linux-ext4 - Re: [RFC] ext4 metadata checksumming design

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Message-ID: <20110818181423.GR20655@tux1.beaverton.ibm.com>
Date:	Thu, 18 Aug 2011 11:14:23 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Joel Becker <jlbec@...lplan.org>,
	Mingming Cao <cmm@...ibm.com>,
	Amir Goldstein <amir73il@...il.com>,
	Coly Li <colyli@...il.com>, Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC] ext4 metadata checksumming design

On Thu, Aug 18, 2011 at 12:16:00AM -0600, Andreas Dilger wrote:
> On 2011-08-16, at 9:25 PM, "Darrick J. Wong" <djwong@...ibm.com> wrote:
> > - Extended attribute blocks that are stored in the inode table -- the h_magic
> >  field is written by the kernel, but neither the kernel nor e2fsprogs ever
> >  actually read this field.  The field could be reused to checksum the extra
> >  space since (as far as I can tell) EAs are the only user of that empty space.
> 
> I haven't had a chance to read the document you wrote, but wanted to comment
> on xattrs. There is a hash field for each xattr (including internal xattrs),
> and one for the external xattr blocks that can be used to validate the xattr
> value.
> 
> In addition to the hash for the in-inode xattrs, the inode hash itself would
> serve to validate the xattr values. 
> 
> I have a patch for e2fsprogs that checks the xattr hash for in-inode xattrs
> (currently it is always 0).

I surveyed the h_hash/e_hash calculation code; it only covers the name and
value fields.  Do we care about checksum protection for the extra fields in
struct ext4_xattr_header and struct ext4_xattr_entry?  I think it would be
useful to be able to check the sanity of h_refcount and h_blocks.  Possibly
that extends to e_value_* as well, though the hash probably covers it.  Also,
there's no hardware acceleration available for the xattr hash, though I doubt
xattrs are especially performance sensitive.

--D
> 
> > Please have a look at the design document and please feel free to suggest any
> > changes.
> 
> Hopefully soon. 
> 
> Cheers, Andreas--
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