Ext3 users - Sep 2015 - parse raw image to read block group desc table!

Hi,

I am writing a piece of code to open a raw image file of a virtual machine
which has ubuntu installed in it. The virtual disk is formatted using MBR
partitioning method and has 3 primary and 1 extended partition.

I want to open up that file and read the block group descriptor table and
inode table for each partition. I have written some lines of code and
successfully able to read the partition entries and the superblock of each
partition.

But now I am stuck and unable to read  block group descriptor table and
inode table. Shall I post my code here? Any suggestion or resource would be
of great help.

Thanks,
Gagan
Mob: +91 97049 28427
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On Sep 28, 2015, at 01:07, gagan chhabra <gagan.13031990 at gmail.com>
wrote:
> 
> Hi,
> 
> I am writing a piece of code to open a raw image file of a virtual machine
which has ubuntu installed in it. The virtual disk is formatted using MBR
partitioning method and has 3 primary and 1 extended partition.
> 
> I want to open up that file and read the block group descriptor table and
inode table for each partition. I have written some lines of code and
successfully able to read the partition entries and the superblock of each
partition.
First question is why you are writing your own code to do this? Much
better would be to attach a loop device to the image file to handle
the partition table offset, then use libext2fs to open and access the
filesystem any way you want.

I don't think it is possible to directly access a filesystem within an image
directly, but if that is a requirement and loop devices cannot be used,
then this would be WAY easier to fix in libext2fs compared to rewriting
the full ext3/4 access yourself. 

That ensures it will be able to handle the myriad of different ext3/4
options and combinations thereof, both now and in the future. 

Cheers, Andreas
> But now I am stuck and unable to read  block group descriptor table and
inode table. Shall I post my code here? Any suggestion or resource would be of
great help.
> 
> Thanks,
> Gagan
> Mob: +91 97049 28427
> 
> _______________________________________________
> Ext3-users mailing list
> Ext3-users at redhat.com
> https://www.redhat.com/mailman/listinfo/ext3-users
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As Andreas mentioned, using libext2fs (http://giis.co.in/libext2fs.pdf) is
100 times easier than writing your own code
and I learned this hard way :p First two links are insane, I highly
recommend not to open them ;)  third link uses libext2fs.

https://github.com/Lakshmipathi/giis/blob/master/src/init.c#L195
http://giis.co.in/Kick_start.html

https://github.com/Lakshmipathi/giis-ext4/blob/master/src/giis-ext4.c



----
Cheers,
Lakshmipathi.G
FOSS Programmer.
www.giis.co.in