On 1/18/25 13:21, Linus Torvalds 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.
Yes, turns out I can reproduce the problem by adding WARN_ON() ahead
of similar clamp() calls (see below). However, I can only reproduce it
with gcc 13.3 for parisc. I don't see the problem with other cross compilers
(I tried arm, powerpc, and loongarch64). Compilers are weird :-(.
I am not sure what to do here. That kind of problem seems difficult
to avoid, and I am sure we will hit it again elsewhere. Should I declare
gcc 13.x off limits for parisc builds ?
Guenter
---
diff --git a/drivers/input/mousedev.c b/drivers/input/mousedev.c
index 505c562a5daa..71c0da31a9d2 100644
--- a/drivers/input/mousedev.c
+++ b/drivers/input/mousedev.c
@@ -179,6 +179,7 @@ static void mousedev_abs_event(struct input_dev *dev, struct mousedev *mousedev,
if (size == 0)
size = xres ? : 1;
+ WARN_ON(min > max);
value = clamp(value, min, max);
mousedev->packet.x = ((value - min) * xres) / size;
