[PATCH] drm: Prevent early NULL dereference of fb_helper->fb

[Daniel Vetter]

On Wed, Jul 19, 2017 at 04:24:50PM +0100, Chris Wilson wrote:
> Prior to the fbdev being asynchronously configured on boot, the
> fb_helper->fb may be unset.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101767
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Thierry Reding <treding at nvidia.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> ---
> This may not be necessary with the recent async rework? One hopes.

It shouldn't be possible to call into check_var (or any other fb_ops
callback) before we register the framebuffer, and we don't do that before
the fb is real.

The bug description also says that after several suspend/resume cycles
does it blow up like that, so very unlikely fbdev isn't set up yet. And
the NULL pointer isn't really NULL.

There's something going wrong for sure, but I don't think it's this.
-Daniel

> ---
>  drivers/gpu/drm/drm_fb_helper.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 42090fe00ef9..a44b6b434fa0 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -1502,6 +1502,9 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
>  	if (var->pixclock != 0 || in_dbg_master())
>  		return -EINVAL;
>  
> +	if (!fb) /* too early, fbdev is not yet setup */
> +		return -EINVAL;
> +
>  	/*
>  	 * Changes struct fb_var_screeninfo are currently not pushed back
>  	 * to KMS, hence fail if different settings are requested.
> -- 
> 2.13.3
> 

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