linux-kernel - m88ds3103: Undefined division

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Message-ID: <56C6EA14.7080405@lysator.liu.se>
Date:	Fri, 19 Feb 2016 11:10:28 +0100
From:	Peter Rosin <peda@...ator.liu.se>
To:	Antti Palosaari <crope@....fi>
CC:	linux-media@...r.kernel.org,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	linux-kernel@...r.kernel.org
Subject: m88ds3103: Undefined division

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...

Cheers,
Peter