linux-kernel - Re: [PATCH v2 2/2] drm/vc4: hdmi: Fix pointer dereference before check

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Message-ID: <20221117172849.hk7bgahjbvycml5v@google.com>
Date:   Thu, 17 Nov 2022 09:28:49 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     José Expósito <jose.exposito89@...il.com>
Cc:     mripard@...nel.org, emma@...olt.net, airlied@...il.com,
        daniel@...ll.ch, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v2 2/2] drm/vc4: hdmi: Fix pointer dereference before
 check

On Thu, Nov 10, 2022 at 02:47:52PM +0100, José Expósito wrote:
> Commit 6bed2ea3cb38 ("drm/vc4: hdmi: Reset link on hotplug") introduced
> the vc4_hdmi_reset_link() function. This function dereferences the
> "connector" pointer before checking whether it is NULL or not.
> 
> Rework variable assignment to avoid this issue.
> 
> Fixes: 6bed2ea3cb38 ("drm/vc4: hdmi: Reset link on hotplug")
> Signed-off-by: José Expósito <jose.exposito89@...il.com>
> ---
>  drivers/gpu/drm/vc4/vc4_hdmi.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index a49f88e5d2b9..6b223a5fcf6f 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -318,8 +318,8 @@ static int reset_pipe(struct drm_crtc *crtc,
>  static int vc4_hdmi_reset_link(struct drm_connector *connector,
>  			       struct drm_modeset_acquire_ctx *ctx)
>  {
> -	struct drm_device *drm = connector->dev;
> -	struct vc4_hdmi *vc4_hdmi = connector_to_vc4_hdmi(connector);
> +	struct drm_device *drm;
> +	struct vc4_hdmi *vc4_hdmi;

Hi, I think this change, or another in this area recently, is causing
the following warning. PTAL

drivers/gpu/drm/vc4/vc4_hdmi.c:351:14: warning: variable 'vc4_hdmi' is uninitialized when used here [-Wuninitialized]
        mutex_lock(&vc4_hdmi->mutex);
                    ^~~~~~~~
drivers/gpu/drm/vc4/vc4_hdmi.c:322:27: note: initialize the variable 'vc4_hdmi' to silence this warning
        struct vc4_hdmi *vc4_hdmi;
                                 ^
                                  = NULL

>  	struct drm_connector_state *conn_state;
>  	struct drm_crtc_state *crtc_state;
>  	struct drm_crtc *crtc;
> @@ -330,6 +330,7 @@ static int vc4_hdmi_reset_link(struct drm_connector *connector,
>  	if (!connector)
>  		return 0;
>  
> +	drm = connector->dev;
>  	ret = drm_modeset_lock(&drm->mode_config.connection_mutex, ctx);
>  	if (ret)
>  		return ret;
> @@ -347,6 +348,7 @@ static int vc4_hdmi_reset_link(struct drm_connector *connector,
>  	if (!crtc_state->active)
>  		return 0;
>  
> +	vc4_hdmi = connector_to_vc4_hdmi(connector);
>  	if (!vc4_hdmi_supports_scrambling(vc4_hdmi))
>  		return 0;
>  
> -- 
> 2.25.1
> 
>