Re: [PATCH] drm/amd/display/dc/dce: Fix multiple potential integer overflows
From: Gustavo A. R. Silva
Date: Wed Jul 04 2018 - 09:13:39 EST
On 07/04/2018 02:38 AM, Michel DÃnzer wrote:
> On 2018-07-04 03:13 AM, Gustavo A. R. Silva wrote:
>> Add suffix ULL to constant 5 and cast variables target_pix_clk_khz and
>> feedback_divider to uint64_t in order to avoid multiple potential integer
>> overflows and give the compiler complete information about the proper
>> arithmetic to use.
>> Notice that such constant and variables are used in contexts that
>> expect expressions of type uint64_t (64 bits, unsigned). The current
>> casts to uint64_t effectively apply to each expression as a whole,
>> but they do not prevent them from being evaluated using 32-bit
>> arithmetic instead of 64-bit arithmetic.
>> Also, once the expressions are properly evaluated using 64-bit
>> arithmentic, there is no need for the parentheses that enclose
>> Addresses-Coverity-ID: 1460245 ("Unintentional integer overflow")
>> Addresses-Coverity-ID: 1460286 ("Unintentional integer overflow")
>> Addresses-Coverity-ID: 1460401 ("Unintentional integer overflow")
>> Fixes: 4562236b3bc0 ("drm/amd/dc: Add dc display driver (v2)")
>> Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
>> @@ -145,8 +145,8 @@ static bool calculate_fb_and_fractional_fb_divider(
>> * of fractional feedback decimal point and the fractional FB Divider precision
>> * is 2 then the equation becomes (ullfeedbackDivider + 5*100) / (10*100))*/
>> - feedback_divider += (uint64_t)
>> - (5 * calc_pll_cs->fract_fb_divider_precision_factor);
>> + feedback_divider += 5UL *
>> + calc_pll_cs->fract_fb_divider_precision_factor;
> This should be 5ULL, as the commit log says, otherwise it's still only
> 32 bits on 32-bit platforms.
That's correct. Thanks for the report.
I'll send v2 shortly.