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

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Date: Tue, 13 Feb 2024 14:59:37 +0100
From: Daniel Vetter <daniel@...ll.ch>
To: Rob Clark <robdclark@...il.com>
Cc: dri-devel@...ts.freedesktop.org, Rob Clark <robdclark@...omium.org>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
	Jani Nikula <jani.nikula@...el.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/crtc: fix uninitialized variable use even harder

On Mon, Feb 12, 2024 at 01:55:34PM -0800, Rob Clark wrote:
> From: Rob Clark <robdclark@...omium.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@...omium.org>

Cc: stable@...r.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@...ll.ch>

Cheers, Sima

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

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