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

Dan Carpenter
2017-08-10T13:14:39Z

On Thu, Aug 10, 2017 at 03:02:53PM +0200, Christian König wrote:
> 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?

That wouldn't silence it and I think that's not super pretty either.

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

I'm just going to ignore the warning. This driver isn't part of my
.config so I'm not really compiling it the way it was designed which
means I don't have the cross function database enabled. Probably if I
compiled this normally, I wouldn't even get the warning.

regards,
dan carpenter