[PATCH v11 09/15] iio: afe: rescale: reduce risk of integer overflow

Liam Beguin
2021-12-22T03:47:16Z

From: Liam Beguin <lvb@xiphos.com>

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 <lvb@xiphos.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
---
drivers/iio/afe/iio-rescale.c | 23 ++++++++++++++++++-----
1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index ca8fd69bfe46..0000a58bab9d 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c
@@ -23,21 +23,31 @@ 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);
@@ -49,7 +59,10 @@ 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 = 1 << *val2;

rem2 = *val % (int)tmp;
*val = *val / (int)tmp;
--
2.34.0