tracing: Fix print_fields() and use best filter

When using user_events along with the new print_fields() functionality a few issues were discovered. When printing out fields, the __rel_loc field types were printing out the wrong array values. Also, user_events wasn't setting the best filter type, so __rel_loc data was marked as FILTER_OTHER vs FILTER_RDYN_STRING when chars were used. This resulted in strings being printed out as array of bytes vs a string.

After applying this series user_events will output strings correctly for __rel_loc via /sys/kernel/tracing/trace outputs. All events that utilize print_fields() will print the correct array/string for __data_loc and __rel_loc data, when it's enabled.

Beau Belgrave (2):
tracing/user_events: Set event filter_type from type
tracing: Fix print_fields() for __dyn_loc/__rel_loc

 kernel/trace/trace_events_user.c |  3 +++
 kernel/trace/trace_output.c      | 10 ++++++----
 2 files changed, 9 insertions(+), 4 deletions(-)

base-commit: 88fe1ec75fcb296579e05eaf3807da3ee83137e4

2.25.1

Users expect that events can be filtered by the kernel. User events currently sets all event fields as FILTER_OTHER which limits to binary filters only. When strings are being used, functionality is reduced.

Use filter_assign_type() to find the most appropriate filter type for each field in user events to ensure full kernel capabilities.

---
 kernel/trace/trace_events_user.c | 3 +++
 1 file changed, 3 insertions(+)

index cc8c6d8b69b5..eadb58a3efba 100644
@@ -918,6 +918,9 @@ static int user_event_add_field(struct user_event *user, const char *type,
 	field->is_signed = is_signed;
 	field->filter_type = filter_type;
 
+	if (filter_type == FILTER_OTHER)
+		field->filter_type = filter_assign_type(type);
+
 	list_add(&field->link, &user->fields);
 
 	/*

2.25.1

Both print_fields() and print_array() do not handle if dynamic data ends at the last byte of the payload for both __dyn_loc and __rel_loc field types. For __rel_loc, the offset was off by 4 bytes, leading to incorrect strings and data being printed out. In print_array() the buffer pos was missed from being advanced, which results in the first payload byte being used as the offset base instead of the field offset.

Advance __rel_loc offset by 4 to ensure correct offset and advance pos to the field offset to ensure correct data is displayed when printing arrays. Change >= to > when checking if data is in-bounds, since it's valid for dynamic data to include the last byte of the payload.

Example outputs for event format:

        field:unsigned short common_type;       offset:0;       size:2; signed:0;
        field:unsigned char common_flags;       offset:2;       size:1; signed:0;
        field:unsigned char common_preempt_count;       offset:3;       size:1; signed:0;
        field:int common_pid;   offset:4;       size:4; signed:1;

        field:__rel_loc char text[];  offset:8;      size:4; signed:1;

Output before:
tp_rel_loc: text=<OVERFLOW>

Output after:
tp_rel_loc: text=Test

---
 kernel/trace/trace_output.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

index 780c6971c944..952cc8aa8e59 100644
@@ -819,13 +819,15 @@ static void print_array(struct trace_iterator *iter, void *pos,
 	len = *(int *)pos >> 16;
 
 	if (field)
-		offset += field->offset;
+		offset += field->offset + sizeof(int);
 
-	if (offset + len >= iter->ent_size) {
+	if (offset + len > iter->ent_size) {
 		trace_seq_puts(&iter->seq, "<OVERFLOW>");
 		return;
 	}
 
+	pos = (void *)iter->ent + offset;
+
 	for (i = 0; i < len; i++, pos++) {
 		if (i)
 			trace_seq_putc(&iter->seq, ',');

@@ -861,9 +863,9 @@ static void print_fields(struct trace_iterator *iter, struct trace_event_call *c
 			len = *(int *)pos >> 16;
 
 			if (field->filter_type == FILTER_RDYN_STRING)
-				offset += field->offset;
+				offset += field->offset + sizeof(int);
 
-			if (offset + len >= iter->ent_size) {
+			if (offset + len > iter->ent_size) {
 				trace_seq_puts(&iter->seq, "<OVERFLOW>");
 				break;
 			}

2.25.1