[PATCH] drm/crtc: fix uninitialized variable use even harder

[PATCH] drm/crtc: fix uninitialized variable use even harder

[Rob Clark]

From: Rob Clark <robdclark@chromium.org>

DRM_MODESET_LOCK_ALL_BEGIN() has a hidden trap-door (aka retry loop),
which means we can't rely too much on variable initializers.

Fixes: 6e455f5dcdd1 ("drm/crtc: fix uninitialized variable use")
Signed-off-by: Rob Clark <robdclark@chromium.org>
---
I have mixed feelings about DRM_MODESET_LOCK_ALL_BEGIN() (and friends)
magic.  On one hand it simplifies the deadlock/back dance.  OTOH it
conceals a nasty sharp edge.  Maybe it is better to have the complicated
restart path a bit more explicit, like it was originally.

 drivers/gpu/drm/drm_crtc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index cb90e70d85e8..65f9f66933bb 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -904,6 +904,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
 	connector_set = NULL;
 	fb = NULL;
 	mode = NULL;
+	num_connectors = 0;
 
 	DRM_MODESET_LOCK_ALL_END(dev, ctx, ret);
 
-- 
2.43.0

Re: [PATCH] drm/crtc: fix uninitialized variable use even harder

[Rob Clark]

On Mon, Feb 12, 2024 at 1:55 PM Rob Clark <robdclark@gmail.com> wrote:
>
> From: Rob Clark <robdclark@chromium.org>
>
> DRM_MODESET_LOCK_ALL_BEGIN() has a hidden trap-door (aka retry loop),
> which means we can't rely too much on variable initializers.
>
> Fixes: 6e455f5dcdd1 ("drm/crtc: fix uninitialized variable use")
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
> I have mixed feelings about DRM_MODESET_LOCK_ALL_BEGIN() (and friends)
> magic.  On one hand it simplifies the deadlock/back dance.  OTOH it
> conceals a nasty sharp edge.  Maybe it is better to have the complicated
> restart path a bit more explicit, like it was originally.

I should also point out, had drm-misc-next been using gitlab MRs and
gitlab CI, we would have caught this ;-)

BR,
-R

>  drivers/gpu/drm/drm_crtc.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index cb90e70d85e8..65f9f66933bb 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -904,6 +904,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
>         connector_set = NULL;
>         fb = NULL;
>         mode = NULL;
> +       num_connectors = 0;
>
>         DRM_MODESET_LOCK_ALL_END(dev, ctx, ret);
>
> --
> 2.43.0
>

Re: [PATCH] drm/crtc: fix uninitialized variable use even harder

[Abhinav Kumar]

On 2/12/2024 1:55 PM, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> DRM_MODESET_LOCK_ALL_BEGIN() has a hidden trap-door (aka retry loop),
> which means we can't rely too much on variable initializers.
> 
> Fixes: 6e455f5dcdd1 ("drm/crtc: fix uninitialized variable use")
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
> I have mixed feelings about DRM_MODESET_LOCK_ALL_BEGIN() (and friends)
> magic.  On one hand it simplifies the deadlock/back dance.  OTOH it
> conceals a nasty sharp edge.  Maybe it is better to have the complicated
> restart path a bit more explicit, like it was originally.
> 
>   drivers/gpu/drm/drm_crtc.c | 1 +
>   1 file changed, 1 insertion(+)
> 

Nice catch !!

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>

Re: [PATCH] drm/crtc: fix uninitialized variable use even harder

[Dmitry Baryshkov]

On Mon, 12 Feb 2024 at 23:55, Rob Clark <robdclark@gmail.com> wrote:
>
> From: Rob Clark <robdclark@chromium.org>
>
> DRM_MODESET_LOCK_ALL_BEGIN() has a hidden trap-door (aka retry loop),
> which means we can't rely too much on variable initializers.
>
> Fixes: 6e455f5dcdd1 ("drm/crtc: fix uninitialized variable use")
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
> I have mixed feelings about DRM_MODESET_LOCK_ALL_BEGIN() (and friends)
> magic.  On one hand it simplifies the deadlock/back dance.  OTOH it
> conceals a nasty sharp edge.  Maybe it is better to have the complicated
> restart path a bit more explicit, like it was originally.
>
>  drivers/gpu/drm/drm_crtc.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index cb90e70d85e8..65f9f66933bb 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -904,6 +904,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
>         connector_set = NULL;
>         fb = NULL;
>         mode = NULL;
> +       num_connectors = 0;

Nit: I think we should move all this next to the
DRM_MODESET_LOCK_ALL_BEGIN() and drop initialisation from the prologue
of the function, but it's definitely a separate and more intrusive
story.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

