| Message ID | 20210721030613.3105327-9-liambeguin@gmail.com |
|---|---|
| State | Superseded |
| Headers | show |
| Series | iio: afe: add temperature rescaling support | expand |
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; >
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; > >
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; >>> >
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 --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;