linux-kernel - Re: [PATCH] iommu/vt-d: Fix shift overflow in qi_flush_dev

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Message-ID: <b8d7c8fd-8370-a0f0-e5db-f211423d7b9b@amd.com>
Date:   Wed, 13 Dec 2017 11:31:02 -0600
From:   "Hook, Gary" <ghook@....com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>, iommu@...ts.linux-foundation.org,
        dwmw2@...radead.org, linux-kernel@...r.kernel.org,
        tursulin@...ulin.net
Subject: Re: [PATCH] iommu/vt-d: Fix shift overflow in qi_flush_dev_iotlb

On 12/13/2017 11:15 AM, Alex Williamson wrote:
> On Wed, 13 Dec 2017 10:41:47 -0600
> "Hook, Gary" <ghook@....com> wrote:
> 
>> On 12/13/2017 9:58 AM, Alex Williamson wrote:
>>> On Wed, 13 Dec 2017 15:13:55 +0800
>>> Peter Xu <peterx@...hat.com> wrote:
>>>    
>>>> On Tue, Dec 12, 2017 at 03:43:08PM -0700, Alex Williamson wrote:
>>>>
>>>> [...]
>>>>   
>>>>> diff --git a/drivers/iommu/dmar.c b/drivers/iommu/dmar.c
>>>>> index 9a7ffd13c7f0..87888b102057 100644
>>>>> --- a/drivers/iommu/dmar.c
>>>>> +++ b/drivers/iommu/dmar.c
>>>>> @@ -1345,7 +1345,9 @@ void qi_flush_dev_iotlb(struct intel_iommu *iommu, u16 sid, u16 qdep,
>>>>>    	struct qi_desc desc;
>>>>>    
>>>>>    	if (mask) {
>>>>> -		BUG_ON(addr & ((1 << (VTD_PAGE_SHIFT + mask)) - 1));
>>>>> +		BUG_ON((mask > MAX_AGAW_PFN_WIDTH) ||
>>>>> +		       ((mask == MAX_AGAW_PFN_WIDTH) && addr) ||
>>>>> +		       (addr & ((1 << (VTD_PAGE_SHIFT + mask)) - 1)));
>>>>
>>>> Could it work if we just use 1ULL instead of 1 here?  Thanks,
>>>
>>> In either case we're talking about shifting off the end of the
>>> variable, which I understand to be undefined.  Right?  Thanks,
>>
>> How so? Bits fall off the left (MSB) end, zeroes fill in the right (LSB)
>> end. I believe that behavior is pretty set.
> 
> Maybe I'm relying too much on stackoverflow, but:
> 
> https://stackoverflow.com/questions/11270492/what-does-the-c-standard-say-about-bitshifting-more-bits-than-the-width-of-type

No, probably not. I don't have my copy of c99 handy, so can't check it. 
But it is beyond me why any compiler implementation would choose to use 
a rotate instead of a shift... probably a performance issue.

So, yeah, when you have silly parameters, you get what you get.

I'll stick to my suggestion. Which seems unambiguous... but I could be 
wrong.