linux-kernel - Re: [PATCH][V2] backlight: sky81452: insure while loop does not allow negative array indexing

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Message-ID: <20200227145741.v3r4qzkfsrwpy33h@holly.lan>
Date:   Thu, 27 Feb 2020 14:57:41 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Colin King <colin.king@...onical.com>
Cc:     Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Gyungoh Yoo <jack.yoo@...worksinc.com>,
        Bryan Wu <cooloney@...il.com>, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][V2] backlight: sky81452: insure while loop does not
 allow negative array indexing

On Thu, Feb 27, 2020 at 11:46:23AM +0000, Daniel Thompson wrote:
> On Wed, Feb 26, 2020 at 07:58:26PM +0000, Colin King wrote:
> > From: Colin Ian King <colin.king@...onical.com>
> > 
> > In the unlikely event that num_entry is zero, the while loop
> > pre-decrements num_entry to cause negative array indexing into the
> > array sources. Fix this by iterating only if num_entry >= 0.
> > 
> > Addresses-Coverity: ("Out-of-bounds read")
> > Fixes: f705806c9f35 ("backlight: Add support Skyworks SKY81452 backlight driver")
> > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > ---
> > 
> > V2: fix typo in commit subject line
> 
> Isn't the correct spelling "ensure"?
> 
> 
> > ---
> >  drivers/video/backlight/sky81452-backlight.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/video/backlight/sky81452-backlight.c b/drivers/video/backlight/sky81452-backlight.c
> > index 2355f00f5773..f456930ce78e 100644
> > --- a/drivers/video/backlight/sky81452-backlight.c
> > +++ b/drivers/video/backlight/sky81452-backlight.c
> > @@ -200,7 +200,7 @@ static struct sky81452_bl_platform_data *sky81452_bl_parse_dt(
> >  		}
> >  
> >  		pdata->enable = 0;
> > -		while (--num_entry)
> > +		while (--num_entry >= 0)
> >  			pdata->enable |= (1 << sources[num_entry]);
> 
> This look still looks buggy to me (so I'd second Walter's request to
> change it to a for loop). If the code genuinely does not contain a
> bug then it probably needs a prominent comment explaining why it is
> correct not to honour sources[0]!

Ignore the "still looks buggy". A mental mis-step when switching
contexts...

I think my English is still correct though ;-)


Daniel.