Re: [PATCH] driver core: platform: don't oops on unbound devices

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello Dmitry,

On Sat, Dec 12, 2020 at 11:49:26PM +0300, Dmitry Baryshkov wrote:
> On Sat, 12 Dec 2020 at 18:39, Uwe Kleine-König
> <u.kleine-koenig@pengutronix.de> wrote:
> > On Sat, Dec 12, 2020 at 12:41:32PM +0100, Greg Kroah-Hartman wrote:
> > > On Sat, Dec 12, 2020 at 04:14:26AM +0300, Dmitry Baryshkov wrote:
> > > > Platform code stopped checking if the device is bound to the actual
> > > > platform driver, thus calling non-existing drv->shutdown(). Verify that
> > > > _dev->driver is not NULL before calling remove/shutdown callbacks.
> > > >
> > > > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> > > > Fixes: 9c30921fe799 ("driver core: platform: use bus_type functions")
> > > > ---
> > > >  drivers/base/platform.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> > > > index 0358dc3ea3ad..93f44e69b472 100644
> > > > --- a/drivers/base/platform.c
> > > > +++ b/drivers/base/platform.c
> > > > @@ -1342,7 +1342,7 @@ static int platform_remove(struct device *_dev)
> > > >     struct platform_device *dev = to_platform_device(_dev);
> > > >     int ret = 0;
> > > >
> > > > -   if (drv->remove)
> > > > +   if (_dev->driver && drv->remove)
> > > >             ret = drv->remove(dev);
> > > >     dev_pm_domain_detach(_dev, true);
> > >
> > > I don't object to this, but it always feels odd to be doing pointer math
> > > on a NULL value, wait until the static-checkers get ahold of this and
> > > you get crazy emails saying you are crashing the kernel (hint, they are
> > > broken).
> >
> > I think you refer to the line
> >
> >         struct platform_driver *drv = to_platform_driver(_dev->driver);
> >
> > which when _dev->driver is NULL results in drv being something really
> > big?!
> 
> Yes. To remove pointer math on NULL value I can move the check for
> _dev->driver before calculating drv.

Yeah, that would be good.

> > Accoding to my understanding platform_remove() shouldn't be called if
> > the device isn't bound to a driver.
> >
> > > But, I don't see why this check is needed?  If a driver is not bound to
> > > a device, shouldn't this whole function just not be called?  Or error
> > > out at the top?
> > >
> > > Uwe, I'd really like your review/ack of this before taking it.
> >
> > So I agree and have the same question. So I wonder: @Dmitry, did you see
> > a crash? When did it happen?
> 
> The crash happens in the platform_shutdown() function, which gets
> called for unbound devices after commit 9c30921fe ("driver core:
> platform: use bus_type functions").
> I can include crash trace into v2.

Ah, now I understood. I didn't look too closely on your patch, only on
what Greg quoted. So you added a check to platform_remove (which should
be unnecessary) and to platform_shutdown (where I agree the check is
necessary).

> I added a check to platform_remove() as a safety measure. All current
> calls for dev->bus->remove() in dd.c seem to happen only when
> dev->driver is set, but I thought that it might be a good check. I can
> drop it if you'd like.

Yes, I'd like you to drop this. .remove isn't called for devices without
drivers.

Best regards and thanks for cleaning up after me,
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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