From: Eric Sandeen <sandeen@redhat.com>
Subject: Re: File permissions in ext4
Date: Thu, 01 Aug 2013 12:27:56 -0500
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To: Felipe Monteiro de Carvalho <felipemonteiro.carvalho@gmail.com>
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On 8/1/13 7:34 AM, Felipe Monteiro de Carvalho wrote:
> Hello,
> 
> Ok, I found that the permissions that I was looking for are stored in
> ext4_inode.i_mode ... but still there is the question: What is ACL
> used for then, and when is it different than zero?
> 
> thanks =)
> 

It is the block number containing an extended attribute for that inode,
if any; that extended attribute may be a file ACL.

It's a bit odd to call the xattr block "ACL" but it is what it is.  ;)

-Eric