[RFC PATCH 1/1] tracing: support dynamic string field types for synthetic events

[Axel Rasmussen <axelrasmussen@google.com>]

It's typical [1] to define tracepoint string fields as "const char *",
using the __string, __assign_str, and __get_str helpers. For synthetic
event definitions, the only available mechanism to define a string type
is a fixed-size char array ("char[]") [2].

Without this patch, since the type strings aren't identical, and the
sizes don't match (since one is an array, and the other is a "dynamic
string" integer), they are considered incompatible [3].

This patch modifies that check, so as to let us setup synthetic events,
and plumb through string values from typical tracepoints. It turns out
this is already handled correctly, as long as the check during
definition parsing doesn't prevent it.

[1] grep -r "__string" include/trace/events/
[2] See synth_field_is_string in kernel/trace/trace_events_synth.c
[3] See check_synth_field in kernel/trace/trace_events_hist.c

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 kernel/trace/trace_events_hist.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 0b933546142e..e064feb3cc65 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -3280,9 +3280,18 @@ static int check_synth_field(struct synth_event *event,
 	field = event->fields[field_pos];
 
 	if (strcmp(field->type, hist_field->type) != 0) {
-		if (field->size != hist_field->size ||
-		    field->is_signed != hist_field->is_signed)
-			return -EINVAL;
+		/*
+		 * If both are kinds of strings, they match. We can't use
+		 * is_string_field for the hist_field, as it's only sort of
+		 * partially initialized at this point.
+		 */
+		if (strstr(field->type, "char[") == NULL ||
+		    strstr(hist_field->type, "char[") == NULL) {
+			/* They still match if size and signedness match. */
+			if (field->size != hist_field->size ||
+			    field->is_signed != hist_field->is_signed)
+				return -EINVAL;
+		}
 	}
 
 	return 0;
-- 
2.28.0.681.g6f77f65b4e-goog