diff mbox series

[v6,08/13] iio: afe: rescale: reduce risk of integer overflow

Message ID 20210721030613.3105327-9-liambeguin@gmail.com (mailing list archive)
State Changes Requested
Headers show
Series iio: afe: add temperature rescaling support | expand

Commit Message

Liam Beguin July 21, 2021, 3:06 a.m. UTC 
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(-)

Comments

Peter Rosin July 23, 2021, 9:17 p.m. UTC | #1 
On 2021-07-21 05:06, Liam Beguin wrote:
> 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);

Hi!

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

Cheers,
Peter

> +			tmp = div_s64(tmp, factor);
> +			tmp2 = div_s64(tmp2, factor);
> +		}
> +		*val = tmp;
> +		*val2 = tmp2;
>  		return scale_type;
>  	case IIO_VAL_INT:
>  		*val *= rescale->numerator;
>
Liam Beguin July 28, 2021, 12:07 a.m. UTC | #2 
On Fri Jul 23, 2021 at 5:17 PM EDT, Peter Rosin wrote:
> On 2021-07-21 05:06, Liam Beguin wrote:
> > 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);

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
>
> > +			tmp = div_s64(tmp, factor);
> > +			tmp2 = div_s64(tmp2, factor);
> > +		}
> > +		*val = tmp;
> > +		*val2 = tmp2;
> >  		return scale_type;
> >  	case IIO_VAL_INT:
> >  		*val *= rescale->numerator;
> >
Peter Rosin July 28, 2021, 7:47 a.m. UTC | #3 
On 2021-07-28 02:07, Liam Beguin wrote:
> On Fri Jul 23, 2021 at 5:17 PM EDT, Peter Rosin wrote:
>> On 2021-07-21 05:06, Liam Beguin wrote:
>>> 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);
> 
> 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.

Maybe I've misread the code and gcd is in fact working for negative
numbers? However, I imagine it might be arch specific, so testing on
a single arch feels insufficient and deeper analysis is required.

However, looking at lib/math/gcd.c it certainly still looks like
negative values will work very poorly, and there is no macro magic
in include/linux/gcd.h to handle it by wrapping the core C routine.

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

The type of the operands may be s32, but if you look at how those values
are populated, and with what they are populated, I think you will find that
only positive scale factors are sensible for a voltage divider. Using
resistors with so high resistance that s32 is not enough is simply not
supported.

Cheers,
Peter

> Thanks,
> Liam
> 
>>
>> Cheers,
>> Peter
>>
>>> +			tmp = div_s64(tmp, factor);
>>> +			tmp2 = div_s64(tmp2, factor);
>>> +		}
>>> +		*val = tmp;
>>> +		*val2 = tmp2;
>>>  		return scale_type;
>>>  	case IIO_VAL_INT:
>>>  		*val *= rescale->numerator;
>>>
>
Liam Beguin July 29, 2021, 4:02 p.m. UTC | #4 
On Wed Jul 28, 2021 at 3:47 AM EDT, Peter Rosin wrote:
> On 2021-07-28 02:07, Liam Beguin wrote:
> > On Fri Jul 23, 2021 at 5:17 PM EDT, Peter Rosin wrote:
> >> On 2021-07-21 05:06, Liam Beguin wrote:
> >>> 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);
> > 
> > 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.
>
> Maybe I've misread the code and gcd is in fact working for negative
> numbers? However, I imagine it might be arch specific, so testing on
> a single arch feels insufficient and deeper analysis is required.
>
> However, looking at lib/math/gcd.c it certainly still looks like
> negative values will work very poorly, and there is no macro magic
> in include/linux/gcd.h to handle it by wrapping the core C routine.

I agree that looking at lib/math/gcd.c odd things might happen with
negative values. I'll use the the absolute values to calculate the GCD
as it shouldn't affect the value of factor.

>
> > rescale_voltage_divider_props() seems to also use gcd() with signed
> > integers.
>
> The type of the operands may be s32, but if you look at how those values
> are populated, and with what they are populated, I think you will find
> that
> only positive scale factors are sensible for a voltage divider. Using
> resistors with so high resistance that s32 is not enough is simply not
> supported.

That makes sense!

Thanks,
Liam

>
> Cheers,
> Peter
>
> > Thanks,
> > Liam
> > 
> >>
> >> Cheers,
> >> Peter
> >>
> >>> +			tmp = div_s64(tmp, factor);
> >>> +			tmp2 = div_s64(tmp2, factor);
> >>> +		}
> >>> +		*val = tmp;
> >>> +		*val2 = tmp2;
> >>>  		return scale_type;
> >>>  	case IIO_VAL_INT:
> >>>  		*val *= rescale->numerator;
> >>>
> >
diff mbox series

Patch

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;