Re: [PATCH 6/6] drm: Avoid undefined behavior on u16 multiplication

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 08 Sep 2025, Jani Nikula <jani.nikula@linux.intel.com> wrote:
> On Mon, 08 Sep 2025, Krzysztof Karas <krzysztof.karas@intel.com> wrote:
>> Fields hdiplay and vdisplay are defined as u16. Their
>> multiplication causes implicit promotion to signed 32-bit value,
>> which may overflow and cause undefined behavior.
>>
>> The same goes for vpos, which is multiplied by signed integer.
>>
>> Prevent possible undefined behavior by explicitly casting one of
>> the operands to (unsigned int) type.
>>
>> Fixes: 3ed4351a83ca ("drm: Extract drm_vblank.[hc]")
>> Fixes: cc4312127108 ("drm/tinydrm/mipi-dbi: Add mipi_dbi_init_with_formats()")
>> Fixes: 80f7c3f77697 ("drm/vram: Add helpers to validate a display mode's memory requirements")
>> Cc: Thomas Zimmermann <tzimmermann@suse.de>
>> Cc: Noralf Trønnes <noralf@tronnes.org>
>> Cc: Simona Vetter <simona.vetter@ffwll.ch>
>> Cc: <stable@vger.kernel.org> # v4.13+
>> Cc: <stable@vger.kernel.org> # v5.4+
>> Cc: <stable@vger.kernel.org> # v5.7+
>
> The multitude of files, Fixes and subsequent Cc: stables with different
> target kernels is just an indication this should be three separate
> patches.
>
> The cause may be the same, but they're still unrelated fixes.
>
> Moreover, imagine the consequences of one of these fixes ending up being
> incorrect, and you have to revert or fix it. It's a logistics PITA.

Also, depending on things introduced in v5.7 means this can't be
backported to v5.4 either. Not very important here, but something to
consider for the future.

BR,
Jani.


-- 
Jani Nikula, Intel