How to rescue your partitions after upgrade to Fedora 18 Alpha eats them

Tue Sep 25 15:26:30 UTC 2012

Eddie G. O'Connor Jr. wrote:
> On 09/23/2012 07:22 PM, Roger wrote:
>> On 09/24/2012 02:09 AM, Eddie G. O'Connor Jr. wrote:
>>> On 09/23/2012 07:33 AM, Jakub Kicinski wrote:
>>>> Hi all,
>>>>
>>>> I recently had a weird problem after "upgrade" to Fedora 18 Alpha. I
>>>> decided to share the solution, maybe it will help someone.
>>>>
>>>> But first:
>>>>
>>>> 1. The rant
>>>>
>>>> Clearly something bad is going on in Fedora world. I've been
>>>> upgrading to Alphas/Betas for years now and it never caused more
>>>> trouble than occasional need to rebuild some packages manually.
>>>>
>>>> I read this:
>>>> http://lwn.net/Articles/506831/
>>>> but I didn't suspect that not only Rawhide, but also Alpha releases
>>>> became unusable.
>>>>
>>>> First of all PreUpgrade crashes before it starts. I thought, well, I'll
>>>> try DVD. It kind of worked. I was looking for words like "upgrade" or
>>>> "update" in the new and shiny anaconda but no luck. Eventually it
>>>> started the installation (without any "Warning: write changes to
>>>> disk?" or alike). At that point partitions of Fedora 17 where already
>>>> magically renamed to Fedora 18, so I hoped for the best. Unfortunately
>>>> anaconda crashed when it tried to format my Win7 partition to the
>>>> widely used filesystem "None"?! Imagine my amusement when after reboot
>>>> I realised that MBR and GPT where wiped out and all of my partitions
>>>> were nowhere to be found.
>>>>
>>>> 2. The problem
>>>>
>>>> It turned out anaconda not only wipes out MBR but also removes magic
>>>> numbers from partitions superblocks (anyone can explain why?). So the
>>>> good news was my data was still there, the bad news was: none of
>>>> automatic "lost partition detection" tool could find them (at least none
>>>> that I know of).
>>>>
>>>> 3. The solution
>>>>
>>>> If it happens to you as well, here is what you can try to get your
>>>> data back:
>>>>
>>>>   a. Accept that you do all this on your own risk ;)
>>>>   b. Backup your HDD, preferably all of it. Backup can be done with dd.
>>>>   c. Connect damaged HDD to some other Linux box or use a Live
>>>>      distribution.
>>>>   d. Now you need to locate your first partition. There is a good chance
>>>>      that this will be your /boot and Fedora usually locates it at
>>>>      0x100000 (i.e. it starts in 2048 sector). Other possibility is
>>>>      sector 63/64 from pre-GPT world.
>>>>   e. Set magic value in superblock (see below).
>>>>   f. Try mounting partition using offset option in mount.
>>>>   g. Calculate where next partition is most likely to start using block
>>>>      count and block size from superblock.
>>>>   h. Fix your MBR. You can probably do that in parted or some similar
>>>>      tool, but who would trust tools after one just nearly destroyed all
>>>>      your data.
>>>>   i. If you have more partitions to find, go back to (d).
>>>>
>>>>
>>>> Important: All values on disk are LITTLE ENDIAN.
>>>> Important: Try mounting your partitions read-only first.
>>>>
>>>> Notes on EXT4.
>>>> Ext4 is quite easy to find. You will recognize it by two zeroed out
>>>> sectors (1024 bytes of zeroes) at the beginning of every partition.
>>>> NOTE: those zeroes are *in* the partition, do not skip them when trying
>>>> to mount. After zeroes there is a superblock. What you are looking
>>>> for is offset +0x38 in the superblock, where you once had a magic value
>>>> of 0xEF53, you will probably have to put it back there. More information
>>>> on EXT4:
>>>> https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout
>>>>
>>>> Notes on NTFS.
>>>> NTFS partition starts with EB 52 90, there are no zeroes in front. Magic
>>>> value which is removed by anaconda is "4E 54 46 53 20 20 20 20"
>>>> at offset 0x03 into a partition. Its literally "NTFS    " (with spaces).
>>>> More info:
>>>> http://technet.microsoft.com/en-us/library/cc781134%28v=ws.10%29.aspx
>>>>
>>>> Useful tools:
>>>> dd - will help you to copy blocks of data around. If you don't know it
>>>> already, I advise you to read some tutorial and practise first. It's
>>>> easy to destroy thing with one wrong dd.
>>>>
>>>> emacs, okteta etc. - hex editor...
>>>>
>>>> losetup - is a tool which enables you to mount files and drives at
>>>> particular offset as loop devices. Very useful, read more here:
>>>> http://rackerhacker.com/2010/12/14/mounting-a-raw-partition-file-made-with-dd-or-dd_rescue-in-linux/
>>>>
>>>> dumpe2fs, nftsinfo - will decode superblock for you.
>>>>
>>>> bash calculator - $((1+2+3+4*4)) can save you a lot of time.
>>>> bash printf - printf "%d\n" 0x1AB2F or printf "%x\n" $((7863*4096)) is
>>>> handy as well.
>>>>
>>>> Good luck from (a sightly less happy) Fedora user
>>>>     -- Kuba
>>> WOW!.....that's terrible! But I guess that's just one of the reasons I
>>> purposely wait until a release has been out for almost 3 months....before
>>> installing / upgrading. It just makes it a little easier when the glitches
>>> have been made apparent and are either completely worked out or are in the
>>> process of being worked out. In all honesty...I actually prefer to wait a few
>>> releases before upgrading, I kind of have my OWN "schedule" I usually wait
>>> and let one release pass by, then I'll wait for the next version...and 3 to 4
>>> months into _that _I'll upgrade.....this helps me remain a little more
>>> "stable" and not have to scurry around upgrading every 6 months....
>>>
>>>
>>> EGO II
>>>
>>>
>> Yes I am much the same. I'll skip an upgrade or two, wait for stability then
>> after copying all my files, do a fresh install.
>> Roger
>>
>>
> It just seems that.....of all the different methods....THAT one is the safest.
> I've accumulated quite a few "irreplaceable" files throughout the years...and
> I'd hate to lose some or all of them due to a restructuring of the build for the
> installer!..
>
I invested in a Blu-Ray drive and a few blanks, in conjunction with dvdisaster I 
can back up 20GB of data with three levels of error recovery, on a media which 
is not subject to vibration or shock damage (within any reasonable limits), 
water resistant, and immune to magnetic damage and EMP. Stored off-site one disk 
covers all of my "really critical" data.

Investment is <$100 if you shop carefully.

-- 
Bill Davidsen <davidsen at tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot