Commit c4e18497 authored by Guenter Roeck's avatar Guenter Roeck Committed by Linus Torvalds

linux/kernel.h: fix DIV_ROUND_CLOSEST with unsigned divisors

Commit 263a523d ("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: default avatarGuenter Roeck <linux@roeck-us.net>
Reported-by: default avatarJuergen Beisert <jbe@pengutronix.de>
Tested-by: default avatarJuergen Beisert <jbe@pengutronix.de>
Cc: Jean Delvare <khali@linux-fr.org>
Cc: <stable@vger.kernel.org>	[3.7.x]
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 154b454e
...@@ -77,13 +77,15 @@ ...@@ -77,13 +77,15 @@
/* /*
* Divide positive or negative dividend by positive divisor and round * Divide positive or negative dividend by positive divisor and round
* to closest integer. Result is undefined for negative divisors. * 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)( \ #define DIV_ROUND_CLOSEST(x, divisor)( \
{ \ { \
typeof(x) __x = x; \ typeof(x) __x = x; \
typeof(divisor) __d = divisor; \ typeof(divisor) __d = divisor; \
(((typeof(x))-1) > 0 || (__x) > 0) ? \ (((typeof(x))-1) > 0 || \
((typeof(divisor))-1) > 0 || (__x) > 0) ? \
(((__x) + ((__d) / 2)) / (__d)) : \ (((__x) + ((__d) / 2)) / (__d)) : \
(((__x) - ((__d) / 2)) / (__d)); \ (((__x) - ((__d) / 2)) / (__d)); \
} \ } \
......
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