| From c4e18497d8fd92eef2c6e7eadcc1a107ccd115ea Mon Sep 17 00:00:00 2001 |
| From: Guenter Roeck <linux@roeck-us.net> |
| Date: Thu, 20 Dec 2012 15:05:42 -0800 |
| Subject: linux/kernel.h: fix DIV_ROUND_CLOSEST with unsigned divisors |
| |
| From: Guenter Roeck <linux@roeck-us.net> |
| |
| commit c4e18497d8fd92eef2c6e7eadcc1a107ccd115ea upstream. |
| |
| 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> |
| Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
| Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| |
| --- |
| include/linux/kernel.h | 6 ++++-- |
| 1 file changed, 4 insertions(+), 2 deletions(-) |
| |
| --- a/include/linux/kernel.h |
| +++ b/include/linux/kernel.h |
| @@ -77,13 +77,15 @@ |
| |
| /* |
| * 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)( \ |
| { \ |
| typeof(x) __x = x; \ |
| 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)); \ |
| } \ |