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Date:   Thu, 31 Mar 2022 19:18:00 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Won Chung <wonchung@google.com>
Cc:     Benson Leung <bleung@google.com>, Takashi Iwai <tiwai@suse.de>,
        Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        Jaroslav Kysela <perex@perex.cz>,
        Takashi Iwai <tiwai@suse.com>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Benson Leung <bleung@chromium.org>,
        Prashant Malani <pmalani@chromium.org>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH v2] sound/hda: Add NULL check to component match callback
 function
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On Thu, Mar 31, 2022 at 09:58:43AM -0700, Won Chung wrote:
> On Thu, Mar 31, 2022 at 9:38 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Thu, Mar 31, 2022 at 08:33:03AM -0700, Benson Leung wrote:
> > > Hi Takashi,
> > >
> > > On Thu, Mar 31, 2022 at 04:19:15PM +0200, Takashi Iwai wrote:
> > > > On Thu, 31 Mar 2022 15:29:10 +0200,
> > > > Takashi Iwai wrote:
> > > > >
> > > > > On Thu, 31 Mar 2022 11:45:47 +0200,
> > > > > Takashi Iwai wrote:
> > > > > >
> > > > > > On Thu, 31 Mar 2022 11:34:38 +0200,
> > > > > > Heikki Krogerus wrote:
> > > > > > >
> > > > > > > On Thu, Mar 31, 2022 at 11:28:20AM +0200, Takashi Iwai wrote:
> > > > > > > > On Thu, 31 Mar 2022 11:25:43 +0200,
> > > > > > > > Heikki Krogerus wrote:
> > > > > > > > >
> > > > > > > > > On Thu, Mar 31, 2022 at 11:12:55AM +0200, Takashi Iwai wrote:
> > > > > > > > > > > > > -     if (!strcmp(dev->driver->name, "i915") &&
> > > > > > > > > > > > > +     if (dev->driver && !strcmp(dev->driver->name, "i915") &&
> > > > > > > > > > > >
> > > > > > > > > > > > Can NULL dev->driver be really seen?  I thought the components are
> > > > > > > > > > > > added by the drivers, hence they ought to have the driver field set.
> > > > > > > > > > > > But there can be corner cases I overlooked.
> > > > > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > > > > thanks,
> > > > > > > > > > > >
> > > > > > > > > > > > Takashi
> > > > > > > > > > >
> > > > > > > > > > > Hi Takashi,
> > > > > > > > > > >
> > > > > > > > > > > When I try using component_add in a different driver (usb4 in my
> > > > > > > > > > > case), I think dev->driver here is NULL because the i915 drivers do
> > > > > > > > > > > not have their component master fully bound when this new component is
> > > > > > > > > > > registered. When I test it, it seems to be causing a crash.
> > > > > > > > > >
> > > > > > > > > > Hm, from where component_add*() is called?  Basically dev->driver must
> > > > > > > > > > be already set before the corresponding driver gets bound at
> > > > > > > > > > __driver_probe_deviec().  So, if the device is added to component from
> > > > > > > > > > the corresponding driver's probe, dev->driver must be non-NULL.
> > > > > > > > >
> > > > > > > > > The code that declares a device as component does not have to be the
> > > > > > > > > driver of that device.
> > > > > > > > >
> > > > > > > > > In our case the components are USB ports, and they are devices that
> > > > > > > > > are actually never bind to any drivers: drivers/usb/core/port.c
> > > > > > > >
> > > > > > > > OK, that's what I wanted to know.  It'd be helpful if it's more
> > > > > > > > clearly mentioned in the commit log.
> > > > > > >
> > > > > > > Agree.
> > > > > > >
> > > > > > > > BTW, the same problem must be seen in MEI drivers, too.
> > > > > > >
> > > > > > > Wasn't there a patch for those too? I lost track...
> > > > > >
> > > > > > I don't know, I just checked the latest Linus tree.
> > > > > >
> > > > > > And, looking at the HD-audio code, I still wonder how NULL dev->driver
> > > > > > can reach there.  Is there any PCI device that is added to component
> > > > > > without binding to a driver?  We have dev_is_pci() check at the
> > > > > > beginning, so non-PCI devices should bail out there...
> > > > >
> > > > > Further reading on, I'm really confused.  How data=NULL can be passed
> > > > > to this function?  The data argument is the value passed from the
> > > > > component_match_add_typed() call in HD-audio driver, hence it must be
> > > > > always the snd_hdac_bus object.
> > > > >
> > > > > And, I guess the i915 string check can be omitted completely, at
> > > > > least, for HD-audio driver.  It already have a check of the parent of
> > > > > the device and that should be enough.
> > > >
> > > > That said, something like below (supposing data NULL check being
> > > > superfluous), instead.
> > > >
> > > >
> > > > Takashi
> > > >
> > > > --- a/sound/hda/hdac_i915.c
> > > > +++ b/sound/hda/hdac_i915.c
> > > > @@ -102,18 +102,13 @@ static int i915_component_master_match(struct device *dev, int subcomponent,
> > > >     struct pci_dev *hdac_pci, *i915_pci;
> > > >     struct hdac_bus *bus = data;
> > > >
> > > > -   if (!dev_is_pci(dev))
> > > > +   if (subcomponent != I915_COMPONENT_AUDIO || !dev_is_pci(dev))
> > > >             return 0;
> > > >
> > >
> > > If I recall this bug correctly, it's not the usb port perse that is falling
> > > through this !dev_is_pci(dev) check, it's actually the usb4-port in a new
> > > proposed patch by Heikki and Mika to extend the usb type-c component to
> > > encompass the usb4 specific pieces too. Is it possible usb4 ports are considered
> > > pci devices, and that's how we got into this situation?
> > >
> > > Also, a little more background information: This crash happens because in
> > > our kernel configs, we config'd the usb4 driver as =y (built in) instead of
> > > =m module, which meant that the usb4 port's driver was adding a component
> > > likely much earlier than hdac_i915.
> >
> > So is this actually triggering on 5.17 right now?  Or is it due to some
> > other not-applied changes you are testing at the moment?
> >
> > confused,
> >
> > greg k-h
> 
> Hi Greg,
> 
> I believe it is not causing an issue in 5.17 at the moment. It is
> triggered when we try to apply new changes and test it locally.
> (registering a component for usb4_port)

Then why would it ever be needed to be backported to a stable kernel?

Please be more careful.

greg k-h