From: Zheng Liu <gnehzuil.liu@gmail.com>
Subject: Re: Which features should I implement in my ext4 reader?
Date: Mon, 2 Sep 2013 20:07:18 +0800
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Cc: Theodore Ts'o <tytso@mit.edu>, linux-ext4@vger.kernel.org
To: Felipe Monteiro de Carvalho <felipemonteiro.carvalho@gmail.com>
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Hello,

Sorry for my late reply.

On Mon, Sep 02, 2013 at 11:40:15AM +0200, Felipe Monteiro de Carvalho wrote:
> Hello,
> 
> Ok, I found the extra data over 60 bytes ... it is in an extended
> attribute called "data" in the space after the inode. The wiki says it
> should be called "system.data" =(
> 
> Also the wiki says here
> https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#Extended_Attributes
> that "Extended attributes, when >>>not<<<< stored after the inode,
> have a header ext4_xattr_ibody_header that is 4 bytes long:"
> 
> Which in my oppinion is false, it should be: "Extended attributes,
> when stored after the inode, have a header ext4_xattr_ibody_header
> that is 4 bytes long:"  without the "not"
> 
> But I still am not sure of one thing: How can I know how many entries
> of extended attributes there are? Nothing in the wiki explains this =(

You need to traverse the all entries to get this value.

> 
> Otherwise I cannot differentiate between yet another entry and simply
> data from an extended attribute...

You can use attribute name and name index to differentiate these
entries.  You can read the comment at the front of fs/ext4/xattr.c and
it might help you to understand extended attribute.

Regards,
                                                - Zheng