linux-kernel - Re: [PATCH v3 5/5] OF: Introduce utility helper functions

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Date:	Fri, 8 Nov 2013 17:01:26 +0200
From:	Pantelis Antoniou <panto@...oniou-consulting.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Alexander Sverdlin <alexander.sverdlin@....com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <robherring2@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Matt Porter <matt.porter@...aro.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Alison Chaiken <Alison_Chaiken@...tor.com>,
	Dinh Nguyen <dinh.linux@...il.com>,
	Jan Lubbe <jluebbe@...net.de>, Michael Stickel <ms@...able.de>,
	Dirk Behme <dirk.behme@...il.com>,
	Alan Tull <delicious.quinoa@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Michael Bohan <mbohan@...eaurora.org>,
	Ionut Nicu <ioan.nicu.ext@....com>,
	Michal Simek <monstr@...str.eu>,
	Matt Ranostay <mranostay@...il.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 5/5] OF: Introduce utility helper functions

Hi Guenter,

On Nov 8, 2013, at 4:54 PM, Guenter Roeck wrote:

> On 11/08/2013 01:08 AM, Alexander Sverdlin wrote:
>> Hello Pantelis,
>> 
>> On 07/11/13 21:17, ext Pantelis Antoniou wrote:
>>> Introduce helper functions for working with the live DT tree.
>>> 
>>> __of_free_property() frees a dynamically created property
>>> __of_free_tree() recursively frees a device node tree
>>> __of_copy_property() copies a property dynamically
>>> __of_create_empty_node() creates an empty node
>>> __of_find_node_by_full_name() finds the node with the full name
>>> and
>>> of_multi_prop_cmp() performs a multi property compare but without
>>> having to take locks.
>>> 
>>> Signed-off-by: Pantelis Antoniou <panto@...oniou-consulting.com>
>>> ---
>>>  drivers/of/Makefile |   2 +-
>>>  drivers/of/util.c   | 253 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>  include/linux/of.h  |  59 ++++++++++++
>>>  3 files changed, 313 insertions(+), 1 deletion(-)
>>>  create mode 100644 drivers/of/util.c
>>> 
>> 
>> ...
>> 
>>> +struct property *__of_copy_property(const struct property *prop, gfp_t flags)
>>> +{
>>> +	struct property *propn;
>>> +
>>> +	propn = kzalloc(sizeof(*prop), flags);
>>> +	if (propn == NULL)
>>> +		return NULL;
>>> +
>>> +	propn->name = kstrdup(prop->name, flags);
>>> +	if (propn->name == NULL)
>>> +		goto err_fail_name;
>>> +
>>> +	if (prop->length > 0) {
>>         ^^^^^^^^^^^^^^^^^^^^^^^
>> As Ioan already mentioned, this is really a problem.
>> There is a bunch of places, where properties without values are used.
>> Like gpio-controller; ranges; interrupt-controller;
>> Refer, for example, to of_irq_map_raw() which checks
>> of_get_property(ipar, "interrupt-controller", NULL) != NULL
>> and some other occurrences of exactly same construct.
>> This will simply be broken for merged device-tree parts.
>> 
> 
> Folks,
> 
> it might help if you explain what exactly is broken, and how to fix it.
> It is not as if the property is not copied, only its value
> is not copied. And the value does not exist.
> 
> What do you think the code needs to do differently ? Obviously it can
> not copy a non-existing value. So what would have to be in the else case ?
> 

This was fixed by Ionut, so let me copy his email verbatim here.

> We're using the device tree overlay patches applied on top of a
> vanilla kernel. While working with it I discovered a problem
> with the __of_copy_property() function. When copying properties
> that have no value, such as "interrupt-controller;" their value
> will be set to NULL.
> 
> By contrast, if such a property exists in the initial device tree 
> at boot time it will have length 0 and value set to the empty
> string.
> 
> Some parts of the linux kernel code actually rely on this.
> For example in drivers/of/irq.c, of_irq_map_raw() checks that
> a node is an interrupt controller by calling:
> 
> of_get_property(ipar, "interrupt-controller", NULL)
> 
> and checking that it returns a non-null value.
> 
> For fixing this, we can safely use kmalloc with size 0 which
> will return ZERO_SIZE_PTR like in the patch below. This will 
> cause all the tests for non-null values to pass.

It's a bug fix. Zero length properties exist, but their
value pointer is not set to NULL.

So when copying them we have to follow the same principle,
i.e. property length set to 0, but not a NULL value pointer.

Anyway, this is addressed in the next version of patch series
which I will post shortly.


> Thanks,
> Guenter
> 
>>> +		propn->value = kmalloc(prop->length, flags);
>>> +		if (propn->value == NULL)
>>> +			goto err_fail_value;
>>> +		memcpy(propn->value, prop->value, prop->length);
>>> +		propn->length = prop->length;
>>> +	}
>>> +
>>> +	/* mark the property as dynamic */
>>> +	of_property_set_flag(propn, OF_DYNAMIC);
>>> +
>>> +	return propn;
>>> +
>>> +err_fail_value:
>>> +	kfree(propn->name);
>>> +err_fail_name:
>>> +	kfree(propn);
>>> +	return NULL;
>>> +}
>>> +
>> 
>> ...
>> 
> 

Regards

-- Pantelis

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