linux-kernel - Re: [PATCH] powerpc: powernv: Fix refcount leak bug in opal-powercap

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Message-ID: <fe306bbf-288d-8d8e-1aae-08ad7d8c870c@wanadoo.fr>
Date:   Sat, 18 Jun 2022 09:38:54 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Liang He <windhl@....com>
Cc:     nick.child@....com, linux-kernel@...r.kernel.org, paulus@...ba.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: powernv: Fix refcount leak bug in opal-powercap

Le 17/06/2022 à 07:42, Liang He a écrit :
> 
> 
> 
> At 2022-06-17 13:01:27, "Christophe JAILLET" <christophe.jaillet@...adoo.fr> wrote:
>> Le 17/06/2022 à 06:20, Liang He a écrit :
>>> In opal_powercap_init(), of_find_compatible_node() will return
>>> a node pointer with refcount incremented. We should use of_node_put()
>>> in fail path or when it is not used anymore.
>>>
>>> Besides, for_each_child_of_node() will automatically *inc* and *dec*
>>> refcount during iteration. However, we should add the of_node_put()
>>> if there is a break.
>>
>> Hi,
>>
>> I'm not sure that your patch is right here. Because of this *inc* and
>> *dec* things, do we still need to of_node_put(powercap) once we have
>> entered for_each_child_of_node?
>>
>> I think that this reference will be released on the first iteration of
>> the loop.
>>
> 
> Hi, CJ,
> 
> Thanks for your reply and I want have a discuss.
> 
> Based on my review on the src of 'of_get_next_child',  there is only
> *inc* for next and *dec* for pre as follow.
> 
> (|node| == powercap)
> ======__of_get_next_child( |node|, prev)======
>       ...
>          next = prev? prev->sibling:|node|->child;
> 	of_node_get(next);
> 	of_node_put(prev);
>       ...
> =========================
> 
> However, there is no any code to release the |node| (i.e., *powercap*).
> 
> Am I right?   If I am wrong, please correct me, thanks.

You are right.
I mis-read __of_get_next_child(().

CJ


> 
>>
>> Maybe of_node_put(powercap) should be duplicated everywhere it is
>> relevant and removed from the error handling path?
>> Or an additional reference should be taken before the loop?
>> Or adding a new label with "powercap = NULL" and branching there when
>> needed?
>>
>> CJ
> 
> If my understanding is right, I think current patch is right.
> 
> Otherwise, I will make a new patch to handle that, Thanks.
> 
> Liang
> 
>>
>>>
>>> Signed-off-by: Liang He <windhl@....com>
>>> ---
>>>    arch/powerpc/platforms/powernv/opal-powercap.c | 5 ++++-
>>>    1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/powerpc/platforms/powernv/opal-powercap.c b/arch/powerpc/platforms/powernv/opal-powercap.c
>>> index 64506b46e77b..b102477d3f95 100644
>>> --- a/arch/powerpc/platforms/powernv/opal-powercap.c
>>> +++ b/arch/powerpc/platforms/powernv/opal-powercap.c
>>> @@ -153,7 +153,7 @@ void __init opal_powercap_init(void)
>>>    	pcaps = kcalloc(of_get_child_count(powercap), sizeof(*pcaps),
>>>    			GFP_KERNEL);
>>>    	if (!pcaps)
>>> -		return;
>>> +		goto out_powercap;
>>>    
>>>    	powercap_kobj = kobject_create_and_add("powercap", opal_kobj);
>>>    	if (!powercap_kobj) {
>>> @@ -236,6 +236,9 @@ void __init opal_powercap_init(void)
>>>    		kfree(pcaps[i].pg.name);
>>>    	}
>>>    	kobject_put(powercap_kobj);
>>> +	of_node_put(node);
>>>    out_pcaps:
>>>    	kfree(pcaps);
>>> +out_powercap:
>>> +	of_node_put(powercap);
>>>    }