[PATCH v2] fix overflow in idle exit_latency

[Rafael J. Wysocki]

On Tue, Dec 19, 2023 at 5:24 PM David Laight <David.Laight at aculab.com> wrote:
>
> From: Bo Ye
> > Sent: 19 December 2023 03:15
> >
> > From: C Cheng <C.Cheng at mediatek.com>
> >
> > In detail:
> >
> > In C language, when you perform a multiplication operation, if
> > both operands are of int type, the multiplication operation is
> > performed on the int type, and then the result is converted to
> > the target type. This means that if the product of int type
> > multiplication exceeds the range that int type can represent,
> >  an overflow will occur even if you store the result in a
> > variable of int64_t type.
> >
> > For a multiplication of two int values, it is better to use
> > mul_u32_u32() rather than s->exit_latency_ns = s->exit_latency *
> > NSEC_PER_USEC to avoid potential overflow happenning.
> >
> > Signed-off-by: C Cheng <C.Cheng at mediatek.com>
> > Signed-off-by: Bo Ye <bo.ye at mediatek.com>
> > ---
> > Change in v2:
> > -remove Change-ID
> > -correct patch author name
> > -replace multiplication of two int values with mul_u32_u32
> > -refine commit message
> > ---
> >  drivers/cpuidle/driver.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c
> > index d9cda7f6ccb9..cf5873cc45dc 100644
> > --- a/drivers/cpuidle/driver.c
> > +++ b/drivers/cpuidle/driver.c
> > @@ -16,6 +16,7 @@
> >  #include <linux/cpumask.h>
> >  #include <linux/tick.h>
> >  #include <linux/cpu.h>
> > +#include <linux/math64.h>
> >
> >  #include "cpuidle.h"
> >
> > @@ -187,7 +188,7 @@ static void __cpuidle_driver_init(struct cpuidle_driver *drv)
> >                       s->target_residency = div_u64(s->target_residency_ns, NSEC_PER_USEC);
> >
> >               if (s->exit_latency > 0)
> > -                     s->exit_latency_ns = s->exit_latency * NSEC_PER_USEC;
> > +                     s->exit_latency_ns = mul_u32_u32(s->exit_latency, NSEC_PER_USEC);
>
> Just force either side of the multiply to a 64bit unsigned type.
> The compiler will then DTRT which is likely to be better code than
> whatever mul_u32_u32() generates.

So why is it there?

The default implementation of mul_u32_u32() is to cast its first
argument to u64 before the multiplication AFAICS.

> In any case is the 'exit_latency' ever going to be greater than 4 seconds?
> In which case the 32bit multiply will never overflow.

So this is more of a theoretical thing found by some static analysis
tool or similar.

> >               else if (s->exit_latency_ns < 0)
> >                       s->exit_latency_ns =  0;
> >               else
> > --