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

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Date:   Mon, 17 May 2021 16:36:43 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Colin King <colin.king@...onical.com>,
        Shubhrajyoti Datta <shubhrajyoti.datta@...inx.com>,
        Srinivas Neeli <srinivas.neeli@...inx.com>,
        Michal Simek <michal.simek@...inx.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        kernel-janitors <kernel-janitors@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] gpio: xilinx: Fix potential integer overflow on
 shift of a u32 int

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@...cle.com> 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.

regards,
dan carpenter

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;
}