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

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Message-ID: <CAHp75Ve-YWh_sfupwQV0xxL7Vk8GNObJ+6O29RqRMXCgAmemCw@mail.gmail.com>
Date:   Mon, 17 May 2021 10:07:20 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.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 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?

-- 
With Best Regards,
Andy Shevchenko