Re: [PATCH] drm/fb-helper: Fix height, width, and accel_flags in fb_var

[Daniel Vetter]

On Tue, Apr 18, 2023 at 08:42:46PM +0200, Geert Uytterhoeven wrote:
> Fbtest contains some very simple validation of the fbdev userspace API
> contract.  When used with shmob-drm, it reports the following warnings
> and errors:
> 
>     height changed from 68 to 0
>     height was rounded down
>     width changed from 111 to 0
>     width was rounded down
>     accel_flags changed from 0 to 1
> 
> The first part happens because __fill_var() resets the physical
> dimensions of the first connector, as filled in by drm_setup_crtcs_fb().
> Fix this by retaining the original values.
> 
> The last part happens because __fill_var() forces the FB_ACCELF_TEXT
> flag on, while fbtest disables all acceleration on purpose, so it can
> draw safely to the frame buffer.  Fix this by setting accel_flags to
> zero, as DRM does not implement any text console acceleration.
> Note that this issue can also be seen in the output of fbset, which
> reports "accel true".
> 
> Fixes: ee4cce0a8f03a333 ("drm/fb-helper: fix input validation gaps in check_var")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_fb_helper.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 64458982be40c468..ed6ad787915f0b8f 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -1537,17 +1537,19 @@ static void drm_fb_helper_fill_pixel_fmt(struct fb_var_screeninfo *var,
>  	}
>  }
>  
> -static void __fill_var(struct fb_var_screeninfo *var,
> +static void __fill_var(struct fb_var_screeninfo *var, struct fb_info *info,
>  		       struct drm_framebuffer *fb)
>  {
>  	int i;
>  
>  	var->xres_virtual = fb->width;
>  	var->yres_virtual = fb->height;
> -	var->accel_flags = FB_ACCELF_TEXT;
> +	var->accel_flags = 0;
>  	var->bits_per_pixel = drm_format_info_bpp(fb->format, 0);
>  
> -	var->height = var->width = 0;
> +	var->height = info->var.height;
> +	var->width = info->var.width;
> +
>  	var->left_margin = var->right_margin = 0;
>  	var->upper_margin = var->lower_margin = 0;
>  	var->hsync_len = var->vsync_len = 0;
> @@ -1610,7 +1612,7 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
>  		return -EINVAL;
>  	}
>  
> -	__fill_var(var, fb);
> +	__fill_var(var, info, fb);
>  
>  	/*
>  	 * fb_pan_display() validates this, but fb_set_par() doesn't and just
> @@ -2066,7 +2068,7 @@ static void drm_fb_helper_fill_var(struct fb_info *info,
>  	info->pseudo_palette = fb_helper->pseudo_palette;
>  	info->var.xoffset = 0;
>  	info->var.yoffset = 0;
> -	__fill_var(&info->var, fb);
> +	__fill_var(&info->var, info, fb);

Bit a bikeshed since it zeroed-allocated anyway, but I'd pass NULL here
for info and catch that in __fill_var and then keep the explicit = 0;

Either way Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

>  	info->var.activate = FB_ACTIVATE_NOW;
>  
>  	drm_fb_helper_fill_pixel_fmt(&info->var, format);
> -- 
> 2.34.1
> 

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