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

Christoph Hellwig
2013-11-04T09:16:44Z

On Wed, Oct 30, 2013 at 03:31:18PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@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.