linux-kernel - Re: [PATCH] gpiolib: check for overflow when reading the 'ngpios' property

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Date:   Mon, 7 Mar 2022 12:23:56 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpiolib: check for overflow when reading the 'ngpios'
 property

On Mon, Mar 07, 2022 at 11:08:51AM +0100, Bartosz Golaszewski wrote:
> On Mon, Mar 7, 2022 at 10:53 AM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> > On Mon, Mar 07, 2022 at 12:23:03AM +0200, Andy Shevchenko wrote:
> > > On Mon, Mar 7, 2022 at 12:19 AM Andy Shevchenko
> > > <andy.shevchenko@...il.com> wrote:
> > > > On Mon, Mar 7, 2022 at 12:11 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:

> > > > > The ngpio fields both in struct gpio_device as well as gpio_chip are
> > > > > 16-bit unsigned integers. Let's not risk an overflow and check if the
> > > > > property value represented as a 32-bit unsigned integer is not greater
> > > > > than U16_MAX.
> > > >
> > > > ...
> > > >
> > > > > +               if (ngpios > U16_MAX) {
> > > > > +                       ret = EINVAL;
> > > > > +                       goto err_free_descs;
> > > > > +               }
> > > >
> > > > I don't think it's a fatal error in this case. I would perhaps print a
> > > > warning and simply use a masked (which is done implicitly by an
> > > > assignment of the different type) value. Note, the above is buggy on
> > > > the buggy DTs, where the upper part of the value is not used. After
> > > > this patch you effectively make a regression on, yes, broken DTs.
> > >
> > > Like
> > >
> > >     if (ngpios > U16_MAX)
> > >         chip_warn(gc, "line cnt %u is greater than supported; use
> > > %u\n", ngpios, (u16)ngpio);
> >
> > Or to be on safer side move this after == 0 check as
> >
> >         if (gc->ngpio != ngpios)
> >                 chip_warn(gc, "line cnt %u is greater than supported; use %u\n", ngpios, gc->ngpio);
> >
> 
> ngpios is not necessarily used so this check must be in the scope of
> the device property read (inside the if (gc->ngpio == 0) { block).

Can be done as

        if (gc->ngpio == 0) {
		...
	} else {
		ngpios = gc->ngpio;
	}

        if (gc->ngpio == 0) {
		...
	}

	if (gc->ngpio != ngpios)
		chip_warn(gc, "line cnt %u is greater than supported; use %u\n", ngpios, gc->ngpio);

The point of this exercise is to avoid hard coded type of the variable in a
few places, so if gc->ngpio and/or ngpios have changed type in the future,
you don't need to change this code.


-- 
With Best Regards,
Andy Shevchenko