Hi Steven,
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).
> 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
