linux-kernel - Re: [PATCH 1/2] of: base: Allow more args than MAX_PHANDLE

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Message-ID: <20150716110900.GA30130@suse.de>
Date:	Thu, 16 Jul 2015 13:09:00 +0200
From:	Joerg Roedel <jroedel@...e.de>
To:	Will Deacon <will.deacon@....com>
Cc:	Joerg Roedel <joro@...tes.org>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 1/2] of: base: Allow more args than MAX_PHANDLE_ARGS if
 required

Hi Will,

On Thu, Jul 16, 2015 at 11:23:26AM +0100, Will Deacon wrote:
> On Thu, Jul 16, 2015 at 09:30:43AM +0100, Joerg Roedel wrote:
> > +struct of_phandle_args *of_alloc_phandle_args(int size)
> > +{
> > +	struct of_phandle_args *args;
> > +	int e = max(0, size - MAX_PHANDLE_ARGS);
> > +
> > +	args =  kzalloc(sizeof(struct of_phandle_args) + e * sizeof(uint32_t),
> > +			GFP_KERNEL);
> 
> Should you also update args->args_count to reflect the extended array?

The args_count member just tells us how many of the array elements are
used and not how many there are. So it doesn't need to be updated here.

> That said, extending the fixed-size array member like this feels a bit
> fragile. Does GCC not complain about out-of-bounds accesses if you
> statically address args->args[MAX_PHANDLE_ARGS]? Admittedly, I can't
> think *why* this would be break (things like additional padding will be
> harmless), but I'm not intimate with the C standard.

Yeah, I agree, it is not the best possible solution. But this way I
don't need to update all callers, and thus it works better with our
development model.

But I am open for suggestions on how to solve this problem better. In
fact, my main motivation in sending this was to get the discussion about
an upstreamable solution started :)

Lets see what the device-tree maintainers have to say.

> I guess the more worrying possibility is if somebody adds a new member to
> the end of of_phandle_args.

I should probably add a comment there.



	Joerg

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