Root unbootable Nexus 7?

[krisply]

hello folks
I have a rooted wifi-only N7 (WugFresh'ed) otherwise stock with 4.4.3 upgrade over 4.4.2 (after rooting). The device became unbootable after I edited the /system/build.prop file to include some permanent DNS lines as my DNS stopped working after the device ran out of storage (and stayed that way after reclaiming space).
I managed to introduce a typo in those lines as I was using echo >> on the command line and my eyes are not what they were. I reasoned that as the DNS line concerned only the secondary DNS server that I could afford to reboot.
Wrong!
The device booted as far as the static google logo and then went blank - that's it.
I am assuming my poor editing is causing the hang-up.
I installed MTP on an XP box and the box made suitable noises on N7 bootup, but no new icon under My Computer, but given the unreliable nature of XP/MTP I cannot know for sure it is working correctly so I cannot know whether the lack of icon is due to the device not being far enough into the boot sequence or not.
I can get fastboot and recovery menu (stock).

Now, I cannot afford to lose my data, so my first question is: can I rescue my data?

If it is likely, how should I proceed?
I'm aware of the use of adb for solving boot-looping problems but this doesn't look like a case of that.
I'm aware of CWM/TWRP custom recoveries and CM10 done on a previous phone.
I'm not familiar enough with Android to know how the apps/data/settings and user-data are arranged in the filesystem, but I would like to avoid anything that resets the apps own data / installation, what would be considered the exact opposite of, say, XP behaviour of losing all knowledge of installed apps when only re-installing the OS over an existing one.

I'm reading tons of stuff on here and elsewhere but yet am not able to be confident of the effect of making changes - even as to whether installing CWM could be fatal!

Any suggestions other than the dread hard reset welcome!

 

[krisply]

Can anyone at least say if data loss can be avoided?

 

[breadnatty08]

Welcome to the forums, krisply.
It's a bit beyond my league, so I'll go with perhaps a dumb question. Are you certain you have USB Debugging enabled on your N7? I've only worked with ADB and Win7/8 so I'm not sure of the differences with XP.

 

[krisply]

thanks BN. I'm pretty sure debugging is turned off - I believe it needs to be on for adb to work, no? I'm now trying to install MTP on a vista system - naively I believed it would be easier than on an XP one!
My understanding is also that only with debugging off will MTP work.
I'm assuming at the moment that the boot is not far enough along to allow MTP access anyway and so I would like to know if any flashing will erase the existing filesystem or re-create it.

 

[Madbat]

When you rooted, did you install twrp recovery? Did you make a backup? If yes, boot into twrp and restore just the "system" from that backup. It won't mess with data.

 

[krisply]

thanks MB. there is only stock recovery and I have no backups.
can I install a custom recovery without disturbing the rest of the system?
can I then make a backup with it?

in regards to MTP: I have not been able to (as far as I can tell with an incompletely booting N7) get it going on either an XP or vista install. Would it in fact be straightforward to mount
the nexus FS (ext3?) if plugged into a linux box?

 

[krisply]

I have now setup an ubuntu system with various MTP implementations and the results are not good.
I am much more confident that the detail being spat out by linux shows the N7 not playing nicely, due I imagine to being too early in the boot process.

So no direct way of accessing the filesystem. That leaves the possibility of custom recovery.

Can anyone say whether installing TWRP or CWM will total the sdcard content/filesystem?

 

[Madbat]

I have now setup an ubuntu system with various MTP implementations and the results are not good.
I am much more confident that the detail being spat out by linux shows the N7 not playing nicely, due I imagine to being too early in the boot process.

So no direct way of accessing the filesystem. That leaves the possibility of custom recovery.

Can anyone say whether installing TWRP or CWM will total the sdcard content/filesystem?

Installing twrp only installs recovery, it does not touch any data. In fact, the most recent twrp allows mtp connection while in recovery. You can install it in wugs.
Never start messing with build props, or any system files really, without a custom recovery and making a backup first. This is android 101.
But now installing twrp you should be able to access your files through mtp while booted into recovery. After you get whatever data you're trying to get, you can flash a zip of 4.4.4 in twrp, or just fastboot flash a factory image and start over.
If you have data that you "cannot afford to lose" I would not start messing with the system files without a backup. Backup, backup, backup!

 

[jhawkkw]

If you can't get mtp to work, TWRP recovery also supports adb (and doesn't require enabling it through developer options). So you should be able to access the internal sdcard folder from the command line and invoke an adb pull command and thus copy the whole sdcard folder to your computer. This won't save app data, which would be saved by invoking the adb backup command

 

[krisply]

thanks guys, just what I wanted to hear but was too afraid to experiment!
I have a backup of /sdcard underway through TWRP.
I pulled/edited/pushed the /system/build.prop file to remove my added lines
and fully expected the lump to spring back into life as that was the only change I made - it didn't!
So now I am considering the stock images....

As I don't know if there is any difference in doing such and, say, replacing a linux kernel (no impact at all on the rest of the filesystem) I am still worried whether
apps will need re-installing, and if they did whether the existing config data for them would be overwritten .......
I am particularly concerned about my PGP and bitcoin keys under app control!
As as .img file I would guess it is written as a file into the filesystem and mounts at a point in the filesystem
as a filesytem in it's own right (and includes the kernel image) but doesn't alter the filesystem in any way other
way than in the normal course of kernel operation (ie. /proc, etc)
But that would _only_ be an educated *guess*!!!!

MB - if I flash a stock 4.4.3 image will I lose *anything*?

 

[krisply]

I have just tried the 'temporary' booting feature of WUG to try a stock 4.4.3 img and
the N7 still hangs. I would take that to mean the hold-up is something in the solid-state filesystem, a corrupted file perhaps.

 

[jhawkkw]

One ad-hoc fix that could also be applied is to download the factory image and flash the stock system.img, and boot.img files manually to avoid flashing the userdata.img file that would overwrite all the app data. I've done this to apply OS updates before the OTA has been pushed and not have to worry about losing data.

 

[Madbat]

One ad-hoc fix that could also be applied is to download the factory image and flash the stock system.img, and boot.img files manually to avoid flashing the userdata.img file that would overwrite all the app data. I've done this to apply OS updates before the OTA has been pushed and not have to worry about losing data.

That was going to be my advice as well. Only flash stock system.img and boot.img. data should stay intact. If you're not familiar with fastboot flash, wigs has an option to flash stock without wiping data. In options/flash stock, just select "without wiping data". If you know fastboot, all the better.

 

[krisply]

Deep joy! It's back!
As instructed I flashed just the system partition. With fastboot.
I now have TWRP installed too although Wug has it covered with a non-flash recovery boot.
And I got some insight into the original problem that caused all this: the loss of DNS in the wifi - after the flash it came back with wifi symbol in white; but I found the flash had upset the rooting (I thought that only effected the bootloader?!) so I re-rooted through wug which duly flashed a modded boot image; DNS worked for 5 minutes after that then the symbol turned orange and it worked no more, even after rebooting. So rooting strongly suspected. Also playstore no longer downloads.

But a result none the less. Thank you guys