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@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;
--
Regards,
Laurent Pinchart
