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

[Andy Shevchenko]

On Tue, Aug 15, 2023 at 05:43:53PM +0300, Andy Shevchenko wrote:
> On Tue, Aug 15, 2023 at 03:07:50PM +0200, Linus Walleij wrote:
> > On Tue, Aug 15, 2023 at 2:57 PM Andy Shevchenko
> > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > On Tue, Aug 15, 2023 at 01:40:22PM +0200, Linus Walleij wrote:
> > > > On Tue, Aug 15, 2023 at 11:50 AM Andy Shevchenko
> > > > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > > > On Fri, Aug 11, 2023 at 09:30:34PM +0200, Bartosz Golaszewski wrote:

...

> > > > > > +     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?
> > >
> > > Basically the Q is
> > > "if desc is non-NULL, does it guarantee that gdev is non-NULL either?"
> > 
> > gdev->desc is assigned in one single spot, which is in
> > gpiochip_add_data_with_key():
> > 
> >        for (i = 0; i < gc->ngpio; i++)
> >                 gdev->descs[i].gdev = gdev;
> > 
> > It is never assigned anywhere else, so I guess yes.
> > 
> > We may also ask if it is ever invalid (i.e. if desc->gdev can point to
> > junk).
> > 
> > A gdev turns to junk when its reference count goes down to zero
> > and gpiodev_release() is called effectively calling kfree() on the
> > struct gpio_device *.
> > 
> > But that can only happen as a result of module_put() getting
> > called, pulling the references down to zero. Which is what we
> > are discussing. The line after module_put(), desc->gdev
> > *could* be NULL.
> 
> Yes.
> 
> > But then we just call gpio_device_put(desc->gdev) which is
> > just a call to device_put(), which is NULL-tolerant.
> 
> But gpio_device_put() does not NULL tolerant.
> So, oops in this line then.

That said, this gpiod_free() function needs a lot of additional comments to
explain all this.

-- 
With Best Regards,
Andy Shevchenko