linux-kernel - Re: [PATCHv5 4/6] drm/omap: fix incorrect union usage

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Message-ID: <20181123173310.GI53235@atomide.com>
Date:   Fri, 23 Nov 2018 09:33:10 -0800
From:   Tony Lindgren <tony@...mide.com>
To:     Sebastian Reichel <sebastian.reichel@...labora.com>
Cc:     Sebastian Reichel <sre@...nel.org>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Pavel Machek <pavel@....cz>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        "H. Nikolaus Schaller" <hns@...delico.com>,
        dri-devel@...ts.freedesktop.org, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCHv5 4/6] drm/omap: fix incorrect union usage

* Sebastian Reichel <sebastian.reichel@...labora.com> [181121 16:09]:
> The DSI encoder sets dssdev->ops->dsi.set_config, which is stored at the
> same offset as dssdev->ops->hdmi.set_hdmi_mode. The code in omap_encoder
> only checks if dssdev->ops->hdmi.set_hdmi_mode is NULL. Due to the way
> union works, it won't be NULL if dsi.set_config is set. This means
> dsi_set_config will be called with config=hdmi_mode=false=NULL parameter
> resulting in a NULL dereference. Also the dereference happens while
> console is locked, so kernel hangs without any debug output without
> "fb.lockless_register_fb=1" parameter.
> 
> This restructures the code, so that the HDMI mode is only configured
> for HDMI output types. The new function also has a safe-guard directly
> before accessing the union, that can be optimized away by the compiler
> when the function is inlined and HDMI type has already been checked.
> 
> Fixes: 83910ad3f51fb ("drm/omap: Move most omap_dss_driver operations to omap_dss_device_ops")
> Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>

Works for me:

Tested-by: Tony Lindgren <tony@...mide.com>