Re: [PATCH] iio: inkern: Avoid risky abs() usage in iio_multiply_value()

[David Laight]

On Tue, 31 Mar 2026 21:34:06 +0300
Andy Shevchenko <andriy.shevchenko@xxxxxxxxx> wrote:

> On Tue, Mar 31, 2026 at 04:26:35PM +0100, David Laight wrote:
> > On Tue, 31 Mar 2026 12:29:22 +0300
> > Andy Shevchenko <andriy.shevchenko@xxxxxxxxx> wrote:  
> > > On Tue, Mar 31, 2026 at 10:49:59AM +0200, Romain Gantois wrote:  
> 
> > > > iio_multiply_value() passes integers val and val2 directly to abs(). This
> > > > is problematic because if a signed argument to abs is the lowest value for
> > > > its type, then the result is undefined due to overflow.
> > > > 
> > > > Cast val and val2 to s64 before passing them to abs() to avoid this issue.    
> 
> ...
> 
> > I've just looked at the 'work of art' that is abs().
> > What is wrong with:
> > #define abs(x) (sizeof(x) == sizeof(long long) ? __abs(long long, x) : \
> > 		__abs(int, x))
> > #define __abs(type, x) \
> > 	({ type __abs_x = (x); __abs_x < 0 ? -__abs_x : __abs_x;})
> > 
> > It is just as broken for u128.
> > It will use the correct signedness for char (but it is unsigned now).
> > It doesn't cast back to char, but that is entirely pointless unless code
> > looks at the type of the expression, the return value itself is always
> > promoted to int before being used.
> > 
> > Actually replace the -__abs_x (UB for INT_MIN) with the safe:
> > 	(unsigned type)-(__abs_x + 1) + 1
> > and the return type will be unsigned with a correct value for -INT_MIN.
> > (Oh and the compiler sees through the mess.)  
> 
> And this is definitely wrong. We must keep type, because abs() might be used in
> the comparisons with signed or as parameter to multiplication or division where
> sign has to be preserved.

Thinks.... (bad at 11pm)
IIRC -INT_MIN is UB, but (~INT_MIN + 1) is fine provided -fno-strict-overflow
is set - which it is for kernel builds.
At least that guarantees the abs(-INT_MIN) == INT_MIN which is about the best
you can do.
It isn't as if it is ever going to happen.
There are all sorts of ways to break things in a driver.

	David