tracing: Error if a trace event has an array for a __field()

A __field() in the TRACE_EVENT() macro is used to set up the fields of the
trace event data. It is for single storage units (word, char, int,
pointer, etc) and not for complex structures or arrays. Unfortunately,
there's nothing preventing the build from accepting:

__field(int, arr[5]);

from building. It will turn into a array value. This use to work fine, as
the offset and size use to be determined by the macro using the field name,
but things have changed and the offset and size are now determined by the
type. So the above would only be size 4, and the next field will be
located 4 bytes from it (instead of 20).

The proper way to declare static arrays is to use the __array() macro.

Instead of __field(int, arr[5]) it should be __array(int, arr, 5).

Add some macro tricks to the building of a trace event from the
TRACE_EVENT() macro such that __field(int, arr[5]) will fail to build. A
comment by the failure will explain why the build failed.

Link: https://lore.kernel.org/lkml/[e-mail]/
Link: https://lore.kernel.org/linux-trace-kernel/[e-mail]

Reported-by: Douglas RAILLARD <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>
Acked-by: Masami Hiramatsu (Google) <[e-mail]>

include/trace/stages/stage5_get_offsets.h

···
 #undef __entry
 #define __entry entry
 
+/*
+ * Fields should never declare an array: i.e. __field(int, arr[5])
+ * If they do, it will cause issues in parsing and possibly corrupt the
+ * events. To prevent that from happening, test the sizeof() a fictitious
+ * type called "struct _test_no_array_##item" which will fail if "item"
+ * contains array elements (like "arr[5]").
+ *
+ * If you hit this, use __array(int, arr, 5) instead.
+ */
 #undef __field
-#define __field(type, item)
+#define __field(type, item)					\
+	{ (void)sizeof(struct _test_no_array_##item *); }
 
 #undef __field_ext
-#define __field_ext(type, item, filter_type)
+#define __field_ext(type, item, filter_type)			\
+	{ (void)sizeof(struct _test_no_array_##item *); }
 
 #undef __field_struct
-#define __field_struct(type, item)
+#define __field_struct(type, item)				\
+	{ (void)sizeof(struct _test_no_array_##item *); }
 
 #undef __field_struct_ext
-#define __field_struct_ext(type, item, filter_type)
+#define __field_struct_ext(type, item, filter_type)		\
+	{ (void)sizeof(struct _test_no_array_##item *); }
 
 #undef __array
 #define __array(type, item, len)