linux-kernel - Re: m88ds3103: Undefined division

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Message-ID: <56C6F292.3090508@iki.fi>
Date:	Fri, 19 Feb 2016 12:46:42 +0200
From:	Antti Palosaari <crope@....fi>
To:	Peter Rosin <peda@...ator.liu.se>
Cc:	linux-media@...r.kernel.org,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	linux-kernel@...r.kernel.org
Subject: Re: m88ds3103: Undefined division

On 02/19/2016 12:10 PM, Peter Rosin wrote:
> Hi!
>
> I'm looking at this code in drivers/media/dvb-frontends/m88ds3103.c in
> the m88ds3103_set_frontend() function, line 600 (give or take):
>
> 	s32tmp = 0x10000 * (tuner_frequency - c->frequency);
> 	s32tmp = DIV_ROUND_CLOSEST(s32tmp, priv->mclk_khz);
> 	if (s32tmp < 0)
> 		s32tmp += 0x10000;
>
> There is code that tries to handle negative s32tmp, so I assume that
> negative s32tmp is a possibility. Further, priv->mclk_khz is an unsigned
> type as far as I can tell. But then we have this comment for the
> DIV_ROUND_CLOSEST macro:
>
> /*
>   * Divide positive or negative dividend by positive divisor and round
>   * to closest integer. Result is undefined for negative divisors and
>   * for negative dividends if the divisor variable type is unsigned.
>   */
> #define DIV_ROUND_CLOSEST(x, divisor)(                  \
>
> I don't know how bad this is, and what the consequences of garbage are,
> but from here it looks like a problem waiting to happen...

Divisor type (mclk) needs to be changed signed then somehow...

regards
Antti

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