Re: [PATCH][next] gpio: xilinx: Fix potential integer overflow on shift of a u32 int

[Andy Shevchenko <andy.shevchenko@xxxxxxxxx>]

On Fri, May 14, 2021 at 12:26 PM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
> On Thu, May 13, 2021 at 09:52:27AM +0100, Colin King wrote:

...

> >       const unsigned long offset = (bit % BITS_PER_LONG) & BIT(5);
> >
> >       map[index] &= ~(0xFFFFFFFFul << offset);
> > -     map[index] |= v << offset;
> > +     map[index] |= (unsigned long)v << offset;
>
> Doing a shift by BIT(5) is super weird.

Not the first place in the kernel with such a trick.

>  It looks like a double shift
> bug and should probably trigger a static checker warning.  It's like
> when people do BIT(BIT(5)).
>
> It would be more readable to write it as:
>
>         int shift = (bit % BITS_PER_LONG) ? 32 : 0;

Usually this code is in a kinda fast path. Have you checked if the
compiler generates the same or better code when you are using ternary?

-- 
With Best Regards,
Andy Shevchenko