[PATCH] perf trace: avoid garbage when not printing an trace event's arguments

[Benjamin Peterson <benjamin@engflow.com>]

trace__fprintf_tp_fields may not print any tracepoint arguments. E.g., if the
argument values are all zero. Previously, this would result in a totally
uninitialized buffer being passed to fprintf, which could lead to garbage on the
console. Fix the problem by passing the number of initialized bytes fprintf.

Remove the return value of trace__fprintf_tp_fields, since it was meaningless
and ignored.

Fixes: f11b2803bb88 ("perf trace: Allow choosing how to augment the tracepoint arguments")
Signed-off-by: Benjamin Peterson <benjamin@engflow.com>
---
 tools/perf/builtin-trace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
index d3f11b90d025..e93f795e680f 100644
--- a/tools/perf/builtin-trace.c
+++ b/tools/perf/builtin-trace.c
@@ -3021,7 +3021,7 @@ static void bpf_output__fprintf(struct trace *trace,
 	++trace->nr_events_printed;
 }
 
-static size_t trace__fprintf_tp_fields(struct trace *trace, struct evsel *evsel, struct perf_sample *sample,
+static void trace__fprintf_tp_fields(struct trace *trace, struct evsel *evsel, struct perf_sample *sample,
 				       struct thread *thread, void *augmented_args, int augmented_args_size)
 {
 	char bf[2048];
@@ -3087,7 +3087,7 @@ static size_t trace__fprintf_tp_fields(struct trace *trace, struct evsel *evsel,
 		printed += syscall_arg_fmt__scnprintf_val(arg, bf + printed, size - printed, &syscall_arg, val);
 	}
 
-	return printed + fprintf(trace->output, "%s", bf);
+	fprintf(trace->output, "%.*s", printed, bf);
 }
 
 static int trace__event_handler(struct trace *trace, struct evsel *evsel,
-- 
2.39.5