Re: [RFC PATCH v2 08/74] livetree: Improve get_node_by_phandle()

[Herve Codina <herve.codina@bootlin.com>]

Hi David,

On Fri, 23 Oct 2026 14:25:29 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Aug 26, 2026 at 11:48:37AM +0200, Herve Codina wrote:
> > get_node_by_phandle() allows to get a node based on its phandle value.
> > It checks the phandle value against value available in internal node
> > structure.
> > 
> > This internal phandle value is updated during process_check() and so,
> > get_node_by_phandle() cannot give correct results before the
> > process_check() call.
> > 
> > Improve get_node_by_phandle() to look at node phandle properties when
> > the internal phandle value is not valid.
> > 
> > This allows to return a correct matching node even if process_check()
> > was not called yet.  
> 
> The idea is sound - it's always been kind of clunky that we need the
> process_checks() pass to fill the phandles before we can use them.
> But, there's at least one significant catch:
> 
> [Aside: you might notice the way it's done violates the principle I
>  stated elsewhere of avoiding structures where you must know where you
>  are in the flow to know which parts are populated.  My type
>  discipline has improved since I first wrote this code :)]
> 
> We need to gracefully handle the case of malformed phandle /
> linux,phandle properties; that's actually the reason for the weird
> placement of this in checks.c.  By treating it as a "check" we handle
> this case by failing the check.  Other checks that rely on phandles
> then have the 'explicit_phandles' check as a prereq so they don't need
> to verify this again.  We can still report errors for other checks
> that don't rely on that (which is the whole point of the checks
> mechanism).
> 
> That's not the only way to do it, of course, but we need something to
> check for that error case.  Further notes on the code.
> 
> > With the recently introduced FDT_PROPDATA_HANDLE dtb tag, this will be
> > needed to update internal phandle references before the call to
> > process_check(). Indeed, this tag allows to identify phandles and
> > internal references need to be updated based on the phandle value before
> > the process_check() call.
> > 
> > Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> > ---
> >  livetree.c | 21 ++++++++++++++++++++-
> >  1 file changed, 20 insertions(+), 1 deletion(-)
> > 
> > diff --git a/livetree.c b/livetree.c
> > index abe73de1..a9f8c5d2 100644
> > --- a/livetree.c
> > +++ b/livetree.c
> > @@ -628,16 +628,35 @@ struct node *get_node_by_label(struct node *tree, const char *label)
> >  	return NULL;
> >  }
> >  
> > +static cell_t get_node_phandle_existing(struct node *node)
> > +{
> > +	struct property *prop;
> > +
> > +	if (phandle_is_valid(node->phandle))
> > +		return node->phandle;
> > +
> > +	prop = get_property(node, "phandle");
> > +	if (!prop) {
> > +		prop = get_property(node, "linux,phandle");
> > +		if (!prop)
> > +			return 0;
> > +	}
> > +
> > +	return propval_cell(prop);  
> 
> In the case of a malformed property, this could SEGV.

Ok, will update to
--- 8< ---
	if (prop->val.len != sizeof(cell_t))
		return 0;

	return propval_cell(prop)
--- 8< ---

> 
> > +}
> > +
> >  struct node *get_node_by_phandle(struct node *tree, cell_t phandle)
> >  {
> >  	struct node *child, *node;
> > +	cell_t tree_phandle;
> >  
> >  	if (!phandle_is_valid(phandle)) {
> >  		assert(generate_fixups);
> >  		return NULL;
> >  	}
> >  
> > -	if (tree->phandle == phandle) {
> > +	tree_phandle = get_node_phandle_existing(tree);  
> 
> This means that every time we search for a phandle, for every node
> scanned before the destination with no phandle, we do a full property
> scan to check that no, it still doesn't have a phandle.
> 
> I don't care that much about dtc performance, but that's still a bit
> much to swallow.  I think we need to cache negative lookups to avoid
> that.  Happily, since there are two invalid phandle values this
> shouldn't be too hard.  We could, say, use tree->phandle == 0 to mean
> we don't know if there's a phandle, and tree->phandle == -1 to mean
> we've scanned the properties and we know there isn't one.
> 
> In fact, caching property lookups might be the better way to do this.
> Rather than having an explicit pass where tree->phandle is populated,
> we could have get_node_by_phandle() cache its property scan (success
> or failure) in tree->phandle.
> 
> I think that's a more elegant approach than the current one, but we do
> still need to handle the malformed property case.

Well, get_node_phandle_existing() can be improved:
---- 8< ----
static cell_t get_node_phandle_existing(struct node *node)
{
	cell_t phandle1, phandle2;
	struct property *prop1, *prop2;

	if (phandle_is_valid(node->phandle))
		return node->phandle;
	if (node->phandle == ~0UL) {
		/*
		 * We've already looked at existing properties and no valid
		 * phandle value were present. Avoid looking at them again
		 * and just return the invalid phandle value.
		 */
		return 0;
	}

	/* Try to get it from the 'phandle' property */
	phandle1 = 0; /* Assume invalid */
	prop1 = get_property(node, "phandle");
	if (prop1) {
		if (prop1->val.len == sizeof(cell_t))
			phandle1 = propval_cell(prop1); /* Well formed -> Get it */
	}

	/* Try to get it from the 'linux,phandle' property */
	phandle2 = 0; /* Assume invalid */
	prop2 = get_property(node, "linux,phandle");
	if (prop2) {
		if (prop2->val.len == sizeof(cell_t))
			phandle2 = propval_cell(prop2); /* Well formed -> Get it */
	}

	if (prop1 && prop2) {
		/*
		 * Both property are present. They values must be the same
		 * and valid.
		 */
		if (phandle1 == phandle2) && phandle_is_valid(phandle1)) {
			/* Ok, Cache this correct phandle value and return it */
			node->phandle = phandle1;
			return phandle1;
		}
	} else if (prop1) {
		/*
		 * Only the 'phandle' property is present.
		 * Check its value validity
		 */
		if (phandle_is_valid(phandle1)) {
			/* Ok, Cache this correct phandle value and return it */
			node->phandle = phandle1;
			return phandle1;
		}
	} else if (prop2) {
		/*
		 * Only the 'linux,phandle' property is present.
		 * Check its value validity
		 */
		if (phandle_is_valid(phandle2)) {
			/* Ok, Cache this correct phandle value and return it */
			node->phandle = phandle2;
			return phandle2;
		}
	}

	/*
	 * No valid phandle value were found. Cache this information and return
	 * the invalid phandle value.
	 */
	node->phandle == ~0UL;
	return 0;
}
--- 8< ---

In get_node_by_phandle(), no change compared to my proposition and so the
full function is:
--- 8< ---
struct node *get_node_by_phandle(struct node *tree, cell_t phandle)
{
        struct node *child, *node;
        cell_t tree_phandle;

        if (!phandle_is_valid(phandle)) {
                assert(generate_fixups);
                return NULL;
        }

        tree_phandle = get_node_phandle_existing(tree);
        if (phandle_is_valid(tree_phandle) && tree_phandle == phandle) {
                if (tree->deleted)
                        return NULL;
                return tree;
        }

        for_each_child(tree, child) {
                node = get_node_by_phandle(child, phandle);
                if (node)
                        return node;
        }

        return NULL;
}
--- 8< ---

Caching should work correctly without any other modification.

I let you double-check on your side. Let me know about your opinion.

Best regards,
Hervé