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

[Dave Gordon <david.s.gordon@intel.com>]

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

.Dave.
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