Re: [PATCH] drm/gem: Fix inconsistent plane dimension calculation in drm_gem_fb_init_with_funcs()

[ashutosh desai <ashutoshdesai993@gmail.com>]

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Hi Jani,

Thank you for your question. It is a valid one. I have used AI to a small
extent to help me understand the subsystem because it was something new for
me. I haven't used AI to identify any bugs or create patches. This type of
bug is something that I am always on the lookout for (size checking and
boundary checks). So, these became evident during my review. Since AI
wasn’t used for code generation, I didn’t think attribution was required,
but please let me know if you’d prefer otherwise.

Would be happy to walk through any of the patches if it is helpful.

Best regards,
Ashutosh

On Fri, Apr 10, 2026 at 1:26 AM Jani Nikula <jani.nikula@linux.intel.com>
wrote:

> On Thu, 09 Apr 2026, Ashutosh Desai <ashutoshdesai993@gmail.com> wrote:
> > drm_gem_fb_init_with_funcs() computes sub-sampled plane dimensions
> > using plain integer division:
> >
> >   unsigned int width  = mode_cmd->width  / (i ? info->hsub : 1);
> >   unsigned int height = mode_cmd->height / (i ? info->vsub : 1);
> >
> > However, the ioctl-level framebuffer_check() in drm_framebuffer.c uses
> > drm_format_info_plane_width/height() which round up dimensions via
> > DIV_ROUND_UP(). This inconsistency corrupts the subsequent GEM object
> > size check for certain pixel format and dimension combinations.
> >
> > For example, with NV12 (vsub=2) and a 1-pixel-tall framebuffer the
> > GEM size validation path sees height=0 instead of height=1. The
> > expression (height - 1) then wraps to UINT_MAX as an unsigned int,
> > causing min_size to overflow and wrap back to a small value. A tiny
> > GEM object therefore passes the size guard, yet when the GPU accesses
> > the chroma plane it will read or write memory beyond the object's
> > bounds.
> >
> > Fix by replacing the open-coded divisions with
> drm_format_info_plane_width()
> > and drm_format_info_plane_height(), which use DIV_ROUND_UP() and match
> > the calculation already used in framebuffer_check().
> >
> > Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
>
> Hello Ashutosh -
>
> Receiving a handful of highly polished patches in quick succession on
> various parts of the drm subsystem from someone who has no commits in
> the kernel and has no previous interactions on the mailing lists is
> virtually unheard of.
>
> I have to ask, did you use AI coding assistants? Please read the kernel
> documentation on AI coding assistants and attribution [1].
>
>
> BR,
> Jani.
>
>
> [1] https://docs.kernel.org/process/coding-assistants.html
>
>
> > ---
> >  drivers/gpu/drm/drm_gem_framebuffer_helper.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> > index 9166c353f..88808e972 100644
> > --- a/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> > +++ b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> > @@ -172,8 +172,8 @@ int drm_gem_fb_init_with_funcs(struct drm_device
> *dev,
> >       }
> >
> >       for (i = 0; i < info->num_planes; i++) {
> > -             unsigned int width = mode_cmd->width / (i ? info->hsub :
> 1);
> > -             unsigned int height = mode_cmd->height / (i ? info->vsub :
> 1);
> > +             unsigned int width = drm_format_info_plane_width(info,
> mode_cmd->width, i);
> > +             unsigned int height = drm_format_info_plane_height(info,
> mode_cmd->height, i);
> >               unsigned int min_size;
> >
> >               objs[i] = drm_gem_object_lookup(file,
> mode_cmd->handles[i]);
>
> --
> Jani Nikula, Intel
>

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