linux-kernel - Re: [PATCH] drm/of: free the iterator object on failure

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Message-ID: <5c3db755-c3fb-dfe7-3d23-bbbcc81af3d8@arm.com>
Date:   Tue, 13 Jul 2021 17:16:16 +0100
From:   Steven Price <steven.price@....com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Biju Das <biju.das.jz@...renesas.com>
Subject: Re: [PATCH] drm/of: free the iterator object on failure

On 12/07/2021 22:55, Laurent Pinchart wrote:
> Hi Steven,

Hi Laurent,

> On Mon, Jul 12, 2021 at 10:31:52PM +0100, Steven Price wrote:
>> On 12/07/2021 17:50, Laurent Pinchart wrote:
>>> On Mon, Jul 12, 2021 at 04:57:58PM +0100, Steven Price wrote:
>>>> When bailing out due to the sanity check the iterator value needs to be
>>>> freed because the early return prevents for_each_child_of_node() from
>>>> doing the dereference itself.
>>>>
>>>> Fixes: 4ee48cc5586b ("drm: of: Fix double-free bug")
>>>
>>> I don't think the Fixes tag is correct, the issue was already present
>>> before 4ee48cc5586b. The fix looks right though.
>>
>> I'm not sure quite what you mean by "already present". As I understand
>> it the timeline was:
>>
>> 1. 6529007522de drm: of: Add drm_of_lvds_get_dual_link_pixel_order
>>    The function was originally added. This made the mistake twice of
>>    calling of_node_put() on the wrong variable (remote_port rather than
>>    endpoint).
> 
> Correct.
> 
>> 2. 4ee48cc5586b drm: of: Fix double-free bug
>>    One of the of_node_put() calls was removed as it was a double-free.
>>    This left the first incorrect of_node_put() in place, and the second
>>    is now a straight leak.
> 
> That's right, but this commit didn't introduce the leak, it was already
> there in 6529007522de (in addition to the double-free).

Ah, I see what you mean. My thought process was that the original
comment had the bug "using the wrong variable", and (2) (partially)
fixed that but in the process introduced a new bug (a memory leak). But
I guess technically the memory leak was there from the beginning.

The other reason I referenced (2) in the Fixes line is because this
patch depends on patch (2), whereas it won't apply cleanly without.

However I don't think it really matters either way: (2) has already been
backported, and either way this needs fixing if either (1) or (2) are
present.

Would you like me to resend with a "Fixes: 6529007522de drm: of: Add
drm_of_lvds_get_dual_link_pixel_order", or are you happy to just fix
this up when merging?

Thanks,

Steve

>> 3. b557a5f8da57 drm/of: free the right object
>>    This (correctly) fixes the first of_node_put() to free endpoint. And
>>    the post from Daniel was what caused me to look.
>>
>> 4. This patch
>>    Reintroduces the of_node_put() removed in (2) but putting endpoint
>>    rather than remote_port.
>>
>> I've put (2) in the Fixes line as this patch is fixing the leak
>> introduced by that patch, but that in itself was of course 'fixing' the
>> double free of the original patch.
>>
>>>> Signed-off-by: Steven Price <steven.price@....com>
>>>> ---
>>>>  drivers/gpu/drm/drm_of.c | 4 +++-
>>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>>
>>>> Daniel's email[1] made me take a look at this function and it appears
>>>> that for_each_child_of_node()'s interface had caused a bad bug fix due
>>>> to the hidden reference counting in the iterator.
>>>>
>>>> [1] https://lore.kernel.org/r/YOxQ5TbkNrqCGBDJ%40phenom.ffwll.local
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
>>>> index 197c57477344..997b8827fed2 100644
>>>> --- a/drivers/gpu/drm/drm_of.c
>>>> +++ b/drivers/gpu/drm/drm_of.c
>>>> @@ -331,8 +331,10 @@ static int drm_of_lvds_get_remote_pixels_type(
>>>>  		 * configurations by passing the endpoints explicitly to
>>>>  		 * drm_of_lvds_get_dual_link_pixel_order().
>>>>  		 */
>>>> -		if (!current_pt || pixels_type != current_pt)
>>>> +		if (!current_pt || pixels_type != current_pt) {
>>>> +			of_node_put(endpoint);
>>>>  			return -EINVAL;
>>>> +		}
>>>>  	}
>>>>  
>>>>  	return pixels_type;
>