libtracefs: Check README to know if we should do old onmatch format

Older kernels use:

  onmatch(..).synth_event(...)

Where the "synth_event" is the synthetic event that was created. The new
format uses:

  onmatch(..).trace(synth_event,...)

Where it uses the new "trace" command and the synthetic event is the first
parameter.

The tracefs README can determine which format is used. Search the README
and if the string "trace(<synthetic_event>,param list)" is found, then we
know that the kernel uses the new format. Yes, the README in the tracefs
file system *is* an ABI.

Link: https://lore.kernel.org/linux-trace-devel/20220131163642.2754485-4-rostedt@goodmis.org

Fixes: 7c9000c7495b9 ("libtracefs: Create a way to create a synthetic event")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 35 insertions, 3 deletions

diff --git a/src/tracefs-hist.c b/src/tracefs-hist.c
index 622fe5c..ec63b88 100644
--- a/src/tracefs-hist.c
+++ b/src/tracefs-hist.c
@@ -674,6 +674,7 @@ struct action {
  * @end_filters: The fields in the end event to record
  * @start_parens: Current parenthesis level for start event
  * @end_parens: Current parenthesis level for end event
+ * @new_format: onmatch().trace(synth_event,..) or onmatch().synth_event(...)
  */
 struct tracefs_synth {
 	struct tracefs_instance *instance;
@@ -702,6 +703,7 @@ struct tracefs_synth {
 	int			*start_type;
 	char			arg_name[16];
 	int			arg_cnt;
+	bool			new_format;
 };
 
  /*
@@ -949,6 +951,26 @@ static int alloc_synthetic_event(struct tracefs_synth *synth)
 	return synth->dyn_event ? 0 : -1;
 }
 
+/*
+ * See if it is onmatch().trace(synth_event,...) or
+ *   onmatch().synth_event(...)
+ */
+static bool has_new_format()
+{
+	char *readme;
+	char *p;
+	int size;
+
+	readme = tracefs_instance_file_read(NULL, "README", &size);
+	if (!readme)
+		return false;
+
+	p = strstr(readme, "trace(<synthetic_event>,param list)");
+	free(readme);
+
+	return p != NULL;
+}
+
 /**
  * tracefs_synth_alloc - create a new tracefs_synth instance
  * @tep: The tep handle that holds the events to work on
@@ -1037,6 +1059,8 @@ struct tracefs_synth *tracefs_synth_alloc(struct tep_handle *tep,
 		synth = NULL;
 	}
 
+	synth->new_format = has_new_format();
+
 	return synth;
 }
 
@@ -1730,13 +1754,21 @@ static char *create_trace(char *hist, struct tracefs_synth *synth)
 	char *name;
 	int i;
 
-	hist = append_string(hist, NULL, ".trace(");
-	hist = append_string(hist, NULL, synth->name);
+	if (synth->new_format) {
+		hist = append_string(hist, NULL, ".trace(");
+		hist = append_string(hist, NULL, synth->name);
+		hist = append_string(hist, NULL, ",");
+	} else {
+		hist = append_string(hist, NULL, ".");
+		hist = append_string(hist, NULL, synth->name);
+		hist = append_string(hist, NULL, "(");
+	}
 
 	for (i = 0; synth->synthetic_args && synth->synthetic_args[i]; i++) {
 		name = synth->synthetic_args[i];
 
-		hist = append_string(hist, NULL, ",");
+		if (i)
+			hist = append_string(hist, NULL, ",");
 		hist = append_string(hist, NULL, name);
 	}