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

[Dmitry Torokhov]

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?
And would you object to unifying this?

Thanks.

-- 
Dmitry

index 0ca3c0908b0c..3b4c394138e2 100644
--- a/drivers/base/property.c
+++ b/drivers/base/property.c
@@ -18,6 +18,28 @@
 #include <linux/string.h>
 #include <linux/types.h>
 
+#define FWNODE_ITERATE(n, result, cont_val, op, ...)				\
+({										\
+	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;									\
+})
+
 struct fwnode_handle *__dev_fwnode(struct device *dev)
 {
 	return IS_ENABLED(CONFIG_OF) && dev->of_node ?
@@ -57,16 +79,14 @@ EXPORT_SYMBOL_GPL(device_property_present);
 bool fwnode_property_present(const struct fwnode_handle *fwnode,
 			     const char *propname)
 {
+	int error;
 	bool ret;
 
-	if (IS_ERR_OR_NULL(fwnode))
+	error = FWNODE_ITERATE(fwnode, ret, false, property_present, propname);
+	if (error)
 		return false;
 
-	ret = fwnode_call_bool_op(fwnode, property_present, propname);
-	if (ret)
-		return ret;
-
-	return fwnode_call_bool_op(fwnode->secondary, property_present, propname);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(fwnode_property_present);
 
@@ -259,18 +279,15 @@ static int fwnode_property_read_int_array(const struct fwnode_handle *fwnode,
 					  unsigned int elem_size, void *val,
 					  size_t nval)
 {
+	int error;
 	int ret;
 
-	if (IS_ERR_OR_NULL(fwnode))
-		return -EINVAL;
-
-	ret = fwnode_call_int_op(fwnode, property_read_int_array, propname,
-				 elem_size, val, nval);
-	if (ret != -EINVAL)
-		return ret;
+	error = FWNODE_ITERATE(fwnode, ret, -EINVAL, property_read_int_array,
+			       propname, elem_size, val, nval);
+	if (error)
+		return error;
 
-	return fwnode_call_int_op(fwnode->secondary, property_read_int_array, propname,
-				  elem_size, val, nval);
+	return ret;
 }
 
 /**
@@ -414,18 +431,15 @@ int fwnode_property_read_string_array(const struct fwnode_handle *fwnode,
 				      const char *propname, const char **val,
 				      size_t nval)
 {
+	int error;
 	int ret;
 
-	if (IS_ERR_OR_NULL(fwnode))
-		return -EINVAL;
-
-	ret = fwnode_call_int_op(fwnode, property_read_string_array, propname,
-				 val, nval);
-	if (ret != -EINVAL)
-		return ret;
+	error = FWNODE_ITERATE(fwnode, ret, -EINVAL, property_read_string_array,
+			       propname, val, nval);
+	if (error)
+		return error;
 
-	return fwnode_call_int_op(fwnode->secondary, property_read_string_array, propname,
-				  val, nval);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(fwnode_property_read_string_array);