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@xxxxxxx>
>>>> ---
>>>> 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;
>
