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

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

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

...

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.

 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