linux-kernel - Re: [PATCH v2 1/2] driver core: platform: Clarify that IRQ 0 is invalid

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Date:   Mon, 4 May 2020 13:08:22 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Aman Sharma <amanharitsh123@...il.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 1/2] driver core: platform: Clarify that IRQ 0 is
 invalid

On Sat, May 02, 2020 at 08:15:37AM +0200, Greg Kroah-Hartman wrote:
> On Fri, May 01, 2020 at 05:40:41PM -0500, Bjorn Helgaas wrote:
> > From: Bjorn Helgaas <bhelgaas@...gle.com>
> > 
> > These interfaces return a negative error number or an IRQ:
> > 
> >   platform_get_irq()
> >   platform_get_irq_optional()
> >   platform_get_irq_byname()
> >   platform_get_irq_byname_optional()
> > 
> > The function comments suggest checking for error like this:
> > 
> >   irq = platform_get_irq(...);
> >   if (irq < 0)
> >     return irq;
> > 
> > which is what most callers (~900 of 1400) do, so it's implicit that IRQ 0
> > is invalid.  But some callers check for "irq <= 0", and it's not obvious
> > from the source that we never return an IRQ 0.
> > 
> > Make this more explicit by updating the comments to say that an IRQ number
> > is always non-zero and adding a WARN() if we ever do return zero.  If we do
> > return IRQ 0, it likely indicates a bug in the arch-specific parts of
> > platform_get_irq().
> 
> I worry about adding WARN() as there are systems that do panic_on_warn()
> and syzbot trips over this as well.  I don't think that for this issue
> it would be a problem, but what really is this warning about that
> someone could do anything with?
> 
> Other than that minor thing, this looks good to me, thanks for finally
> clearing this up.

What I'm concerned about is an arch that returns 0.  Most drivers
don't check for 0 so they'll just try to use it, and things will fail
in some obscure way.  My assumption is that if there really is no IRQ,
we should return -ENOENT or similar instead of 0.

I could be convinced that it's not worth warning about at all, or we
could do something like the following:

diff --git a/drivers/base/platform.c b/drivers/base/platform.c
index 084cf1d23d3f..4afa5875e14d 100644
--- a/drivers/base/platform.c
+++ b/drivers/base/platform.c
@@ -220,7 +220,11 @@ int platform_get_irq_optional(struct platform_device *dev, unsigned int num)
 	ret = -ENXIO;
 #endif
 out:
-	WARN(ret == 0, "0 is an invalid IRQ number\n");
+	/* Returning zero here is likely a bug in the arch IRQ code */
+	if (ret == 0) {
+		pr_warn("0 is an invalid IRQ number\n");
+		dump_stack();
+	}
 	return ret;
 }
 EXPORT_SYMBOL_GPL(platform_get_irq_optional);
@@ -312,7 +316,11 @@ static int __platform_get_irq_byname(struct platform_device *dev,
 
 	r = platform_get_resource_byname(dev, IORESOURCE_IRQ, name);
 	if (r) {
-		WARN(r->start == 0, "0 is an invalid IRQ number\n");
+		/* Returning zero here is likely a bug in the arch IRQ code */
+		if (r->start == 0) {
+			pr_warn("0 is an invalid IRQ number\n");
+			dump_stack();
+		}
 		return r->start;
 	}