[PATCH 2/2] tracing: Fix print_fields() for __dyn_loc/__rel_loc

[Beau Belgrave]

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

Fixes: 80a76994b2d8 ("tracing: Add "fields" option to show raw trace event fields")
Reported-by: Doug Cook <dcook@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx>
---
 kernel/trace/trace_output.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index 780c6971c944..952cc8aa8e59 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -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