linux-kernel - Re: [PATCH v2] drm/amd/display/dc/dce: Fix multiple potential integer overflows

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Message-ID: <23116da0-5dd7-b8fb-8450-a927a5647fb3@embeddedor.com>
Date:   Thu, 19 Jul 2018 11:51:12 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Michel Dänzer <michel@...nzer.net>,
        Harry Wentland <harry.wentland@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        David Airlie <airlied@...ux.ie>
Cc:     amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] drm/amd/display/dc/dce: Fix multiple potential integer
 overflows

Hi Alex, Harry,

I wonder if this patch should have been tagged for stable.

Thanks
--
Gustavo

On 07/04/2018 08:22 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
> them.
> 
> 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@...eddedor.com>
> ---
> Changes in v2:
>  - Add suffix ULL to constant 5 instead of UL. Thanks to Michel Dänzer
>    for pointing this out.
> 
>  drivers/gpu/drm/amd/display/dc/dce/dce_clock_source.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/dce/dce_clock_source.c b/drivers/gpu/drm/amd/display/dc/dce/dce_clock_source.c
> index 88b09dd..ca13775 100644
> --- a/drivers/gpu/drm/amd/display/dc/dce/dce_clock_source.c
> +++ b/drivers/gpu/drm/amd/display/dc/dce/dce_clock_source.c
> @@ -133,7 +133,7 @@ static bool calculate_fb_and_fractional_fb_divider(
>  	uint64_t feedback_divider;
>  
>  	feedback_divider =
> -		(uint64_t)(target_pix_clk_khz * ref_divider * post_divider);
> +		(uint64_t)target_pix_clk_khz * ref_divider * post_divider;
>  	feedback_divider *= 10;
>  	/* additional factor, since we divide by 10 afterwards */
>  	feedback_divider *= (uint64_t)(calc_pll_cs->fract_fb_divider_factor);
> @@ -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 += 5ULL *
> +			    calc_pll_cs->fract_fb_divider_precision_factor;
>  	feedback_divider =
>  		div_u64(feedback_divider,
>  			calc_pll_cs->fract_fb_divider_precision_factor * 10);
> @@ -203,8 +203,8 @@ static bool calc_fb_divider_checking_tolerance(
>  			&fract_feedback_divider);
>  
>  	/*Actual calculated value*/
> -	actual_calc_clk_khz = (uint64_t)(feedback_divider *
> -					calc_pll_cs->fract_fb_divider_factor) +
> +	actual_calc_clk_khz = (uint64_t)feedback_divider *
> +					calc_pll_cs->fract_fb_divider_factor +
>  							fract_feedback_divider;
>  	actual_calc_clk_khz *= calc_pll_cs->ref_freq_khz;
>  	actual_calc_clk_khz =
>