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

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Date:   Wed, 4 Jul 2018 07:51:06 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Michel Dänzer <michel@...nzer.net>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        David Airlie <airlied@...ux.ie>
Cc:     dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/amd/display/dc/dce: Fix multiple potential integer
 overflows

Hi Michel,

On 07/04/2018 02:38 AM, Michel Dänzer 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
>> 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>
>>
>> [...]
>>  
>> @@ -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.

Thanks
--
Gustavo