On Thu Jul 15, 2021 at 6:23 AM EDT, Peter Rosin wrote:

On 2021-07-15 05:12, 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 774eb3044edd..4c3cfd4d5181 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 +68,16 @@ 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;
+			if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) ||
+			check_mul_overflow(*val2, rescale->denominator, (s32 *)&tmp2)) {

Hi Peter,

The white space should be like this, methinks.

if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) ||
check_mul_overflow(*val2, rescale->denominator, (s32 *)&tmp2))
{

Sorry about that... Like I said in the cover letter, I'm working on
getting kunit tests running for the iio-rescale. At the moment it still
requires copying part of the code over and sure enough I forgot to copy
some of it back. My apologies for the noise...

This is what I meant to send:

case IIO_VAL_FRACTIONAL:
	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);
		do_div(tmp, factor);
		do_div(tmp2, factor);
	}

	*val = tmp;
	*val2 = tmp2;
	return ret;

I'll also move the opening bracket on a new line if you prefer.

quoted

+				factor = gcd(tmp, tmp2);

And I just realized, gcd() works on unsigned values which is a bit safer
for the
scale factor. But here, for the actual values, more care is needed.

I added negative test cases to take this into account. I'll update and
resend. I'm going to find a way to get the test cases ready for the next
revision.

quoted

+				do_div(tmp, factor);
+				do_div(tmp2, factor);
+			}
+			*val = tmp;
+			*val2 = tmp2;

And beside the above points, the whole mechanism seems broken. The
returned value
in the third argument to check_mul_overflow isn't useful if there is an
overflow.
Yet, the code continues to use tmp and tmp2 in case of overflow. And why
do you
first multiply tmp and tmp2 without checks, only to then do the same mul
again
but with checks? Or have I completely misunderstood how
check_mul_overflow
works?

Again, my apologies for this. It's not what I meant to send.
Hopefully the snippet above makes more sense.

Thanks for your time,
Liam

Cheers,
Peter

quoted

 			return ret;
 		case IIO_VAL_INT:
 			*val *= rescale->numerator;