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Old 09-23-2012, 11:33 AM
Jakub Kicinski
 
Default How to rescue your partitions after upgrade to Fedora 18 Alpha eats them

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
" 0x1AB2F or printf "%x
" $((7863*4096)) is
handy as well.

Good luck from (a sightly less happy) Fedora user
-- Kuba
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