linux-kernel - [PATCH v4 04/10] iio: afe: rescale: reduce risk of integer overflow

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Message-Id: <20210706160942.3181474-5-liambeguin@gmail.com>
Date:   Tue,  6 Jul 2021 12:09:36 -0400
From:   Liam Beguin <liambeguin@...il.com>
To:     liambeguin@...il.com, peda@...ntia.se, jic23@...nel.org,
        lars@...afoo.de, pmeerw@...erw.net
Cc:     linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
        devicetree@...r.kernel.org, robh+dt@...nel.org
Subject: [PATCH v4 04/10] iio: afe: rescale: reduce risk of integer overflow

From: Liam Beguin <lvb@...hos.com>

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.

Signed-off-by: Liam Beguin <lvb@...hos.com>
---
 drivers/iio/afe/iio-rescale.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 774eb3044edd..ba3bdcc69b16 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c
@@ -39,7 +39,8 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 			    int *val, int *val2, long mask)
 {
 	struct rescale *rescale = iio_priv(indio_dev);
-	unsigned long long tmp;
+	s64 tmp, tmp2;
+	u32 factor;
 	int ret;
 
 	switch (mask) {
@@ -67,8 +68,13 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 		}
 		switch (ret) {
 		case IIO_VAL_FRACTIONAL:
-			*val *= rescale->numerator;
-			*val2 *= rescale->denominator;
+			tmp = (s64)*val * rescale->numerator;
+			tmp2 = (s64)*val2 * rescale->denominator;
+			factor = gcd(tmp, tmp2);
+			do_div(tmp, factor);
+			*val = tmp;
+			do_div(tmp2, factor);
+			*val2 = tmp2;
 			return ret;
 		case IIO_VAL_INT:
 			*val *= rescale->numerator;
-- 
2.30.1.489.g328c10930387