efi: Replace efi_memory_attributes_table_t 0-sized array with flexible array

While efi_memory_attributes_table_t::entry isn't used directly as an
array, it is used as a base for pointer arithmetic. The type is wrong
as it's not technically an array of efi_memory_desc_t's; they could be
larger. Regardless, leave the type unchanged and remove the old style
"0" array size. Additionally replace the open-coded entry offset code
with the existing efi_memdesc_ptr() helper.
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/memattr.c

@@ -164,7 +164,7 @@ int __init efi_memattr_apply_permissions(struct mm_struct *mm,
 		bool valid;
 		char buf[64];
 
-		valid = entry_is_valid((void *)tbl->entry + i * tbl->desc_size,
+		valid = entry_is_valid(efi_memdesc_ptr(tbl->entry, tbl->desc_size, i),
 				       &md);
 		size = md.num_pages << EFI_PAGE_SHIFT;
 		if (efi_enabled(EFI_DBG) || !valid)

include/linux/efi.h

@@ -608,7 +608,11 @@ typedef struct {
 	u32 num_entries;
 	u32 desc_size;
 	u32 flags;
-	efi_memory_desc_t entry[0];
+	/*
+	 * There are @num_entries following, each of size @desc_size bytes,
+	 * including an efi_memory_desc_t header. See efi_memdesc_ptr().
+	 */
+	efi_memory_desc_t entry[];
 } efi_memory_attributes_table_t;
 
 typedef struct {