linux-kernel - Re: [PATCH v3] gpiolib: fix reference leaks when removing GPIO chips still in use

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Message-ID: <CACRpkdZ32gW3YgQKPbWTnoRwxjXkViendGMhrAxfyp+W4NbqkA@mail.gmail.com>
Date:   Tue, 15 Aug 2023 13:40:22 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Bartosz Golaszewski <brgl@...ev.pl>,
        Kent Gibson <warthog618@...il.com>, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v3] gpiolib: fix reference leaks when removing GPIO chips
 still in use

On Tue, Aug 15, 2023 at 11:50 AM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
> On Fri, Aug 11, 2023 at 09:30:34PM +0200, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >
> > After we remove a GPIO chip that still has some requested descriptors,
> > gpiod_free_commit() will fail and we will never put the references to the
> > GPIO device and the owning module in gpiod_free().
> >
> > Rework this function to:
> > - not warn on desc == NULL as this is a use-case on which most free
> >   functions silently return
> > - put the references to desc->gdev and desc->gdev->owner unconditionally
> >   so that the release callback actually gets called when the remaining
> >   references are dropped by external GPIO users
>
> ...
>
> > -     if (desc && desc->gdev && gpiod_free_commit(desc)) {
>
> The commit message doesn't explain disappearing of gdev check.
>
> > -             module_put(desc->gdev->owner);
> > -             gpio_device_put(desc->gdev);
> > -     } else {
> > +     /*
> > +      * We must not use VALIDATE_DESC_VOID() as the underlying gdev->chip
> > +      * may already be NULL but we still want to put the references.
> > +      */
> > +     if (!desc)
> > +             return;
> > +
> > +     if (!gpiod_free_commit(desc))
> >               WARN_ON(extra_checks);
> > -     }
> > +
> > +     module_put(desc->gdev->owner);
> > +     gpio_device_put(desc->gdev);
> >  }
>
> So, if gdev can be NULL, you will get an Oops with new code.

I read it such that gdev->chip can be NULL, but not gdev,
and desc->gdev->owner is fine to reference?

Yours,
Linus Walleij