drm/radeon: Replace one-element array with flexible-array member

One-element arrays are deprecated, and we are replacing them with
flexible array members instead. So, replace one-element array with
flexible-array member in struct _ATOM_FAKE_EDID_PATCH_RECORD and
refactor the rest of the code accordingly.

It's worth mentioning that doing a build before/after this patch results
in no binary output differences.

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: KSPP#79
Link: KSPP#239
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]

Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Paulo Miguel Almeida <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

Author: PauloMigAlmeida

drivers/gpu/drm/radeon/atombios.h

@@ -3615,7 +3615,7 @@ typedef struct _ATOM_FAKE_EDID_PATCH_RECORD
 {
   UCHAR ucRecordType;
   UCHAR ucFakeEDIDLength;
-  UCHAR ucFakeEDIDString[1];    // This actually has ucFakeEdidLength elements.
+  UCHAR ucFakeEDIDString[];    // This actually has ucFakeEdidLength elements.
 } ATOM_FAKE_EDID_PATCH_RECORD;
 
 typedef struct  _ATOM_PANEL_RESOLUTION_PATCH_RECORD

drivers/gpu/drm/radeon/radeon_atombios.c

@@ -1727,8 +1727,11 @@ struct radeon_encoder_atom_dig *radeon_atombios_get_lvds_info(struct
 						}
 					}
 					record += fake_edid_record->ucFakeEDIDLength ?
-						fake_edid_record->ucFakeEDIDLength + 2 :
-						sizeof(ATOM_FAKE_EDID_PATCH_RECORD);
+						  struct_size(fake_edid_record,
+							      ucFakeEDIDString,
+							      fake_edid_record->ucFakeEDIDLength) :
+						  /* empty fake edid record must be 3 bytes long */
+						  sizeof(ATOM_FAKE_EDID_PATCH_RECORD) + 1;
 					break;
 				case LCD_PANEL_RESOLUTION_RECORD_TYPE:
 					panel_res_record = (ATOM_PANEL_RESOLUTION_PATCH_RECORD *)record;