tracing: Fix TASK_COMM_LEN in trace event format file

After commit 3087c61e ("tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN"),
the content of the format file under
/sys/kernel/tracing/events/task/task_newtask was changed from
  field:char comm[16];    offset:12;    size:16;    signed:0;
to
  field:char comm[TASK_COMM_LEN];    offset:12;    size:16;    signed:0;

John reported that this change breaks older versions of perfetto.
Then Mathieu pointed out that this behavioral change was caused by the
use of __stringify(_len), which happens to work on macros, but not on enum
labels. And he also gave the suggestion on how to fix it:
  :One possible solution to make this more robust would be to extend
  :struct trace_event_fields with one more field that indicates the length
  :of an array as an actual integer, without storing it in its stringified
  :form in the type, and do the formatting in f_show where it belongs.

The result as follows after this change,
$ cat /sys/kernel/tracing/events/task/task_newtask/format
        field:char comm[16];    offset:12;      size:16;        signed:0;

Link: https://lore.kernel.org/lkml/Y+QaZtz55LIirsUO@google.com/
Link: https://lore.kernel.org/linux-trace-kernel/20230210155921.4610-1-laoar.shao@gmail.com/
Link: https://lore.kernel.org/linux-trace-kernel/20230212151303.12353-1-laoar.shao@gmail.com

Cc: stable@vger.kernel.org
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Kajetan Puchalski <kajetan.puchalski@arm.com>
CC: Qais Yousef <qyousef@layalina.io>
Fixes: 3087c61e ("tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN")
Reported-by: John Stultz <jstultz@google.com>
Debugged-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/linux/trace_events.h

@@ -270,6 +270,7 @@ struct trace_event_fields {
 			const int  align;
 			const int  is_signed;
 			const int  filter_type;
+			const int  len;
 		};
 		int (*define_fields)(struct trace_event_call *);
 	};

include/trace/stages/stage4_event_fields.h

@@ -26,7 +26,8 @@
 #define __array(_type, _item, _len) {					\
 	.type = #_type"["__stringify(_len)"]", .name = #_item,		\
 	.size = sizeof(_type[_len]), .align = ALIGN_STRUCTFIELD(_type),	\
-	.is_signed = is_signed_type(_type), .filter_type = FILTER_OTHER },
+	.is_signed = is_signed_type(_type), .filter_type = FILTER_OTHER,\
+	.len = _len },
 
 #undef __dynamic_array
 #define __dynamic_array(_type, _item, _len) {				\

kernel/trace/trace.h

@@ -1282,6 +1282,7 @@ struct ftrace_event_field {
 	int			offset;
 	int			size;
 	int			is_signed;
+	int			len;
 };
 
 struct prog_entry;

kernel/trace/trace_events.c

@@ -114,7 +114,7 @@ trace_find_event_field(struct trace_event_call *call, char *name)
 
 static int __trace_define_field(struct list_head *head, const char *type,
 				const char *name, int offset, int size,
-				int is_signed, int filter_type)
+				int is_signed, int filter_type, int len)
 {
 	struct ftrace_event_field *field;
 
@@ -133,6 +133,7 @@ static int __trace_define_field(struct list_head *head, const char *type,
 	field->offset = offset;
 	field->size = size;
 	field->is_signed = is_signed;
+	field->len = len;
 
 	list_add(&field->link, head);
 
@@ -150,14 +151,28 @@ int trace_define_field(struct trace_event_call *call, const char *type,
 
 	head = trace_get_fields(call);
 	return __trace_define_field(head, type, name, offset, size,
-				    is_signed, filter_type);
+				    is_signed, filter_type, 0);
 }
 EXPORT_SYMBOL_GPL(trace_define_field);
 
+int trace_define_field_ext(struct trace_event_call *call, const char *type,
+		       const char *name, int offset, int size, int is_signed,
+		       int filter_type, int len)
+{
+	struct list_head *head;
+
+	if (WARN_ON(!call->class))
+		return 0;
+
+	head = trace_get_fields(call);
+	return __trace_define_field(head, type, name, offset, size,
+				    is_signed, filter_type, len);
+}
+
 #define __generic_field(type, item, filter_type)			\
 	ret = __trace_define_field(&ftrace_generic_fields, #type,	\
 				   #item, 0, 0, is_signed_type(type),	\
-				   filter_type);			\
+				   filter_type, 0);			\
 	if (ret)							\
 		return ret;
 
@@ -166,7 +181,7 @@ EXPORT_SYMBOL_GPL(trace_define_field);
 				   "common_" #item,			\
 				   offsetof(typeof(ent), item),		\
 				   sizeof(ent.item),			\
-				   is_signed_type(type), FILTER_OTHER);	\
+				   is_signed_type(type), FILTER_OTHER, 0);	\
 	if (ret)							\
 		return ret;
 
@@ -1588,12 +1603,17 @@ static int f_show(struct seq_file *m, void *v)
 		seq_printf(m, "\tfield:%s %s;\toffset:%u;\tsize:%u;\tsigned:%d;\n",
 			   field->type, field->name, field->offset,
 			   field->size, !!field->is_signed);
-	else
-		seq_printf(m, "\tfield:%.*s %s%s;\toffset:%u;\tsize:%u;\tsigned:%d;\n",
+	else if (field->len)
+		seq_printf(m, "\tfield:%.*s %s[%d];\toffset:%u;\tsize:%u;\tsigned:%d;\n",
 			   (int)(array_descriptor - field->type),
 			   field->type, field->name,
-			   array_descriptor, field->offset,
+			   field->len, field->offset,
 			   field->size, !!field->is_signed);
+	else
+		seq_printf(m, "\tfield:%.*s %s[];\toffset:%u;\tsize:%u;\tsigned:%d;\n",
+				(int)(array_descriptor - field->type),
+				field->type, field->name,
+				field->offset, field->size, !!field->is_signed);
 
 	return 0;
 }
@@ -2379,9 +2399,10 @@ event_define_fields(struct trace_event_call *call)
 			}
 
 			offset = ALIGN(offset, field->align);
-			ret = trace_define_field(call, field->type, field->name,
+			ret = trace_define_field_ext(call, field->type, field->name,
 						 offset, field->size,
-						 field->is_signed, field->filter_type);
+						 field->is_signed, field->filter_type,
+						 field->len);
 			if (WARN_ON_ONCE(ret)) {
 				pr_err("error code is %d\n", ret);
 				break;

kernel/trace/trace_export.c

@@ -111,7 +111,8 @@ static void __always_unused ____ftrace_check_##name(void)		\
 #define __array(_type, _item, _len) {					\
 	.type = #_type"["__stringify(_len)"]", .name = #_item,		\
 	.size = sizeof(_type[_len]), .align = __alignof__(_type),	\
-	is_signed_type(_type), .filter_type = FILTER_OTHER },
+	is_signed_type(_type), .filter_type = FILTER_OTHER,			\
+	.len = _len },
 
 #undef __array_desc
 #define __array_desc(_type, _container, _item, _len) __array(_type, _item, _len)