On Fri Jul 23, 2021 at 5:17 PM EDT, Peter Rosin wrote:

On 2021-07-21 05:06, Liam Beguin wrote:

quoted

From: Liam Beguin <redacted>

Reduce the risk of integer overflow by doing the scale calculation with
64bit integers and looking for a Greatest Common Divider for both parts
of the fractional value when required.

Signed-off-by: Liam Beguin <redacted>
---
 drivers/iio/afe/iio-rescale.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 6f6a711ae3ae..35fa3b4e53e0 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c

@@ -21,12 +21,21 @@
 int rescale_process_scale(struct rescale *rescale, int scale_type,
 			  int *val, int *val2)
 {
-	unsigned long long tmp;
+	s64 tmp, tmp2;
+	u32 factor;
 
 	switch (scale_type) {
 	case IIO_VAL_FRACTIONAL:
-		*val *= rescale->numerator;
-		*val2 *= rescale->denominator;
+		if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) ||
+		    check_mul_overflow(*val2, rescale->denominator, (s32 *)&tmp2)) {
+			tmp = (s64)*val * rescale->numerator;
+			tmp2 = (s64)*val2 * rescale->denominator;
+			factor = gcd(tmp, tmp2);

Hi Peter,

Hi!

Reiterating that gcd() only works for unsigned operands, so this is
broken for
negative values.

Apologies, I didn't mean to make it seem like I ignored your comments. I
should've added a note. After you pointed out that gcd() only works for
unsigned elements, I added test cases for negative values, and all tests
passed. I'll look into it more.

rescale_voltage_divider_props() seems to also use gcd() with signed
integers.

Thanks,
Liam

Cheers,
Peter

quoted

+			tmp = div_s64(tmp, factor);
+			tmp2 = div_s64(tmp2, factor);
+		}
+		*val = tmp;
+		*val2 = tmp2;
 		return scale_type;
 	case IIO_VAL_INT:
 		*val *= rescale->numerator;