Re: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow

[Guenter Roeck]

On 10/2/19 2:43 PM, Nick Desaulniers wrote:
> On Mon, Sep 30, 2019 at 5:01 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > Again, I fail to understand why waiting for a multiple of 20 seconds
> > under any circumstances would make any sense. Maybe the idea was
> > to divide us by 1000 before entering the second loop ?
>
> Yes, that's very clearly a mistake of mine.
>
> > Looking into the code, there is no need to use udelay() in the first
> > place. It should be possible to replace the longer waits with
> > usleep_range(). Something like
> >
> >                  if (us < some_low_value)        // eg. 0x80
> >                          delay(us)
>
> Did you mean udelay here?
Yes

> >                  else
> >                          usleep_range(us, us * 2);
> >
> > should do, and at the same time prevent the system from turning
> > into a space heater.
>
> The issue would persist with the above if udelay remains in a loop
> that gets fully unrolled.  That's while I "peel" the loop into two
> loops over different ranges with different bodies.

Sorry, you lost me. If calls to udelay() with even small delay
parameters for some compiler-related reason no longer work, trying
to fix the problem with some odd driver code is most definitely not
a real solution.

> I think I should iterate in the first loop until the number of `us` is
> greater than 1000 (us per ms)(which is less of a magical constant and
> doesn't expose internal implementation details of udelay), then start
> the second loop (dividing us by 1000).  What do you think, Guenter?

We should have no second loop, period.

Again, a hot delay loop of 128 ms (actually, more like 245 ms,
adding all delays together) is clearly wrong. Those udelay() calls
in the driver should really be replaced with usleep_range().

Guenter