linux-kernel - Re: [PATCH v2 04/13] media: dvb_frontend: don't play tricks with underflow values

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Message-ID: <ab51f981844c700d4e66b366c8d2abde7c5947bf.camel@redhat.com>
Date: Fri, 18 Oct 2024 13:44:20 +0200
From: Philipp Stanner <pstanner@...hat.com>
To: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
Cc: Hans Verkuil <hverkuil@...all.nl>, Kevin Hao <haokexin@...il.com>, 
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org, Kees Cook
	 <keescook@...omium.org>
Subject: Re: [PATCH v2 04/13] media: dvb_frontend: don't play tricks with
 underflow values

On Fri, 2024-10-18 at 07:53 +0200, Mauro Carvalho Chehab wrote:
> fepriv->auto_sub_step is unsigned. Setting it to -1 is just a
> trick to avoid calling continue, as reported by Coverity.
> 
> It relies to have this code just afterwards:
> 
> 	if (!ready) fepriv->auto_sub_step++;
> 
> Simplify the code by simply setting it to zero and use
> continue to return to the while loop.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

Oh wow, back to the big-bang-commit ^^'

So is this a bug or not? It seems to me that the uint underflows to
UINT_MAX, and then wrapps around to 0 again through the ++..

I take the liberty of ++CCing Kees, since I heard him talk a lot about
overflowing on Plumbers.

If it's not a bug, I would not use "Fixes". If it is a bug, it should
be backported to stable, agreed?

Plus, is there a report-link somewhere by Coverty that could be linked
with "Closes: "?

> Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>

Anyways, this in my eyes does what it's intended to do:

Reviewed-by: Philipp Stanner <pstanner@...hat.com>

> ---
>  drivers/media/dvb-core/dvb_frontend.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/dvb-core/dvb_frontend.c
> b/drivers/media/dvb-core/dvb_frontend.c
> index d48f48fda87c..c9283100332a 100644
> --- a/drivers/media/dvb-core/dvb_frontend.c
> +++ b/drivers/media/dvb-core/dvb_frontend.c
> @@ -443,8 +443,8 @@ static int dvb_frontend_swzigzag_autotune(struct
> dvb_frontend *fe, int check_wra
>  
>  		default:
>  			fepriv->auto_step++;
> -			fepriv->auto_sub_step = -1; /* it'll be
> incremented to 0 in a moment */
> -			break;
> +			fepriv->auto_sub_step = 0;
> +			continue;
>  		}
>  
>  		if (!ready) fepriv->auto_sub_step++;