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

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:

quoted

On Fri, May 14, 2021 at 12:26 PM Dan Carpenter [off-list ref] wrote:

quoted

On Thu, May 13, 2021 at 09:52:27AM +0100, Colin King wrote:

...

quoted

quoted

      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.

quoted

 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