linux-kernel - Re: [PATCH 1/2] drm/panic: Ensure drm_panic

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Message-ID: <76fb4c29-2c7b-4971-92d6-365b5782aec6@redhat.com>
Date: Tue, 6 Jan 2026 22:32:58 +0100
From: Jocelyn Falempe <jfalempe@...hat.com>
To: Nathan Chancellor <nathan@...nel.org>,
 Javier Martinez Canillas <javierm@...hat.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Tvrtko Ursulin <tvrtko.ursulin@...lia.com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] drm/panic: Ensure drm_panic_type is initialized to a
 valid value

On 06/01/2026 07:19, Nathan Chancellor wrote:
> If a user has set an invalid CONFIG_DRM_PANIC_SCREEN value (such as
> "qr_code" without CONFIG_DRM_PANIC_SCREEN_QR_CODE=y due to missing or
> incorrect Rust dependencies), there is a panic when accessing
> /sys/module/drm/parameters/panic_screen:
> 
>    [   12.218375] BUG: unable to handle page fault for address: 0000000796dd8818
>    [   12.219737] #PF: supervisor read access in kernel mode
>    ...
>    [   12.227167] RIP: 0010:drm_panic_type_get+0x1b/0x30
> 
> If drm_panic_type_set() does not find a valid drm_panic_type enumeration
> in drm_panic_type_map based on the provided value, it does not change
> drm_panic_type from the default -1 value, which is not a valid index for
> accessing drm_panic_type_map in drm_panic_type_get(), resulting in the
> crash.
> 
> Validate the value of CONFIG_DRM_PANIC_SCREEN at boot time via the
> return value of drm_panic_type_set() in drm_panic_init() and explicitly
> fall back to the default of "user" with a message to the user so that
> they can adjust their configuration or fix missing dependencies.
> 
>    [    0.800697] Unsupported value for CONFIG_DRM_PANIC_SCREEN ('qr_code'), falling back to 'user'...
> 

Good catch, thanks for the fix.

Reviewed-by: Jocelyn Falempe <jfalempe@...hat.com>

> Fixes: e85e9ccf3f84 ("drm/panic: Report invalid or unsupported panic modes")
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> ---
>   drivers/gpu/drm/drm_panic.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_panic.c b/drivers/gpu/drm/drm_panic.c
> index eb7ef17b9c71..0cd574dd9d88 100644
> --- a/drivers/gpu/drm/drm_panic.c
> +++ b/drivers/gpu/drm/drm_panic.c
> @@ -1072,8 +1072,11 @@ void drm_panic_unregister(struct drm_device *dev)
>    */
>   void __init drm_panic_init(void)
>   {
> -	if (drm_panic_type == -1)
> -		drm_panic_type_set(CONFIG_DRM_PANIC_SCREEN, NULL);
> +	if (drm_panic_type == -1 && drm_panic_type_set(CONFIG_DRM_PANIC_SCREEN, NULL)) {
> +		pr_warn("Unsupported value for CONFIG_DRM_PANIC_SCREEN ('%s'), falling back to 'user'...\n",
> +			CONFIG_DRM_PANIC_SCREEN);
> +		drm_panic_type = DRM_PANIC_TYPE_USER;
> +	}
>   	drm_panic_qr_init();
>   }
>   
>