Re: [PATCH 1/2] xfs: add CRC infrastructure

[Dave Chinner <david@fromorbit.com>]

On Thu, Nov 08, 2012 at 08:42:56AM -0600, Mark Tinguely wrote:
> On 11/07/12 07:37, Dave Chinner wrote:
> >+/*
> >+ * Calculate the intermediate checksum for a buffer that has the CRC field
> >+ * inside it.  The offset of the 32bit crc fields is passed as the
> >+ * cksum_offset parameter.
> >+ */
> >+static inline __uint32_t
> >+xfs_start_cksum(char *buffer, size_t length, unsigned long cksum_offset)
> >+{
> >+	__uint32_t zero = 0;
> >+	__uint32_t crc;
> >+
> >+	/* Calculate CRC up to the checksum. */
> >+	crc = crc32c(XFS_CRC_SEED, buffer, cksum_offset);
> >+
> >+	/* Skip checksum field */
> >+	crc = crc32c(crc,&zero, sizeof(__u32));
> >+
> >+	/* Calculate the rest of the CRC. */
> >+	return crc32c(crc,&buffer[cksum_offset + sizeof(__be32)],
> >+		      length - (cksum_offset + sizeof(__be32)));
> >+}
> 
> Why not zero the cksum field and then calculate the crc on the whole buffer?

Because we use the same calculation function for verification of the
CRC and we do not want to zero the CRC field when verifying it. It
would make subsequent verification calls on an unmodified buffer
fail.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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