Subject: [PATCH 1/2] of: base: Allow more args than MAX_PHANDLE_ARGS if required

From: [email-masked] (Joerg Roedel)
Date: Thu, 16 Jul 2015 13:09:00 +0200

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