[PATCH 1/2] drm/panic: Ensure drm_panic_type is initialized to a valid value

[Nathan Chancellor <nathan@kernel.org>]

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>
---
 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();
 }
 

-- 
2.52.0