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

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Message-ID: <CAMRc=Mf8RRkhnwTQuEgr5iavaSpxtvouJ9TrpaGeG6k9+Sq6aA@mail.gmail.com>
Date:   Thu, 2 Dec 2021 11:26:03 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
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 12:18 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> 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!)

Ok, this is the thing I didn't realize, the property can indeed be 0
so your approach is correct.

Bart

> 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
>
>