linux-kernel - Re: [PATCH] drm/amd/powerplay: ensure loop does not wraparound on decrement

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Date:   Thu, 18 May 2017 09:15:51 +0200
From:   walter harms <wharms@....de>
To:     Colin King <colin.king@...onical.com>
CC:     Alex Deucher <alexander.deucher@....com>,
        Christian König 
        <christian.koenig@....com>, David Airlie <airlied@...ux.ie>,
        Rex Zhu <Rex.Zhu@....com>,
        Eric Huang <JinHuiEric.Huang@....com>,
        Huang Rui <ray.huang@....com>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/amd/powerplay: ensure loop does not wraparound on
 decrement



Am 17.05.2017 20:13, schrieb Colin King:
> From: Colin Ian King <colin.king@...onical.com>
> 
> The current for loop decrements i when it is zero and this causes
> a wrap-around back to ~0 because i is unsigned. In the unlikely event
> that mask is 0, the loop will run forever. Fix this so we can't loop
> forever.
> 
> Detected by CoverityScan, CID#1435469 ("Unsigned compared against 0")
> 
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  drivers/gpu/drm/amd/powerplay/hwmgr/vega10_hwmgr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/amd/powerplay/hwmgr/vega10_hwmgr.c b/drivers/gpu/drm/amd/powerplay/hwmgr/vega10_hwmgr.c
> index ad30f5d3a10d..d92c9b9b15be 100644
> --- a/drivers/gpu/drm/amd/powerplay/hwmgr/vega10_hwmgr.c
> +++ b/drivers/gpu/drm/amd/powerplay/hwmgr/vega10_hwmgr.c
> @@ -4199,7 +4199,7 @@ static int vega10_force_clock_level(struct pp_hwmgr *hwmgr,
>  		}
>  		data->smc_state_table.gfx_boot_level = i;
>  
> -		for (i = 31; i >= 0; i--) {
> +		for (i = 32; --i; ) {
>  			if (mask & (1 << i))
>  				break;
>  		}

nitpicking:
we notices at several points that programmers are bad at counting backwards.
Is there a reason not to start with i=0 ?

re,
 wh