linux-kernel - Re: [PATCH] PM / clock_ops: Fix clock error check in __pm_clk

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Message-ID: <55523F67.9040100@linaro.org>
Date:	Tue, 12 May 2015 20:59:03 +0300
From:	"Grygorii.Strashko@...aro.org" <grygorii.strashko@...aro.org>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
CC:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Kevin Hilman <khilman@...aro.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PM / clock_ops: Fix clock error check in __pm_clk_add()

On 05/12/2015 07:42 PM, Dmitry Torokhov wrote:
> On Tue, May 12, 2015 at 04:55:39PM +0300, Grygorii.Strashko@...aro.org wrote:
>> On 05/09/2015 12:05 AM, Dmitry Torokhov wrote:
>>> On Fri, May 08, 2015 at 10:59:04PM +0200, Geert Uytterhoeven wrote:
>>>> On Fri, May 8, 2015 at 7:19 PM, Dmitry Torokhov
>>>> <dmitry.torokhov@...il.com> wrote:
>>>>> On Fri, May 08, 2015 at 10:47:43AM +0200, Geert Uytterhoeven wrote:
>>>>>> In the final iteration of commit 245bd6f6af8a62a2 ("PM / clock_ops: Add
>>>>>> pm_clk_add_clk()"), a refcount increment was added by Grygorii Strashko.
>>>>>> However, the accompanying IS_ERR() check operates on the wrong clock
>>>>>> pointer, which is always zero at this point, i.e. not an error.
>>>>>> This may lead to a NULL pointer dereference later, when __clk_get()
>>>>>> tries to dereference an error pointer.
>>>>>>
>>>>>> Check the passed clock pointer instead to fix this.
>>>>>
>>>>> Frankly I would remove the check altogether. Why do we only check for
>>>>> IS_ERR and not NULL or otherwise validate the pointer? The clk is passed
>>>>
>>>> __clk_get() does the NULL check.
>>>
>>> No, not really. It _handles_ clk being NULL and returns "everything is
>>> fine". In any case it is __clk_get's decision what to do.
>>>
>>> I dislike gratuitous checks of arguments passed in. Instead of relying
>>> on APIs refusing grabage we better not pass garbage to these APIs in the
>>> first place. So I'd change it to trust that we are given a usable
>>> pointer and simply do:
>>>
>>> 	if (!__clk_get(clk)) {
>>> 		kfree(ce);
>>> 		return -ENOENTl
>>> 	}
>>
>> Not sure this is right thing to do, because this API initially
>> was intended to be used as below [1]:
>> 	clk = of_clk_get(dev->of_node, i));
>>   	ret = pm_clk_add_clk(dev, clk);
>>   	clk_put(clk);
>>
>> and of_clk_get may return ERR_PTR().
> 
> Jeez, that sequence was not meant to be taken literally, it does miss
> error handling completely. If you notice the majority of users of this
> API do something like below:
> 
> 	i = 0;
> 	while ((clk = of_clk_get(dev->of_node, i++)) && !IS_ERR(clk)) {
> 		dev_dbg(dev, "adding clock '%s' to list of PM clocks\n",
> 			__clk_get_name(clk));
> 		error = pm_clk_add_clk(dev, clk);
> 		clk_put(clk);
> 		if (error) {
> 			dev_err(dev, "pm_clk_add_clk failed %d\n", error);
> 			pm_clk_destroy(dev);
> 			return error;
> 		}
> 	}
> 
> i.e. it already validates clk pointer before passing it on since it
> needs to know when to stop iterating.

np. It's just my opinion - if you agree that code will just crash
in case of passing invalid @clk argument (in worst case:)

int __clk_get(struct clk *clk)
{
	struct clk_core *core = !clk ? NULL : clk->core;
						^^^ here

let it be.

-- 
regards,
-grygorii
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