linux-ext4 - Re: [PATCH 13/16] ext4: Calculate and verify checksums of extended attribute blocks

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Date:	Fri, 2 Sep 2011 11:43:32 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Andreas Dilger <adilger.kernel@...ger.ca>
Cc:	Theodore Tso <tytso@....edu>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Amir Goldstein <amir73il@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Mingming Cao <cmm@...ibm.com>,
	Joel Becker <jlbec@...lplan.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-ext4@...r.kernel.org, Coly Li <colyli@...il.com>
Subject: Re: [PATCH 13/16] ext4: Calculate and verify checksums of extended
	attribute blocks

On Thu, Sep 01, 2011 at 01:42:26AM -0600, Andreas Dilger wrote:
> On 2011-08-31, at 6:32 PM, Darrick J. Wong wrote:
> > Calculate and verify the checksums of extended attribute blocks.  This only
> > applies to separate EA blocks that are pointed to by inode->i_file_acl (i.e.
> > external EA blocks); the checksum lives in the EA header.
> > 
> > diff --git a/fs/ext4/xattr.h b/fs/ext4/xattr.h
> > index 25b7387..b2b20af 100644
> > --- a/fs/ext4/xattr.h
> > +++ b/fs/ext4/xattr.h
> > @@ -27,7 +27,8 @@ struct ext4_xattr_header {
> > 	__le32	h_refcount;	/* reference count */
> > 	__le32	h_blocks;	/* number of disk blocks used */
> > 	__le32	h_hash;		/* hash value of all attributes */
> > -	__u32	h_reserved[4];	/* zero right now */
> > +	__le32	h_checksum;	/* crc32c(uuid+inum+xattrblock) */
> 
> This comment is incorrect - the inum cannot be part of the checksum if
> the block is shared.  That said, I wouldn't object if the checksum DID
> include the inum if the block was not shared (h_refcount == 1).  Since
> the h_refcount needs to be modified and the checksum recomputed if other
> inodes share this block, this doesn't impose any extra overhead.

Oops, I guess I was typing too fast.  "inum" should be "ea_block_num".  Though
I suspect that in the common case files don't share EA blocks, so we could do
inum when refcount==1.

--D
> 
> Cheers, Andreas
> 
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