[RFC] mess in jbd2_block_tag_csum_verify()

[Al Viro <viro@ZenIV.linux.org.uk>]

You have
	calculated = jbd2_chksum(j, calculated, buf, j->j_blocksize);
	provided = be32_to_cpu(tag->t_checksum);

	return provided == cpu_to_be32(calculated);

in there, which makes no sense whatsoever.  First of all, you are
converting big-endian to native, then another native to big-endian
and compare results.  The bogosity aside, it's equivalent to simply
comparing tag->t_checksum with calculated - cpu_to_be32() is the
same mapping as be32_to_cpu() on all architectures and it's a one-to-one
mapping, at that.

Bogosity, of course, is that tag->t_checksum is apparently big-endian
and definitely a 16bit value.  How in hell is that check going to
yield true?  Note that you are asking for 16 bits out of crc32c result
to be zero, _NOT_ to be ignored.

Producer of that value shoves lower 16 bits of cpu_to_be32(crc) into the
on-disk structure.  Also a bloody bad idea, since the values on little-endian
and big-endian hosts will be different; move the disk from one box to
another and watch the mismatches...

What the hell is going on there?