Messages in this thread | | | Date | Wed, 19 Dec 2012 13:47:21 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors |
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On Wed, 19 Dec 2012 06:40:15 -0800 Guenter Roeck <linux@roeck-us.net> wrote:
> Commit 263a523 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.
The changelog didn't describe the end-user visible effects of the bug. Please always include this information. Because...
The patch is applicable to 3.7.x. Should we backport it?
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