Re: [PATCH] iommu/vt-d: Fix shift overflow in qi_flush_dev_iotlb

[Hook, Gary]

On 12/13/2017 11:15 AM, Alex Williamson wrote:
> On Wed, 13 Dec 2017 10:41:47 -0600
> "Hook, Gary" <ghook@xxxxxxx> wrote:
>
> > On 12/13/2017 9:58 AM, Alex Williamson wrote:
> > > On Wed, 13 Dec 2017 15:13:55 +0800
> > > Peter Xu <peterx@xxxxxxxxxx> 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.