Re: [PATCH] drm/i915/display: Check source height is > 0

[Drew Davenport]

On Tue, Jan 03, 2023 at 12:42:43PM +0200, Juha-Pekka Heikkila wrote:
> Hi Drew,

Hi Juha-Pekka, sorry for the late response since I was on vacation.

> 
> this is good find. I went looking where the problem is in and saw what you
> probably also saw earlier.
> 
> I was wondering if diff below would be better fix? I assume this would end
> up with einval or erange in your case but code flow otherwise would stay as
> is while fixing all future callers for same issue:

Yes, the function you identify below is where I encountered
divide-by-zero errors. If width/height less than 1 is a legitimate use
case, then your proposed solution looks like it would be better. It
should have no risk of regression in userspace either, which is nice.

I tested your patch, and as expected I did not hit the divide-by-zero
error, though the test commit was rejected due to a check further along
inside skl_update_scaler. Perhaps there is some other configuration
which would pass the test commit with a width/height less than 1, but I
didn't dig much further.

> 
> diff --git a/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> index 10e1fc9d0698..a9948e8d3543 100644
> --- a/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> +++ b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> @@ -144,7 +144,7 @@ unsigned int intel_adjusted_rate(const struct drm_rect
> *src,
>                                  const struct drm_rect *dst,
>                                  unsigned int rate)
>  {
> -       unsigned int src_w, src_h, dst_w, dst_h;
> +       unsigned int src_w, src_h, dst_w, dst_h, dst_wh;
> 
>         src_w = drm_rect_width(src) >> 16;
>         src_h = drm_rect_height(src) >> 16;
> @@ -155,8 +155,10 @@ unsigned int intel_adjusted_rate(const struct drm_rect
> *src,
>         dst_w = min(src_w, dst_w);
>         dst_h = min(src_h, dst_h);
> 
> -       return DIV_ROUND_UP_ULL(mul_u32_u32(rate, src_w * src_h),
> -                               dst_w * dst_h);
> +       /* in case src contained only fractional part */
> +       dst_wh = max(dst_w * dst_h, (unsigned) 1);
> +
> +       return DIV_ROUND_UP_ULL(mul_u32_u32(rate, src_w * src_h), dst_wh);
>  }
> 
>  unsigned int intel_plane_pixel_rate(const struct intel_crtc_state
> *crtc_state,
> 
> 
> What do you think? I'll in any case come up with some test for this in igt.

I see that you've posted your fix to the list already. Adding a
test to cover this in IGT also sounds great. Thanks!

Breadcrumbs to Juha-Pekka's patch for anyone following this
thread: https://patchwork.freedesktop.org/series/112396/

> 
> /Juha-Pekka
> 
> On 27.12.2022 7.53, Drew Davenport wrote:
> > The error message suggests that the height of the src rect must be at
> > least 1. Reject source with height of 0.
> > 
> > Signed-off-by: Drew Davenport <ddavenport@xxxxxxxxxxxx>
> > 
> > ---
> > I was investigating some divide-by-zero crash reports on ChromeOS which
> > pointed to the intel_adjusted_rate function. Further prodding showed
> > that I could reproduce this in a simple test program if I made src_h
> > some value less than 1 but greater than 0.
> > 
> > This seemed to be a sensible place to check that the source height is at
> > least 1. I tried to repro this issue on an amd device I had on hand, and
> > the configuration was rejected.
> > 
> > Would it make sense to add a check that source dimensions are at least 1
> > somewhere in core, like in drm_atomic_plane_check? Or is that a valid
> > use case on some devices, and thus any such check should be done on a
> > per-driver basis?
> > 
> > Thanks.
> > 
> >   drivers/gpu/drm/i915/display/skl_universal_plane.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/skl_universal_plane.c b/drivers/gpu/drm/i915/display/skl_universal_plane.c
> > index 4b79c2d2d6177..9b172a1e90deb 100644
> > --- a/drivers/gpu/drm/i915/display/skl_universal_plane.c
> > +++ b/drivers/gpu/drm/i915/display/skl_universal_plane.c
> > @@ -1627,7 +1627,7 @@ static int skl_check_main_surface(struct intel_plane_state *plane_state)
> >   	u32 offset;
> >   	int ret;
> > -	if (w > max_width || w < min_width || h > max_height) {
> > +	if (w > max_width || w < min_width || h > max_height || h < 1) {
> >   		drm_dbg_kms(&dev_priv->drm,
> >   			    "requested Y/RGB source size %dx%d outside limits (min: %dx1 max: %dx%d)\n",
> >   			    w, h, min_width, max_width, max_height);
>