[PATCH] drm/amdgpu/acpi: Add NULL check for event->device_class in amdgpu_atif_handler

[Srinivasan Shanmugam <srinivasan.shanmugam@xxxxxxx>]

This commit addresses a NULL dereference issue in the
amdgpu_atif_handler function.

The issue arises when event->device_class is NULL and is passed to the
DRM_DEBUG_DRIVER macro, which attempts to print the NULL string with the
%s format specifier. This constitutes undefined behavior.

To resolve this, a conditional check is added to ensure that
event->device_class is not NULL before it is passed to the
DRM_DEBUG_DRIVER macro. If it is NULL, the string "NULL" is printed
instead, thereby preventing the NULL dereference.

Fixes the below:
In function ‘amdgpu_atif_handler’,
    inlined from ‘amdgpu_acpi_event’ at drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:1215:9:
./include/drm/drm_print.h:536:49: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
  536 | #define __drm_dbg(cat, fmt, ...)                ___drm_dbg(NULL, cat, fmt, ##__VA_ARGS__)
      |                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/drm/drm_print.h:582:9: note: in expansion of macro ‘__drm_dbg’
  582 |         __drm_dbg(DRM_UT_DRIVER, fmt, ##__VA_ARGS__)
      |         ^~~~~~~~~
drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:452:9: note: in expansion of macro ‘DRM_DEBUG_DRIVER’
  452 |         DRM_DEBUG_DRIVER("event, device_class = %s, type = %#x\n",
      |         ^~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c: In function ‘amdgpu_acpi_event’:
drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:452:49: note: format string is defined here
  452 |         DRM_DEBUG_DRIVER("event, device_class = %s, type = %#x\n",
      |

Cc: Alex Deucher <alexander.deucher@xxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@xxxxxxx>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
index f85ace0384d2..27131ff30579 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
@@ -450,7 +450,7 @@ static int amdgpu_atif_handler(struct amdgpu_device *adev,
 	int count;
 
 	DRM_DEBUG_DRIVER("event, device_class = %s, type = %#x\n",
-			event->device_class, event->type);
+			event->device_class ? event->device_class : "NULL", event->type);
 
 	if (strcmp(event->device_class, ACPI_VIDEO_CLASS) != 0)
 		return NOTIFY_DONE;
-- 
2.34.1