Re: [PATCH] drm/amd/display: Use 2-factor allocator calls

[Kees Cook]

On Thu, Jul 5, 2018 at 6:37 AM, Michel DÃnzer <michel@xxxxxxxxxxx> wrote:
> On 2018-07-04 07:27 PM, Kees Cook wrote:
>> As already done treewide, switch from open-coded multiplication to
>> 2-factor allocation helper.
>>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>  drivers/gpu/drm/amd/display/modules/color/color_gamma.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/amd/display/modules/color/color_gamma.c b/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
>> index 98edaefa2b47..ee69c949bfbf 100644
>> --- a/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
>> +++ b/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
>> @@ -1723,8 +1723,8 @@ bool  mod_color_calculate_curve(enum dc_transfer_func_predefined trans,
>>               kvfree(rgb_regamma);
>>       } else if (trans == TRANSFER_FUNCTION_HLG ||
>>               trans == TRANSFER_FUNCTION_HLG12) {
>> -             rgb_regamma = kvzalloc(sizeof(*rgb_regamma) *
>> -                                    (MAX_HW_POINTS + _EXTRA_POINTS),
>> +             rgb_regamma = kvcalloc(MAX_HW_POINTS + _EXTRA_POINTS,
>> +                                    sizeof(*rgb_regamma),
>>                                      GFP_KERNEL);
>>               if (!rgb_regamma)
>>                       goto rgb_regamma_alloc_fail;
>> @@ -1802,8 +1802,8 @@ bool  mod_color_calculate_degamma_curve(enum dc_transfer_func_predefined trans,
>>               kvfree(rgb_degamma);
>>       } else if (trans == TRANSFER_FUNCTION_HLG ||
>>               trans == TRANSFER_FUNCTION_HLG12) {
>> -             rgb_degamma = kvzalloc(sizeof(*rgb_degamma) *
>> -                                    (MAX_HW_POINTS + _EXTRA_POINTS),
>> +             rgb_degamma = kvcalloc(MAX_HW_POINTS + _EXTRA_POINTS,
>> +                                    sizeof(*rgb_degamma),
>>                                      GFP_KERNEL);
>>               if (!rgb_degamma)
>>                       goto rgb_degamma_alloc_fail;
>>
>
> Since the values are constant, kvcalloc incurs the overflow checking
> overhead for no gain.

The way the macros are designed, they'll get entirely optimized away
in that case:
http://lkml.kernel.org/r/CAGXu5jLw9kMZRkzL_Q7JdyyprntHF6bdVSE_vjyzect3CenDng@xxxxxxxxxxxxxx

Using this patch means if the code ever changes away from constants,
it'll still be safe. And static checkers won't yell about the
open-coded multiplication here. So it's only up-sides. :)

-Kees

-- 
Kees Cook
Pixel Security