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

[Andy Shevchenko <andy.shevchenko@xxxxxxxxx>]

On Mon, May 17, 2021 at 04:36:43PM +0300, Dan Carpenter wrote:
> On Mon, May 17, 2021 at 10:07:20AM +0300, Andy Shevchenko wrote:
> > 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?
> 
> I wrote a little benchmark to see which was faster and they're the same
> as far as I can see.

Thanks for checking.

Besides the fact that offset should be 0 for 32-bit always and if compiler can
proof that...

The test below doesn't take into account the exact trick is used for offset
(i.e. implicit dependency between BITS_PER_LONG, size of unsigned long, and
 using 5th bit out of value). I don't know if compiler can properly optimize
the ternary in this case (but it looks like it should generate the same code).

That said, I would rather to see the diff between assembly of the exact
function before and after your proposal.

> static inline __attribute__((__gnu_inline__)) unsigned long xgpio_set_value_orig(unsigned long *map, int bit, u32 v)
> {
>         int shift = (bit % 64) & ((((1UL))) << (5));
>         return v << shift;
> }
> 
> static inline __attribute__((__gnu_inline__)) unsigned long xgpio_set_value_new(unsigned long *map, int bit, u32 v)
> {
>         int shift = (bit % 64) ? 32 : 0;
>         return v << shift;
> }
> 
> int main(void)
> {
>         int i;
> 
>         for (i = 0; i < INT_MAX; i++)
>                 xgpio_set_value_orig(NULL, i, 0);
> 
> //      for (i = 0; i < INT_MAX; i++)
> //              xgpio_set_value_new(NULL, i, 0);
> 
>         return 0;
> }
> 

-- 
With Best Regards,
Andy Shevchenko