Message-ID: <Z1cxoUFCnkN0Ux24@smile.fi.intel.com>
Date: Mon, 9 Dec 2024 20:06:25 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Daniel Scally <djrscally@...il.com>, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] device property: do not leak child nodes when using
 NULL/error pointers

On Thu, Dec 05, 2024 at 12:57:38PM -0800, Dmitry Torokhov wrote:
> On Wed, Dec 04, 2024 at 03:16:34AM +0200, Andy Shevchenko wrote:
> > 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?
> 
> We could potentially do something like below.
> 
> BTW, do you know why fwnode_property_get_reference_args() returns
> -ENOENT for NULL or error fwnode instead of -EINVAL as the rest of them?

I think we need to ask author, but I believe it's due to the OF analogue.
(Haven't checked myself, though)

> And would you object to unifying this?

...

> +#define FWNODE_ITERATE(n, result, cont_val, op, ...)				\

for_each_fwnode() or alike.

> +({										\
> +	int __ret = -EINVAL;							\
> +	typeof(result) __r;							\
> +										\
> +	for (const struct fwnode_handle *__node = n;				\
> +	     !IS_ERR_OR_NULL(__node);						\
> +	     __node = __node->secondary) {					\
> +	        if (!__node->ops || !__node->ops->op) {				\
> +			__ret = -ENXIO;						\
> +			continue;						\
> +		}								\
> +		__r = __node->ops->op(__node, ## __VA_ARGS__);			\
> +		if (__r != cont_val) {						\
> +			result = __r;						\
> +			__ret = 0;						\
> +			break;							\
> +		}								\
> +	}									\
> +	__ret;									\

With a bit of polishing this may be the way to go.

> +})

-- 
With Best Regards,
Andy Shevchenko

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