>         DRM_MODESET_LOCK_ALL_END(dev, ctx, ret);


-- 
With best wishes
Dmitry

Re: [PATCH] drm/crtc: fix uninitialized variable use even harder

[Dmitry Baryshkov]

On Tue, 13 Feb 2024 at 15:50, Dmitry Baryshkov
<dmitry.baryshkov@linaro.org> wrote:
>
> On Mon, 12 Feb 2024 at 23:55, Rob Clark <robdclark@gmail.com> wrote:
> >
> > From: Rob Clark <robdclark@chromium.org>
> >
> > DRM_MODESET_LOCK_ALL_BEGIN() has a hidden trap-door (aka retry loop),
> > which means we can't rely too much on variable initializers.
> >
> > Fixes: 6e455f5dcdd1 ("drm/crtc: fix uninitialized variable use")
> > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > ---
> > I have mixed feelings about DRM_MODESET_LOCK_ALL_BEGIN() (and friends)
> > magic.  On one hand it simplifies the deadlock/back dance.  OTOH it
> > conceals a nasty sharp edge.  Maybe it is better to have the complicated
> > restart path a bit more explicit, like it was originally.
> >
> >  drivers/gpu/drm/drm_crtc.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> > index cb90e70d85e8..65f9f66933bb 100644
> > --- a/drivers/gpu/drm/drm_crtc.c
> > +++ b/drivers/gpu/drm/drm_crtc.c
> > @@ -904,6 +904,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
> >         connector_set = NULL;
> >         fb = NULL;
> >         mode = NULL;
> > +       num_connectors = 0;
>
> Nit: I think we should move all this next to the
> DRM_MODESET_LOCK_ALL_BEGIN() and drop initialisation from the prologue
> of the function, but it's definitely a separate and more intrusive
> story.
>
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

After running this through CI:

Tested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org> #sc7180, sdm845

> >         DRM_MODESET_LOCK_ALL_END(dev, ctx, ret);
>
>
> --
> With best wishes
> Dmitry



-- 
With best wishes
Dmitry

Re: [PATCH] drm/crtc: fix uninitialized variable use even harder

[Daniel Vetter]

On Mon, Feb 12, 2024 at 01:55:34PM -0800, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> DRM_MODESET_LOCK_ALL_BEGIN() has a hidden trap-door (aka retry loop),
> which means we can't rely too much on variable initializers.
> 
> Fixes: 6e455f5dcdd1 ("drm/crtc: fix uninitialized variable use")
> Signed-off-by: Rob Clark <robdclark@chromium.org>

Cc: stable@vger.kernel.org

I guess at least ...

> ---
> I have mixed feelings about DRM_MODESET_LOCK_ALL_BEGIN() (and friends)
> magic.  On one hand it simplifies the deadlock/back dance.  OTOH it
> conceals a nasty sharp edge.  Maybe it is better to have the complicated
> restart path a bit more explicit, like it was originally.

Alternative would be a very magic for {} macro, but those tend to get
easily defeated with goto/break and friends. Maybe it works now that
linux/cleanup.h has landed, but I'm not sure it's actually better and not
just trading one set of sharp edges for another. Worth a shot in a
prototype at least, if someone's bored.




> 
>  drivers/gpu/drm/drm_crtc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index cb90e70d85e8..65f9f66933bb 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -904,6 +904,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
>  	connector_set = NULL;
>  	fb = NULL;
>  	mode = NULL;
> +	num_connectors = 0;

I think it'd be neater to have all these right below the
DRM_MODESET_LOCK_A_BEGIN instead of duplicated here and at the beginning
of the function. But it's a bit a bikeshed and in some cases (when you use
it later on) gcc might get pissed too. So either way:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Cheers, Sima

>  
>  	DRM_MODESET_LOCK_ALL_END(dev, ctx, ret);
>  
> -- 
> 2.43.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] drm/crtc: fix uninitialized variable use even harder

[Dmitry Baryshkov]

On Mon, 12 Feb 2024 13:55:34 -0800, Rob Clark wrote:
> DRM_MODESET_LOCK_ALL_BEGIN() has a hidden trap-door (aka retry loop),
> which means we can't rely too much on variable initializers.
> 
> 

Applied to drm-misc-fixes, thanks!

[1/1] drm/crtc: fix uninitialized variable use even harder
      commit: b6802b61a9d0e99dcfa6fff7c50db7c48a9623d3

Best regards,
-- 
Dmitry Baryshkov <dmitry.baryshkov@linaro.org>