Re: [Intel-gfx] [PATCH] drm/i915: Remove unused IRQ chip data of HDMI LPE audio

[Takashi Iwai]

On Wed, 13 Dec 2017 12:35:54 +0100,
Thomas Gleixner wrote:
> 
> > > On Mon, 11 Dec 2017, Anand, Jerome wrote:
> > > > > On Fri, 8 Dec 2017, Ville SyrjÃlà wrote:
> > > > >
> > > > > > On Fri, Dec 08, 2017 at 05:33:23PM +0800, Augustine.Chen wrote:
> > > > > > > The chip data of HDMI LPE audio is set to drm_i915_private which
> > > > > > > is not consistent with the expectation by x86 APIC driver.
> > > > > >
> > > > > > Hmm. Why is the apic code looking at data for an irq chip it
> > > > > > hasn't created?
> > > > > >
> > > >
> > > > apic code expects an irq domain to be place as generic approach.
> > > 
> > > APIC code does not even see that interrupt at all. It's completely disconnected.
> > > 
> > 
> > That's the problem - APIC just converts the chip data to its internal
> > format and fails.
> 
> How does APIC code end up to touch that interrupt at all? Call stack please.

It's found in the bugzilla referred in the patch:
  https://bugs.freedesktop.org/show_bug.cgi?id=103731

[   87.353072] irq 298 idata->chip->name hdmi_lpe_audio_irqchip
[   87.353072] irq 298 apic_chip_data
[   87.353073] irq 298 data->domain is NULL
[   87.353120] BUG: unable to handle kernel NULL pointer dereference at (null)
[   87.353132] IP: setup_vector_irq+0x1ba/0x230
[   87.353133] PGD 0

If my understanding is correct, it happens only with 4.14 and earlier
kernels where __setup_vector_irq() loops over the all irqs:

static void __setup_vector_irq(int cpu)
{
	struct apic_chip_data *data;
	struct irq_desc *desc;
	int irq, vector;

	/* Mark the inuse vectors */
	for_each_irq_desc(irq, desc) {
		struct irq_data *idata = irq_desc_get_irq_data(desc);

		data = apic_chip_data(idata);
		if (!data || !cpumask_test_cpu(cpu, data->domain))
			continue;
		....

And since we have assigned a non-APIC chip data in the driver, the
code above refers to a wrong object, leading to Oops.

As a further note, the setup_vector_irq() code has been changed in
4.15, and such a reference won't happen any longer.  So the patch
isn't necessary for now, although it's not bad to take as a cleanup.
And we can eventually put Cc to stable there since it actually works
around the issue above for the older kernels -- of course, with more
detailed descriptions about the background.


thanks,

Takashi