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

Jean Delvare
2012-12-20T11:48:35Z

On Wed, 19 Dec 2012 15:01:44 -0800, Guenter Roeck wrote:
> On Wed, Dec 19, 2012 at 11:21:15PM +0100, Jean Delvare wrote:
> > Hi Guenter,
> >
> > On Wed, 19 Dec 2012 06:40:15 -0800, Guenter Roeck 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.
> >
> > This is surprising to say the least. But if the C standard says so...
>
> Agreed, but it is how it is.
>
> > I wouldn't be surprised if there are bugs because of this in the kernel
> > and in other projects.
>
> Might easily be. This might make a good interview question - I suspect many
> if not most engineers would fail it. At least I would have until yesterday :).

Neither did I. And I'm not sure I'll remember it in one year from now.

> > (...)
> > Thinking a bit more about this... Documenting the non-working cases is
> > great, however I don't really expect all developers to pay attention. I
> > can also imagine variable types changing from signed to unsigned later,
> > and never thinking this can introduce a bug.
> >
> > So, is there nothing we can do to spot at least the second issue at
> > build time? For regular division there's nothing we can do (although I
> > don't understand why gcc doesn't warn...) but here we get the
> > opportunity to report the issue, let's take it.
> >
> > And given that the divisor is almost always a constant,
> > maybe we can check for negative divisors too, this would be safer and
> > the code size increase would probably be very small in practice.
> > Opinions?
>
> Agreed, though we should fix the problem now and think about reporting
> afterwards.

Yes, that's a good plan.

--
Jean Delvare