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From:   Liam Beguin <liambeguin@gmail.com>
To:     liambeguin@gmail.com, peda@axentia.se, jic23@kernel.org,
        lars@metafoo.de, pmeerw@pmeerw.net
Cc:     linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org,
        devicetree@vger.kernel.org, robh+dt@kernel.org
Subject: [PATCH v6 08/13] iio: afe: rescale: reduce risk of integer overflow
Date:   Tue, 20 Jul 2021 23:06:08 -0400
Message-Id: <20210721030613.3105327-9-liambeguin@gmail.com>
In-Reply-To: <20210721030613.3105327-1-liambeguin@gmail.com>
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From: Liam Beguin <lvb@xiphos.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 when required.

Signed-off-by: Liam Beguin <lvb@xiphos.com>
---
 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);
+			tmp = div_s64(tmp, factor);
+			tmp2 = div_s64(tmp2, factor);
+		}
+		*val = tmp;
+		*val2 = tmp2;
 		return scale_type;
 	case IIO_VAL_INT:
 		*val *= rescale->numerator;
-- 
2.30.1.489.g328c10930387