[Intel-gfx] [PATCH] drm/i915: Make sure fb gtt offsets stay within 32bits

[Chris Wilson]

Quoting Ville Syrjälä (2018-09-21 14:06:02)
> On Thu, Sep 20, 2018 at 09:07:35PM +0100, Chris Wilson wrote:
> > Quoting Ville Syrjala (2018-09-20 20:10:18)
> > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > 
> > > Let's try to make sure the fb offset computations never hit
> > > an integer overflow by making sure the entire fb stays
> > > below 32bits. framebuffer_check() in the core already does
> > > the same check, but as it doesn't know about tiling some things
> > > can slip through. Repeat the check in the driver with tiling
> > > taken into account.
> > > 
> > > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/intel_display.c | 18 +++++++++++++++++-
> > >  1 file changed, 17 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > > index e642b7717106..67259c719ffe 100644
> > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > @@ -2400,10 +2400,26 @@ static int intel_fb_offset_to_xy(int *x, int *y,
> > >                                  int color_plane)
> > >  {
> > >         struct drm_i915_private *dev_priv = to_i915(fb->dev);
> > > +       unsigned int height;
> > >  
> > >         if (fb->modifier != DRM_FORMAT_MOD_LINEAR &&
> > > -           fb->offsets[color_plane] % intel_tile_size(dev_priv))
> > > +           fb->offsets[color_plane] % intel_tile_size(dev_priv)) {
> > > +               DRM_DEBUG_KMS("Misaligned offset 0x%08x for color plane %d\n",
> > > +                             fb->offsets[color_plane], color_plane);
> > >                 return -EINVAL;
> > > +       }
> > > +
> > > +       height = drm_framebuffer_plane_height(fb->height, fb, color_plane);
> > > +       height = ALIGN(height, intel_tile_height(fb, color_plane));
> > > +
> > > +       /* Catch potential overflows early */
> > > +       if (mul_u32_u32(height, fb->pitches[color_plane]) +
> > 
> > if (add_overflows(mul_u32_u32(height, fb->pitches[color_plane]),
> >                 fb->offsets[color_plane],
> >                 U32_MAX) {
> > ?
> 
> Didn't know we had that. Looks like what we have right now doesn't quite
> work as it only takes two arguments.

True, we would need the mul_overflows as well I guess (unless we are
happy that's eliminated earlier).
 
> Oh interesting. __builtin_add_overflow_p() & co. work supposedly work on
> bitfields as well.
> 
> Bah. Looks like this stuff generatess worse code than the normal thing:

True. Maybe harmless though since we are on an init path (and who
would reinitialise their fb on every update...) So the dilemma is having
if (add_overflows()) better than a /* check if add overflows */ comment,
and is that worth waiting on better code generation.
-Chris