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Subject: Re: [PATCH] of: overlay: Stop leaking resources on overlay removal
From:   Frank Rowand <frowand.list@gmail.com>
To:     Jan Kiszka <jan.kiszka@siemens.com>,
        Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
        Rob Herring <robh+dt@kernel.org>,
        devicetree <devicetree@vger.kernel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alan Tull <atull@kernel.org>
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 <90e7da9d-d40a-17f1-e627-873f58a3dce5@siemens.com>
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 <17561711-94f7-b3d4-8f06-9cfa69daaf23@gmail.com>
Message-ID: <033fba6b-d93c-249c-d552-1f5336b34387@gmail.com>
Date:   Wed, 25 Apr 2018 17:44:39 -0700
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On 04/25/18 17:32, Frank Rowand wrote:
> Hi Jan,
> 
> On 04/24/18 13:58, Frank Rowand wrote:
>> On 04/24/18 10:50, Jan Kiszka wrote:
>>> On 2018-04-24 19:44, Frank Rowand wrote:
>>>> On 04/24/18 09:19, Jan Kiszka wrote:
>>>>> Only the overlay notifier callbacks have a chance to potentially get
>>>>> hold of references to those two resources, but they do not store them.
>>>>> So it is safe to stop the intentional leaking.
>>>>>
>>>>> See also https://lkml.org/lkml/2018/4/23/1063 and following.
>>>>>
>>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>>>> ---
>>>>>
>>>>> Ideally, we sort out any remaining worries during the 4.17-rc cycle.
>>>>>
>>>>>  drivers/of/overlay.c | 13 ++-----------
>>>>>  1 file changed, 2 insertions(+), 11 deletions(-)
>>>>>
>>>>> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
>>>>> index b35fe88f1851..3553f1f57a62 100644
>>>>> --- a/drivers/of/overlay.c
>>>>> +++ b/drivers/of/overlay.c
>>>>> @@ -671,17 +671,8 @@ static void free_overlay_changeset(struct overlay_changeset *ovcs)
>>>>>  		of_node_put(ovcs->fragments[i].overlay);
>>>>>  	}
>>>>>  	kfree(ovcs->fragments);
>>>>> -
>>>>> -	/*
>>>>> -	 * TODO
>>>>> -	 *
>>>>> -	 * would like to: kfree(ovcs->overlay_tree);
>>>>> -	 * but can not since drivers may have pointers into this data
>>>>> -	 *
>>>>> -	 * would like to: kfree(ovcs->fdt);
>>>>> -	 * but can not since drivers may have pointers into this data
>>>>> -	 */
>>>>> -
>>>>> +	kfree(ovcs->overlay_tree);
>>>>> +	kfree(ovcs->fdt);
>>>>>  	kfree(ovcs);
>>>>>  }
>>>>>  
>>>>>
>>>>
>>>> Nack.  It is premature to submit this while the conversation is
>>>> continuing in the other thread.
>>>>
>>>> I'll continue the conversation in the other thread.
>>>>
>>>
>>> Well, at least the strongest argument has been resolved now, the
>>> notifier topic. Curious to learn what remains. As I noted, we should
>>> work hard to sort out the API regression prior to the release.
>>
>> Nope, the notifier discussion continues in the other thread.
> 
> Thanks for your patience in the other thread.
> 
> As I noted there, I am now willing to accept this patch with some
> small changes.  Please add a minimal section to
> Documentation/devicetree/overlay-notes.txt about overlay notifiers.
> The most important thing to note there is that the overlay notifiers
> are not allowed to retain any pointers into the overlay devicetree.

Please also add a function header comment to of_overlay_notifier_register()
in drivers/of/overlay.c that notes the restriction on the overlay notifier.


> Also, instead of removing the "TODO" comment in free_overlay_changeset(),
> change it to say something to the effect of "there should be no live pointers
> into ovcs->overlay_tree and ovcs->fdt due to the policy that overlay
> notifiers are not allowed to retain pointers into the overlay devicetree".
> 
> I will also add myself to the OPEN FIRMWARE AND DEVICE TREE OVERLAYS
> entry of MAINTAINERS and add a keyword line to catch overlay notifiers.
> 
> I am not happy about freeing the overlay devicetree and overlay fdt
> while overlay notifiers are able to retain pointers into the overlay
> devicetree and overlay fdt, but am willing to accept documentation and
> review as a partial protection until the devicetree access APIs can be
> modified to prevent the notifiers from accessing the pointers.  The
> volume of overlay notifier patches should be small enough to not be
> a review burden.
> 
> -Frank
> 
>