Re: [PATCH 0/6] leds: fix broken devres usage

Andy Shevchenko
2020-06-01T14:08:53Z

On Mon, Jun 1, 2020 at 5:01 PM Johan Hovold <johan@kernel.org> wrote:
>
> On Mon, Jun 01, 2020 at 04:51:01PM +0300, Andy Shevchenko wrote:
> > On Mon, Jun 1, 2020 at 4:42 PM Johan Hovold <johan@kernel.org> wrote:
> > >
> > > Several MFD child drivers register their class devices directly under
> > > the parent device (about half of the MFD LED drivers do so).
> > >
> > > This means you cannot blindly do devres conversions so that
> > > deregistration ends up being tied to the parent device, something which
> > > leads to use-after-free on driver unbind when the class device is
> > > released while still being registered (and, for example, oopses on later
> > > parent MFD driver unbind or LED class callbacks, or resource leaks and
> > > name clashes on child driver reload).
> >
> > Shouldn't MFD take reference count for their children?
>
> That's not the issue here. The child driver is allocating memory for the
> class device (for example using devres), and that will end up being
> freed on unbind while said device is still registered. The child driver
> may then even be unloaded. No extra reference can fix this.

Okay, I didn't still get how dropping devres will help here.

Say, we have

->probe()
{
return devm_foo_register();
}

and no ->remove()

vs.

->probe()
{
return foo_register();
}

->remove()
{
foo_unregister();
}

So, basically what you seem to workaround is that ->remove() is not
getting called?

--
With Best Regards,
Andy Shevchenko