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

[Michel Thierry <michel.thierry@intel.com>]

On 4/30/2015 3:53 PM, 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))
> 	WARN_ON((start + length) & (~0ULL << 32))
> 	WARN_ON((start + length) >> 32) != 0)

Yes, I was planning to just replace _31_ with _32_ and avoid all those 
zeroes...

>
> .Dave.
>
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