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

walter harms
2017-08-10T13:17:43Z

Am 10.08.2017 15:02, schrieb Christian König:
> 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.
>

Why not keeping Dan 1. patch and add a comment that pages_per_frag is always >31 ?

Using 32bit in a 64bit is not forbidden, and changing it causes more problems than
it solves. But doing so should be done in a clean way.

just my 2 cents,
re,
wh