From: Guenter Roeck <[email protected]>
Subject: linux/kernel.h: fix DIV_ROUND_CLOSEST with unsigned divisors
Commit 263a523 ("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 <[email protected]>
Reported-by: Juergen Beisert <[email protected]>
Tested-by: Juergen Beisert <[email protected]>
Cc: Jean Delvare <[email protected]>
Cc: <[email protected]> [3.7.x]
Signed-off-by: Andrew Morton <[email protected]>
---
include/linux/kernel.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff -puN
include/linux/kernel.h~linux-kernelh-fix-div_round_closest-with-unsigned-divisors
include/linux/kernel.h
---
a/include/linux/kernel.h~linux-kernelh-fix-div_round_closest-with-unsigned-divisors
+++ a/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)); \
} \
_
--
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