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Date: Thu, 8 Nov 2001 17:29:05 +0100 (CET)
From: Andreas Gruenbacher <ag@bestbits.at>
To: Luka Renko <luka.renko@hermes.si>
cc: Nathan Scott <nathans@sgi.com>, Linus Torvalds <torvalds@transmeta.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-xfs@hermes.si
Subject: RE: [RFC][PATCH] extended attributes
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On Thu, 8 Nov 2001, Luka Renko wrote:

> In http://acl.bestbits.at/man/extattr.5.html there is a statement:
> 
>    Device special files cannot be associated with extended user attributes 
> 
> What is the reason for this limitation? Why should there be a difference
> between regular files/directories and special files (device files)?

This limitation has been introduced since allowing extended user
attributes for special files leads to an awkward semantic mess: Extended
user attributes are in a sense treaded like file contents (concerning
permissions, for example). Now the "contents" of a special file are a
device, really. The device isn't even located on the filesystem the
special file is on. And the device doesn't have extended attributes.

It is possible to have devices on read-only file systems, which are
readable and writeable. It would not be possible to have extended
attributes in that case.

> I am also thinking in terms of HSM application (or DMAPI if you want). Where
> do you want HSM attributes to be placed? I thought it should be in trusted,
> because we might need access to them from user space. Other option is system
> (that would require accessing them from kernel code) or user (might be
> problematic, since regular user with write permission might remove them...

I am not sure about this. Just note that XFS and current ext2/ext3 don't
yet have the owner and trusted namespaces.

--Andreas.

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