Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

Guenter Roeck
2012-12-19T22:41:18Z

On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote:
> 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...
>
One observed effect 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).

> The patch is applicable to 3.7.x. Should we backport it?
>
Yes. DIV_ROUND_CLOSEST is used throughout the kernel, and impact is
unpredictable.

3.6 needs it as well.