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

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Message-ID: <YKIcKLLP1VXBhYfO@smile.fi.intel.com>
Date:   Mon, 17 May 2021 10:32:56 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Colin King <colin.king@...onical.com>
Cc:     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>,
        "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 10:03:15AM +0300, Andy Shevchenko wrote:
> On Thu, May 13, 2021 at 12:12 PM Colin King <colin.king@...onical.com> wrote:
> >
> > From: Colin Ian King <colin.king@...onical.com>
> >
> > The left shift of the u32 integer v is evaluated using 32 bit
> > arithmetic and then assigned to a u64 integer. There are cases
> > where v will currently overflow on the shift. Avoid this by
> > casting it to unsigned long (same type as map[]) before shifting
> > it.
> >
> > Addresses-Coverity: ("Unintentional integer overflow")
> > Fixes: 02b3f84d9080 ("gpio: xilinx: Switch to use bitmap APIs")
> 
> No, it is a false positive,
> 
> >         const unsigned long offset = (bit % BITS_PER_LONG) & BIT(5);
> 
> See above, offset is 0 when BITS_PER_LONG == 32 and 32 when it's equal to 64.

Should be read as "...and 0 or 32 when..."

> > -       map[index] |= v << offset;
> > +       map[index] |= (unsigned long)v << offset;

-- 
With Best Regards,
Andy Shevchenko