Re: Writing to superblock?

["Niels Böhm" <bitbucket@arcor.de>]

On Friday 20 February 2009, BandiPat wrote:
>
> Anyway, the writer of the script ask me a question I have not been able
> to find anything about, so I thought I would come straight to you guys!
>   I already know that Grub2 will work with XFS and will write to the
> MBR, but he is asking about Superblocks also when using XFS.  Will it?

If I understand you correctly, you talk about writing GRUB to a root sector of 
a partition that contains a XFS filesystem (which happens to coincide with 
the start of the superblock of XFS' first allocation group), right?

I mean the traditional way to execute raw boot code from a partition is to 
chainload its first sector and jump to its first byte, which means there has 
to be boot code at this place or at the very least a jump instruction that 
points to the real boot code.

But I don't think this is possible with XFS since it has its magic number in 
the first 4 bytes. It looks like this:

00000000  58 46 53 42 00 00 10 00  00 00 00 00 00 20 00 00  |XFSB......... ..|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020  19 c5 80 51 80 34 4a c9  b6 aa c5 44 0b 16 c7 7b  |...Q.4J....D...{|
etc.

For this, also have a peek at 
http://xfs.org/index.php/XFS_FAQ#Q:_Does_LILO_work_with_XFS.3F


The alternative would be to find another unused cave with stable location in 
XFS' metadata, although I doubt there is one. But you may want to check more 
deeply at http://oss.sgi.com/projects/xfs/papers/xfs_filesystem_structure.pdf


Regards,
Niels Böhm