Re: [PATCH] drm/i915: Fix 32b overflow check in gen8_ppgtt_alloc_page_directories

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Apr 30, 2015 at 03:53:29PM +0100, Dave Gordon wrote:
> On 30/04/15 15:33, Michel Thierry wrote:
> > On 4/30/2015 3:22 PM, Ville Syrjälä wrote:
> >> On Thu, Apr 30, 2015 at 02:59:34PM +0100, Michel Thierry wrote:
> >>> The patch 69876bed7e008f5fe01538a2d47c09f2862129d0: "drm/i915/gen8:
> >>> page directories rework allocation" added an overflow warning, but the
> >>> mask had an extra 0. Use typo-prone option suggested by Dave instead.
> >>>
> >>> This check will be unnecessary after gen8_alloc_va_range handles more
> >>> than 4 PDPs (48b addressing).
> >>>
> >>> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> >>> Cc: Dave Gordon <david.s.gordon@intel.com>
> >>> Signed-off-by: Michel Thierry <michel.thierry@intel.com>
> >>> ---
> >>>   drivers/gpu/drm/i915/i915_gem_gtt.c | 4 ++--
> >>>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c
> >>> b/drivers/gpu/drm/i915/i915_gem_gtt.c
> >>> index 6fae6bd..6d894fc 100644
> >>> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> >>> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> >>> @@ -756,8 +756,8 @@ static int
> >>> gen8_ppgtt_alloc_page_directories(struct i915_hw_ppgtt *ppgtt,
> >>>
> >>>       WARN_ON(!bitmap_empty(new_pds, GEN8_LEGACY_PDPES));
> >>>
> >>> -    /* FIXME: PPGTT container_of won't work for 64b */
> >>> -    WARN_ON((start + length) > 0x800000000ULL);
> >>> +    /* FIXME: upper bound must not overflow 31 bits  */
> >>> +    WARN_ON((start + length) & (~0ULL << 31));
> >>
> >> Why is it 31 and not 32?
> > 
> > Right, the check really should be (start + length) >= 0x100000000ULL.
> 
> Something with '32' in it might be more obvious and save anyone having
> to count the zeroes ... and a comment that also mentioned the limit:
> 
> 	/* FIXME: for now, upper bound must fit in 32 bits  */
> 
> 	WARN_ON((start + length) >= (1ULL << 32))

This would match the '1ULL<<32' used in the ppgtt init.

> 	WARN_ON((start + length) & (~0ULL << 32))
> 	WARN_ON((start + length) >> 32) != 0)
> 
> .Dave.

-- 
Ville Syrjälä
Intel OTC
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