Re: [PATCH] drm/amdgpu: potential shift wrapping bug

Christian König
2017-08-10T13:02:53Z

Am 10.08.2017 um 14:53 schrieb Dan Carpenter:
> On Thu, Aug 10, 2017 at 02:30:15PM +0200, Christian König wrote:
>> Am 10.08.2017 um 14:16 schrieb Dan Carpenter:
>>> "frag_align" is a u64, so presumably we want to use the high bits as
>>> well instead of shift wrapping.
>>>
>>> Fixes: 6be7adb37d9b ("drm/amdgpu: increase fragmentation size for Vega10 v2")
>>> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
>> The fragment field has only 5bits in hardware and can never be more than 31,
>> so the correct fix would actually be using uint32_t here instead.
>>
> Changing it to uint32_t introduces a new static checker warning:
>
> drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c:1465 amdgpu_vm_frag_ptes()
> warn: was expecting a 64 bit value instead of '~(frag_align - 1)'
>
> Unfortunately, I get so many thousands of those I can't normally even
> review that sort of bug...
>
> Let me resend the original patch but with a modified changelog to say
> that the bug is a false positive.

Ah, yes of course that's why I made it a 64bit value in the first place.

Mhm, could we use something like (u32)(1 << pages_per_frag) instead to
silence the static checker warning?

It doesn't make much sense to use a 64bit shift here.

Christian.

>
> regards,
> dan carpenter
>