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| author | Guenter Roeck <linux@roeck-us.net> | 2012-12-21 03:05:42 +0400 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2012-12-21 05:40:20 +0400 |
| commit | c4e18497d8fd92eef2c6e7eadcc1a107ccd115ea (patch) | |
| tree | 9513b220cbdd633ad7c42299d61f2b46b9fb1561 /include/linux/kernel.h | |
| parent | 154b454edaf6d94a69016db6c342c57fa935bbe9 (diff) | |
| download | linux-c4e18497d8fd92eef2c6e7eadcc1a107ccd115ea.tar.xz | |
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>
Diffstat (limited to 'include/linux/kernel.h')
| -rw-r--r-- | include/linux/kernel.h | 6 |
1 files changed, 4 insertions, 2 deletions
diff --git a/include/linux/kernel.h b/include/linux/kernel.h index d140e8fb075f..c566927efcbd 100644 --- 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)); \ } \ |