linux-kernel - Re: [PATCH v2 4/5] PCI: generic: Correct, and avoid overflow, in bus

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Date:	Wed, 23 Sep 2015 20:35:45 +0100
From:	Will Deacon <will.deacon@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	David Daney <ddaney@...iumnetworks.com>,
	Mark Rutland <Mark.Rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	David Daney <david.daney@...ium.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	David Daney <ddaney.cavm@...il.com>,
	Kumar Gala <galak@...eaurora.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 4/5] PCI: generic: Correct, and avoid overflow, in
 bus_max calculation.

On Wed, Sep 23, 2015 at 08:27:41PM +0100, Arnd Bergmann wrote:
> On Wednesday 23 September 2015 11:21:56 David Daney wrote:
> > >>
> > >>      /* Limit the bus-range to fit within reg */
> > >> -    bus_max = pci->cfg.bus_range->start +
> > >> -              (resource_size(&pci->cfg.res) >> pci->cfg.ops.bus_shift) - 1;
> > >> +    bus_max = (resource_size(&pci->cfg.res) >> pci->cfg.ops.bus_shift) - 1;
> > >> +    if (bus_max > 255)
> > >> +            bus_max = 255;
> > >
> > > I still don't understand the need for this part. If the cfg space is bigger
> > > than bus_max, isn't that simply an invalid resource? Given that the resource
> > > could be broken in other ways too, this check feels more like a specific
> > > workaround rather than generally useful code.
> > 
> > Imagine...
> > 
> >    bus-range [0x80 .. 0xff], this requires a cfg.res that will cover the 
> > entire range of 0..0xff.
> > 
> >    according to the calculations above, (resource_size(&pci->cfg.res) >> 
> > pci->cfg.ops.bus_shift) - 1 will have a value of 0xff, so...
> 
> Extending the computation to 32 bit seems fine, but I'd rather warn loudly
> if the bus range does not fit within the registers.
> 
> Also note that the computation is already correct with my interpretation
> of the reg property.

>From what Lorenzo was saying, ACPI shares the interpretation that David is
implementing here and, given that the DT version seems to be subjective,
aligning this reg property with MMCFG seems to make sense.

Will
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