Re: linux-5.3-rc4/drivers/staging/comedi/drivers/dt3000.c:373: (error) Signed integer overflow for expression 'divider*base'

[Ian Abbott]

On 12/08/2019 08:37, David Binderman wrote:
> Hello there,
>
> Source code is
>
>      prescale = 15;
>      base = timer_base * (1 << prescale);
>      divider = 65535;
>      *nanosec = divider * base;
>
> timer_base seems to be 500 or 100.
> nanosec is a pointer to int, so it can only hold about 2,000,000,000 nanoseconds, or about 2 seconds.

Thanks for the report.

I couldn't reproduce the error with the compiler version I was using, 
but I can see that it is likely to result in an arithmetic overflow.

I think the main problem is that the line (in (dt3k_ns_to_timer()):

      base = timer_base * (1 << prescale);

should be:

      base = timer_base * (prescale + 1);

which matches the earlier instance of this calculation in the same function.

In practice, these lines of code should never be reached due to earlier 
range checks in dt3k_ai_cmdtest().

> Suggest rework code to use longs.

It wouldn't do any harm to change the `int` variables to `unsigned int`.

> Regards
>
> David Binderman
>
>
> time-_Base seems to be 50 or 100.


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