[PATCH 27/30] libxfs: work around do_div() not handling 32 bit numerators

[Christoph Hellwig]

On Wed, Oct 30, 2013 at 03:31:18PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
> The libxfs dquot buffer code uses do_div() with a 32 bit numerator.
> This gives incorrect results as do_div() passes the numerator by
> reference as a pointer to a 64 bit value. Hence it does the division
> using 32 bits of garbage gives the wrong result. The kernel code
> handles 32 bit numerators just fine, so this patch is a temporary
> workaround in the dquot buffer code until we fix do_div() to handle
> 32 bit numerators correctly.

The right fix is to simply stop using do_div here, both in kernel and
userspace.

> 
>  xfs_calc_dquots_per_chunk(
>  	struct xfs_mount	*mp,
>  	unsigned int		nbblks)	/* basic block units */
>  {
> -	unsigned int	ndquots;
> +	uint64_t	ndquots;
>  
>  	ASSERT(nbblks > 0);
>  	ndquots = BBTOB(nbblks);
>  	do_div(ndquots, sizeof(xfs_dqblk_t));
>  
> -	return ndquots;
> +	return (int)ndquots;

	ndquots = BBTOB(nbblks) / sizeof(xfs_dqblk_t);


Also the current xfs_calc_dquots_per_chunk doesn't use the mp argument
anyway, but all callers do.  I think in the end we'd want to take that
in from the calers end end up with a version that includes the
check from the quotainfo.  But for getting userspace up and running my
above minimal version should do it.