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Revision c4e18497d8fd92eef2c6e7eadcc1a107ccd115ea authored by Guenter Roeck on 20 December 2012, 23:05:42 UTC, committed by Linus Torvalds on 21 December 2012, 01:40:20 UTC 
linux/kernel.h: fix DIV_ROUND_CLOSEST with unsigned divisors
 
Commit 263a523d18bc ("linux/kernel.h: Fix warning seen with W=1 due to
change in DIV_ROUND_CLOSEST") fixes a warning seen with W=1 due to
change in DIV_ROUND_CLOSEST.

Unfortunately, the C compiler converts divide operations with unsigned
divisors to unsigned, even if the dividend is signed and negative (for
example, -10 / 5U = 858993457).  The C standard says "If one operand has
unsigned int type, the other operand is converted to unsigned int", so
the compiler is not to blame.  As a result, DIV_ROUND_CLOSEST(0, 2U) and
similar operations now return bad values, since the automatic conversion
of expressions such as "0 - 2U/2" to unsigned was not taken into
account.

Fix by checking for the divisor variable type when deciding which
operation to perform.  This fixes DIV_ROUND_CLOSEST(0, 2U), but still
returns bad values for negative dividends divided by unsigned divisors.
Mark the latter case as unsupported.

One observed effect of this problem is that the s2c_hwmon driver reports
a value of 4198403 instead of 0 if the ADC reads 0.

Other impact is unpredictable.  Problem is seen if the divisor is an
unsigned variable or constant and the dividend is less than (divisor/2).

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: Juergen Beisert <jbe@pengutronix.de>
Tested-by: Juergen Beisert <jbe@pengutronix.de>
Cc: Jean Delvare <khali@linux-fr.org>
Cc: <stable@vger.kernel.org>	[3.7.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent 154b454
Raw File
generic_mpih-add1.c 
/* mpihelp-add_1.c  -  MPI helper functions
 * Copyright (C) 1994, 1996, 1997, 1998,
 *               2000 Free Software Foundation, Inc.
 *
 * This file is part of GnuPG.
 *
 * GnuPG is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * GnuPG is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
 *
 * Note: This code is heavily based on the GNU MP Library.
 *	 Actually it's the same code with only minor changes in the
 *	 way the data is stored; this is to support the abstraction
 *	 of an optional secure memory allocation which may be used
 *	 to avoid revealing of sensitive data due to paging etc.
 *	 The GNU MP Library itself is published under the LGPL;
 *	 however I decided to publish this code under the plain GPL.
 */

#include "mpi-internal.h"
#include "longlong.h"

mpi_limb_t
mpihelp_add_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
	      mpi_ptr_t s2_ptr, mpi_size_t size)
{
	mpi_limb_t x, y, cy;
	mpi_size_t j;

	/* The loop counter and index J goes from -SIZE to -1.  This way
	   the loop becomes faster.  */
	j = -size;

	/* Offset the base pointers to compensate for the negative indices. */
	s1_ptr -= j;
	s2_ptr -= j;
	res_ptr -= j;

	cy = 0;
	do {
		y = s2_ptr[j];
		x = s1_ptr[j];
		y += cy;	/* add previous carry to one addend */
		cy = y < cy;	/* get out carry from that addition */
		y += x;		/* add other addend */
		cy += y < x;	/* get out carry from that add, combine */
		res_ptr[j] = y;
	} while (++j);

	return cy;
}
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