new_boot_busybox.img \ -f bootimg.cfg -k zImage -r initrd.custom.gz This actually failed the first time, since I had to update the `bootimg.cfg` - the `bootsize` parameter had to the updated since the package is bigger now. `abootimg` reports what it needs, so that's easy enough. And now, I boot my custom image... And witness the following... # adb logcat - exec '/system/bin/sh' failed: Permission denied (13) - # adb shell - exec '/system/bin/sh' failed: Permission denied (13) - Hmm... Maybe adbd is not run as root? # adb root restarting adbd as root # adb shell - exec '/system/bin/sh' failed: Permission denied (13) - Fine... I hexedit adbd, and patch /system/bin/sh to be /sbin/sh (I copied the /system/bin/sh from the OTA image to the rootfs of the initrd): Reboot, fastboot... # adb shell - exec '/sbin/sh' failed: Permission denied (13) - Darn. Is this thing able to do anything? # adb pull /proc/partitions 15 KB/s (1272 bytes in 0.079s) It is... let's see: # adb pull /proc/mounts 16 KB/s (1358 bytes in 0.079s) # grep system mounts /dev/block/platform/msm_sdcc.1/by-name/system /system ext4 rw,seclabel,relatime,data=ordered 0 0 OK, so /system *is* mounted. Can I see what's inside? # adb pull /system remote object '/system' does not exist What the... Maybe I can check what /proc/kmsg contains (what "dmesg" would output) # adb pull /proc/kmsg failed to copy '/proc/kmsg' to './kmsg': Operation not permitted Nah, I need to be root to do that. # adb push /sbin/sh /system/bin/sh failed to copy '/sbin/sh' to '/system/bin/sh': Permission denied And that, too. This is turning out to be quite a puzzle... [1]: https://i.sstatic.net/9bIEM.png [2]: https://i.sstatic.net/U7wiX.png