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Date:   Wed, 14 Jul 2021 11:35:13 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Steven Price <steven.price@arm.com>
Cc:     Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@linux.ie>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Biju Das <biju.das.jz@bp.renesas.com>
Subject: Re: [PATCH] drm/of: free the iterator object on failure
Message-ID: <YO6hwQwo1/BaaOZ3@pendragon.ideasonboard.com>
References: <20210712155758.48286-1-steven.price@arm.com>
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Hi Steven,

On Tue, Jul 13, 2021 at 05:16:16PM +0100, Steven Price wrote:
> On 12/07/2021 22:55, Laurent Pinchart wrote:
> > 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?

I don't mind either way, from my point of view it can be fixed up by
whoever will pick the patch up and merge it.

> >> 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@arm.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;

-- 
Regards,

Laurent Pinchart