iio: afe: rescale: reduce risk of integer overflow

Reduce the risk of integer overflow by doing the scale calculation on
a 64-bit integer. Since the rescaling is only performed on *val, reuse
the IIO_VAL_FRACTIONAL_LOG2 case.
Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220213025739.2561834-6-liambeguin@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/afe/iio-rescale.c

@@ -24,21 +24,31 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
 			  int *val, int *val2)
 {
 	s64 tmp;
+	int _val, _val2;
 	s32 rem, rem2;
 	u32 mult;
 	u32 neg;
 
 	switch (scale_type) {
-	case IIO_VAL_FRACTIONAL:
-		*val *= rescale->numerator;
-		*val2 *= rescale->denominator;
-		return scale_type;
 	case IIO_VAL_INT:
 		*val *= rescale->numerator;
 		if (rescale->denominator == 1)
 			return scale_type;
 		*val2 = rescale->denominator;
 		return IIO_VAL_FRACTIONAL;
+	case IIO_VAL_FRACTIONAL:
+		/*
+		 * When the product of both scales doesn't overflow, avoid
+		 * potential accuracy loss (for in kernel consumers) by
+		 * keeping a fractional representation.
+		 */
+		if (!check_mul_overflow(*val, rescale->numerator, &_val) &&
+		    !check_mul_overflow(*val2, rescale->denominator, &_val2)) {
+			*val = _val;
+			*val2 = _val2;
+			return IIO_VAL_FRACTIONAL;
+		}
+		fallthrough;
 	case IIO_VAL_FRACTIONAL_LOG2:
 		tmp = (s64)*val * 1000000000LL;
 		tmp = div_s64(tmp, rescale->denominator);
@@ -50,7 +60,10 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
 		if (!rem)
 			return scale_type;
 
-		tmp = 1 << *val2;
+		if (scale_type == IIO_VAL_FRACTIONAL)
+			tmp = *val2;
+		else
+			tmp = ULL(1) << *val2;
 
 		rem2 = *val % (int)tmp;
 		*val = *val / (int)tmp;