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

[David Laight]

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.  
> 
> ...
> 
> > Fixes: 0f85406bf830 ("iio: consumers: Fix handling of negative channel scale in iio_convert_raw_to_processed()")  
> 
> Doesn't fix any know issue for now.
> 
> ...
> 
> > -		*result = multiplier * abs(val);
> > -		*result += div_s64(multiplier * abs(val2), denominator);
> > +		*result = multiplier * abs((s64)val);
> > +		*result += div_s64(multiplier * abs((s64)val2), denominator);  
> 
> Right, but here we get val and val2 from either static values from the driver
> (when it is SCALE channel), or when channel has PROCESSED support.
> In the latter one it might theoretically be possible to go till the INT_MIN,
> but practically I don't know how, except for the broken driver code in the
> first place. With that being said, I think it's better to validate somewhere
> the multipliers (when it's SCALE or PROCESSED channel). I also noted that
> for the _PROCESSED some drivers keep a garbage in val2. That probably needs
> to be addressed as well (exempli gratia: bmi270_read_raw() does that).
> 

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.)

	David