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 Subject Re: [PATCH 11/28] ext4: Calculate and verify inode checksums From Andreas Dilger <> Date Wed, 12 Oct 2011 18:02:21 -0600
`On 2011-10-12, at 3:03 PM, Darrick J. Wong wrote:> On Wed, Oct 12, 2011 at 12:45:01PM -0700, Andreas Dilger wrote:>>> +static __u32 ext4_inode_csum(struct inode *inode, struct ext4_inode *raw,>>> +			      struct ext4_inode_info *ei)>>> +{>>> +	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);>>> +	__u16 crc_lo;>>> +	__u16 crc_hi = 0;>>> +	__u32 crc;>>> +>>> +	crc_lo = raw->i_checksum_lo;>>> +	raw->i_checksum_lo = 0;>>> +	if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE &&>>> +	    EXT4_FITS_IN_INODE(raw, ei, i_checksum_hi)) {>>> +		crc_hi = raw->i_checksum_hi;>>> +		raw->i_checksum_hi = 0;>>> +	}>>> +>>> +	crc = ext4_chksum(sbi, ei->i_uuid_inum_crc, (__u8 *)raw,>>> +			  EXT4_INODE_SIZE(inode->i_sb));>>> +>>> +	raw->i_checksum_lo = crc_lo;>>> +	if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE &&>>> +	    EXT4_FITS_IN_INODE(raw, ei, i_checksum_hi))>>> +		raw->i_checksum_hi = crc_hi;>>> +>>> +	return crc;>>> +}>> >> This computes both the _lo and _hi parts of the checksum and overwrites>> what is in the inode...> > I don't follow your logic ... for the _lo component, first I save the old> i_checksum_lo contents in crc_lo.  Then I stuff zero into i_checksum_lo.> Next I perform the checksum computation (with the checksum field> effectively "zero") and put the results into crc.  Then I copy whatever> I saved in crc_lo back into i_checksum_lo.> > crc_lo, crc_hi, and crc are three separate variables, and neither crc_lo> nor crc_hi are ever assigned any part of crc.  Therefore crc_lo and> crc_hi should always contain the old checksum contents.> > Did I miss something?  Afaict the contents of raw should be the same> before and after the call to ext4_inode_csum(), but maybe I've been> looking at this too long. :)No, you are right.  I misread the code and thought that crc_lo and crc_hiwere derived from the computed value.Cheers, Andreas`

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