Re: [PATCH 1/2] device property: do not leak child nodes when using NULL/error pointers

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Tue, Dec 03, 2024 at 02:45:49PM -0800, Dmitry Torokhov wrote:
> On Tue, Dec 03, 2024 at 03:27:31PM +0200, Andy Shevchenko wrote:
> > On Mon, Dec 02, 2024 at 09:49:06PM -0800, Dmitry Torokhov wrote:
> > > On Sat, Nov 30, 2024 at 11:44:04PM +0200, Andy Shevchenko wrote:
> > > > On Fri, Nov 29, 2024 at 11:16:54PM -0800, Dmitry Torokhov wrote:
> > > > > On Fri, Nov 29, 2024 at 04:50:15PM +0200, Andy Shevchenko wrote:
> > > > > > On Thu, Nov 28, 2024 at 03:04:50PM -0800, Dmitry Torokhov wrote:
> > > > > > > On Thu, Nov 28, 2024 at 03:13:16PM +0200, Andy Shevchenko wrote:
> > > > > > > > On Wed, Nov 27, 2024 at 09:39:34PM -0800, Dmitry Torokhov wrote:

...

> > > > > > > > > @@ struct fwnode_handle *device_get_next_child_node(const struct device *dev,
> > > > > > > > >  	const struct fwnode_handle *fwnode = dev_fwnode(dev);
> > > > > > > > >  	struct fwnode_handle *next;
> > > > > > > > 
> > > > > > > > > -	if (IS_ERR_OR_NULL(fwnode))
> > > > > > > > > +	if (IS_ERR_OR_NULL(fwnode)) {
> > > > > > > > > +		fwnode_handle_put(child);
> > > > > > > > >  		return NULL;
> > > > > > > > > +	}
> > > > > > > > 
> > > > > > > > >  	/* Try to find a child in primary fwnode */
> > > > > > > > >  	next = fwnode_get_next_child_node(fwnode, child);
> > > > > > > > 
> > > > > > > > So, why not just moving the original check (w/o dropping the reference) here?
> > > > > > > > Wouldn't it have the same effect w/o explicit call to the fwnode_handle_put()?
> > > > > > > 
> > > > > > > Because if you rely on check in fwnode_get_next_child_node() you would
> > > > > > > not know if it returned NULL because there are no more children or
> > > > > > > because the node is invalid. In the latter case you can't dereference
> > > > > > > fwnode->secondary.
> > > > > > 
> > > > > > Yes, so, how does it contradict my proposal?
> > > > > 
> > > > > I guess I misunderstood your proposal then. Could you please explain it
> > > > > in more detail?
> > > > 
> > > > 
> > > > Current code (in steps):
> > > > 	if (IS_ERR_OR_NULL()) check
> > > > 	trying primary
> > > > 	trying secondary if previous is NULL
> > > > 
> > > > 
> > > > My proposal
> > > > 
> > > > 	trying primary
> > > > 	return if not NULL
> > > > 	if (IS_ERR_OR_NULL()) check in its current form (no put op)
> > > > 	trying secondary
> > > > 
> > > > After your first patch IIUC this is possible as trying primary will put child uncoditionally.
> > > 
> > > Ah, I see. No, I do not think this is a good idea: it will make the code
> > > harder to understand for a casual reader: "Why do we check node validity
> > > only after we used it for the first time?"
> > 
> > Theare a re already a few API calls there that are hard to understand, I spent
> > some time on them to get it through and still got it wrong as this series
> > shows. So, I don't think we anyhow change this.
> 
> The fact that some code is confusing does not mean that we should add
> more confusing code. We will not fix everything at once, but we can make
> things better bit by bit.
> 
> Look, the check where it is now makes total sense, you added it there
> yourself! It checks that we are dealing with a valid node and returns
> early. The intent is very easy to understand and the only thing that is
> missing is that "put" operation to satisfy the documented behavior.
> Anything more just makes things more complex for no good reason.

Right, that's why I think we need to go away from open coding the iteration
over the list of nodes (primary, secondary, etc).

> > > For the code not in a hot path there is a lot of value in simplicity.
> > 
> > If you really want to go to this rabbit hole, think how we can get rid of
> > repetitive checks of the secondary or more if any in the future nodes in the
> > list.
> > 
> > So the basic idea is to have this all hidden (to some extent) behind the macro
> > or alike. In the code it would be something as
> > 
> >   for node in primary, secondary, ...
> >     call the API
> >     if (okay)
> > 	return result
> > 
> >   return error
> > 
> > This will indeed help.
> 
> I think this will indeed help if we ever going to have more than primary
> and secondary nodes. It is also tricky if you want to transition
> seamlessly between different types of nodes (i.e. you have ACPI primary
> with OF overlay secondary with swnode as tertiary etc). And you probably
> want to add support for references between different typesof nodes
> (i.e. swnode being able to reference OF device node for example).
> 
> This kind of rework is however out of scope of what I have time to do at
> the moment.

I am not asking you to invest into big rework, the idea is to try to fold the
iterations to a kind of loop. Is it feasible?

Or maybe it can be partially done, so the above becomes something like

call_prmary_op(fwnode, ...)
	if (IS_ERR_OR_NULL(fwnode))
		...
	else
		call_op()

call_secondary_op(fwnode, ...)
	if (IS_ERR_OR_NULL(fwnode))
		...
	else
		call_op()

(with the potential of collapsing one into the other)

and then the above

	next = call_primary_op(wnode, ...);
	if (next)
		return next;

	return call_secondary_op(fwnode, ...);

-- 
With Best Regards,
Andy Shevchenko