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:

quoted

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