Re: [PATCH v2] libfdt: overlay: ensure that existing phandles are not overwritten

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Feb 22, 2024 at 08:22:48AM +0100, Uwe Kleine-König wrote:
> Hello David,
> 
> On Thu, Feb 22, 2024 at 05:32:10PM +1100, David Gibson wrote:
> > On Fri, Feb 16, 2024 at 07:19:20PM +0100, Uwe Kleine-König wrote:
> > > [...]
> > > diff --git a/libfdt/fdt_overlay.c b/libfdt/fdt_overlay.c
> > > index 5c0c3981b89d..914acc5b14a6 100644
> > > --- a/libfdt/fdt_overlay.c
> > > +++ b/libfdt/fdt_overlay.c
> > > @@ -520,6 +520,228 @@ static int overlay_fixup_phandles(void *fdt, void *fdto)
> > >  	return 0;
> > >  }
> > >  
> > 
> > All of these new functions could do with descriptive comments saying
> > what they do (see the existing code for examples).
> 
> Yeah, I was lazy. I didn't expect this patch to go in without comments
> and avoided investing time documenting functions that maybe will change
> in the course of the review-resend loop. :-)
> 
> > > [...]
> > > +static int overlay_update_node_conflicting_references(void *fdto, int tree_node,
> > > +						      int fixup_node,
> > > +						      uint32_t fdt_phandle,
> > > +						      uint32_t fdto_phandle)
> > > +{
> > > +	int fixup_prop;
> > > +	int fixup_child;
> > > +	int ret;
> > > +
> > > +	fdt_for_each_property_offset(fixup_prop, fdto, fixup_node) {
> > > +
> > 
> > Extraneous blank line.
> > 
> > > +		const fdt32_t *fixup_val;
> > > +                const char *tree_val;
> > 
> > libfdt style is to use tab indents, looks like most of this block has
> > space indents instead.
> 
> Huh, I'm surprised about myself that I didn't notice this alone.
> 
> > > +
> > > +			/*
> > > +			 * phandles to fixup can be unaligned.
> > > +			 *
> > > +			 * Use a memcpy for the architectures that do
> > > +			 * not support unaligned accesses.
> > > +			 */
> > > +			memcpy(&adj_val, tree_val + poffset, sizeof(adj_val));
> > 
> > And here you can use fdt32_ld() which has unaligned access handling
> > built in.
> 
> I copied this from somewhere, so I guess there is some potential for
> improvement in existing code. Will take a look.
>  
> > > [...]
> > > +
> > > +static int overlay_prevent_phandle_overwrite_node(void *fdt, int fdtnode,
> > > +						  void *fdto, int fdtonode)
> > 
> > Maybe "overlay_resolve_phandle_conflict()" for a slightly shorter
> > function name.
> 
> Sounds good, wasn't too lucky with
> overlay_prevent_phandle_overwrite_node() either.
>  
> > > +{
> > > +	uint32_t fdt_phandle, fdto_phandle;
> > > +	int fdtochild;
> > > +
> > > +	fdt_phandle = fdt_get_phandle(fdt, fdtnode);
> > > +	fdto_phandle = fdt_get_phandle(fdto, fdtonode);
> > > +
> > > +	if (fdt_phandle && fdto_phandle) {
> > > +		int ret;
> > > +
> > > +		ret = overlay_adjust_local_conflicting_phandle(fdto,
> > > +							       fdt_phandle,
> > > +							       fdto_phandle);
> > > +		if (ret)
> > > +			return ret;
> > 
> > So while, as you've seen, there can be conflicting phandles between
> > the fdt and the fdto, within the fdto a particular phandle should only
> > appear on a single node.  If a phandle is duplicated across nodes
> > witin a single fdto, you have bigger problems.  Therefore, having
> > found a conflict, you're already found the only place you need to
> > change the phandle property itself - you don't need to scan the full
> > fdto again.
> > 
> > So I believe you can replace the above call with just setting the
> > phandle property on fdtonode.
> 
> Oh right!
>  
> > > [...]
> > >  /**
> > >   * overlay_apply_node - Merges a node into the base device tree
> > >   * @fdt: Base Device Tree blob
> > > @@ -824,18 +1046,26 @@ int fdt_overlay_apply(void *fdt, void *fdto)
> > >  	if (ret)
> > >  		goto err;
> > >  
> > > +	/* Increase all phandles in the fdto by delta */
> > >  	ret = overlay_adjust_local_phandles(fdto, delta);
> > >  	if (ret)
> > >  		goto err;
> > >  
> > > +	/* Adapt the phandle values in fdto to the above increase */
> > >  	ret = overlay_update_local_references(fdto, delta);
> > >  	if (ret)
> > >  		goto err;
> > >  
> > > +	/* Update fdto's phandles using symbols from fdt */
> > >  	ret = overlay_fixup_phandles(fdt, fdto);
> > >  	if (ret)
> > >  		goto err;
> > >  
> > > +	/* Don't overwrite phandles in fdt */
> > > +	ret = overlay_prevent_phandle_overwrite(fdt, fdto);
> > > +	if (ret)
> > > +		goto err;
> > 
> > I think this should go above overlay_fixup_phandles(), so that we're
> > completing all the __local_fixups__ based adjustments before going to
> > the __fixups__ based adjustments.
> 
> This is only to please the reader, right? Or is there a corner case I
> failed to see?

Correct.  At least as far as I've seen, too.

> The feedback I removed in this mail I agree with. Expect a v3 soon.
> 
> Thanks for your time to review my patch,
> Uwe
> 



-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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