linux-kernel - Re: [PATCH v3 2/2] gpiolib: check the 'ngpios' property in core gpiolib code

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Date:   Thu, 25 Nov 2021 13:18:08 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Johan Hovold <johan@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] gpiolib: check the 'ngpios' property in core
 gpiolib code

On Thu, Nov 25, 2021 at 11:10:08AM +0100, Bartosz Golaszewski wrote:
> On Wed, Nov 24, 2021 at 3:47 PM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> > On Wed, Nov 24, 2021 at 01:28:50PM +0100, Bartosz Golaszewski wrote:

...

> > > +             ret = device_property_read_u32(&gdev->dev, "ngpios", &ngpios);
> > > +             if (ret) {
> > > +                     if (ret == -ENODATA) {
> > > +                             chip_err(gc, "tried to insert a GPIO chip with zero lines\n");
> > > +                             ret = -EINVAL;
> > > +                     }
> > > +
> > > +                     goto err_free_descs;
> > > +             }
> >
> > And if the property returns 0 in ngpios?
> >
> > What about the modified suggestion from previous version:
> >
> >         if (gc->ngpio == 0) {
> >                 ret = device_property_read_u32(&gdev->dev, "ngpios", &ngpios);

> The comment is a good idea but other than that - it's overcomplicating things.

I don't think so. It is plain and self-explaining each step. See at the end of
the message how.

> >                 if (ret == -ENODATA)
> >                         ngpios = 0;
> >                 else if (ret)
> >                         return ret;

> You still need to goto err_free_descs here.

Right, this doesn't affect the main point / logic here.

> >                 gc->ngpio = ngpios;
> >         }
> >
> >         if (gc->ngpio == 0) {
> 
> Why check that again? We already know the driver set it to 0, we
> checked it a couple lines before. If we can't get the setting from the
> properties then it won't be non 0 here right?

No, it's not right. The check is needed to tell that properties supplied 0.

> >                 chip_err(gc, "tried to insert a GPIO chip with zero lines\n");
> >                 ret = -EINVAL;
> >                 goto err_free_descs;
> >         }
> >
> > ?
> >
> > > +             gc->ngpio = ngpios;
> > >       }
> > >
> > >       if (gc->ngpio > FASTPATH_NGPIO)

> I suggest the following:

It's buggy as submitted version (I actually haven't found any difference in
the code, but comments).

You see, I propose less changes and straight forward logic:

1. Check if the supplied ->ngpio equal to 0
2. If so, try device properties
2.1. If there is no property found, make sure we a) don't use uninitialized
     variable, b) we don't change ->ngpio, so it stays 0
2.2. If there is an error, return it as is to the caller
2.3. Assign ->ngpio by value from property (which very well may be 0!)
3. Check ->ngpio for 0 again, if so, issue a message and return -EINVAL to
the user.

We have three places where ->ngpio can be 0, all of them I covered.

-- 
With Best Regards,
Andy Shevchenko