drm/panic: Ensure drm_panic_type is initialized to a valid value

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'...

Fixes: e85e9ccf3f84 ("drm/panic: Report invalid or unsupported panic modes")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20260105-drm_panic-handle-invalid-drm_panic_screen-v1-1-55228bd4b0f8@kernel.org

diff --git a/drivers/gpu/drm/drm_panic.c b/drivers/gpu/drm/drm_panic.c
index eb7ef17b9c71c35022d7f5eee1f4e0fb43ac25c4..0cd574dd9d883a7ad11d9acd1507b51e4376743a 100644 (file)
--- 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();
 }