On Mon, 20 Jan 2025 12:48:11 +0200
Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote:
> On Sun, 19 Jan 2025, David Laight <david.laight.linux@xxxxxxxxx> wrote:
> > On Sat, 18 Jan 2025 14:58:48 -0800
> > Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> >
> >> On 1/18/25 14:11, David Laight wrote:
> >> > On Sat, 18 Jan 2025 13:21:39 -0800
> >> > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >> >
> >> >> On Sat, 18 Jan 2025 at 09:49, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> >> >>>
> >> >>> No idea why the compiler would know that the values are invalid.
> >> >>
> >> >> It's not that the compiler knows tat they are invalid, but I bet what
> >> >> happens is in scale() (and possibly other places that do similar
> >> >> checks), which does this:
> >> >>
> >> >> WARN_ON(source_min > source_max);
> >> >> ...
> >> >> source_val = clamp(source_val, source_min, source_max);
> >> >>
> >> >> and the compiler notices that the ordering comparison in the first
> >> >> WARN_ON() is the same as the one in clamp(), so it basically converts
> >> >> the logic to
> >> >>
> >> >> if (source_min > source_max) {
> >> >> WARN(..);
> >> >> /* Do the clamp() knowing that source_min > source_max */
> >> >> source_val = clamp(source_val, source_min, source_max);
> >> >> } else {
> >> >> /* Do the clamp knowing that source_min <= source_max */
> >> >> source_val = clamp(source_val, source_min, source_max);
> >> >> }
> >> >>
> >> >> (obviously I dropped the other WARN_ON in the conversion, it wasn't
> >> >> relevant for this case).
> >> >>
> >> >> And now that first clamp() case is done with source_min > source_max,
> >> >> and it triggers that build error because that's invalid.
> >> >>
> >> >> So the condition is not statically true in the *source* code, but in
> >> >> the "I have moved code around to combine tests" case it now *is*
> >> >> statically true as far as the compiler is concerned.
> >> >
> >> > Well spotted :-)
> >> >
> >> > One option would be to move the WARN_ON() below the clamp() and
> >> > add an OPTIMISER_HIDE_VAR(source_max) between them.
> >> >
> >> > Or do something more sensible than the WARN().
> >> > Perhaps return target_min on any such errors?
> >> >
> >>
> >> This helps:
> >>
> >> - WARN_ON(source_min > source_max);
> >> - WARN_ON(target_min > target_max);
> >> -
> >> /* defensive */
> >> source_val = clamp(source_val, source_min, source_max);
> >>
> >> + WARN_ON(source_min > source_max);
> >> + WARN_ON(target_min > target_max);
> >
> > That is a 'quick fix' ...
> >
> > Much better would be to replace the WARN() with (say):
> > if (target_min >= target_max)
> > return target_min;
> > if (source_min >= source_max)
> > return target_min + (target_max - target_min)/2;
> > So that the return values are actually in range (in as much as one is defined).
> > Note that the >= cpmparisons also remove a divide by zero.
>
> I want the loud and early warnings for clear bugs instead of
> "gracefully" silencing the errors only to be found through debugging
> user reports.
A user isn't going to notice a WARN() - not until you tell them to look for it.
In any case even if you output a message you really want to return a 'sane'
value, who knows what effect a very out of range value is going to have.
David
