[PATCH v2 2/3] perf trace: avoid garbage when not printing a syscall's arguments

[Benjamin Peterson <benjamin@engflow.com>]

syscall__scnprintf_args may not place anything in the output buffer
(e.g., because the arguments are all zero). If that happened in
trace__fprintf_sys_enter, its fprintf would receive an unitialized
buffer leading to garbage output.

Fix the problem by passing the (possibly zero) bounds of the argument
buffer to the output fprintf.

Fixes: a98392bb1e169 ("perf trace: Use beautifiers on syscalls:sys_enter_ handlers")
Signed-off-by: Benjamin Peterson <benjamin@engflow.com>
---
 tools/perf/builtin-trace.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
index f6179b13b8b4..28f61d10a2f8 100644
--- a/tools/perf/builtin-trace.c
+++ b/tools/perf/builtin-trace.c
@@ -2702,6 +2702,7 @@ static int trace__fprintf_sys_enter(struct trace *trace, struct evsel *evsel,
 	char msg[1024];
 	void *args, *augmented_args = NULL;
 	int augmented_args_size;
+	size_t printed = 0;
 
 	if (sc == NULL)
 		return -1;
@@ -2717,8 +2718,8 @@ static int trace__fprintf_sys_enter(struct trace *trace, struct evsel *evsel,
 
 	args = perf_evsel__sc_tp_ptr(evsel, args, sample);
 	augmented_args = syscall__augmented_args(sc, sample, &augmented_args_size, trace->raw_augmented_syscalls_args_size);
-	syscall__scnprintf_args(sc, msg, sizeof(msg), args, augmented_args, augmented_args_size, trace, thread);
-	fprintf(trace->output, "%s", msg);
+	printed += syscall__scnprintf_args(sc, msg, sizeof(msg), args, augmented_args, augmented_args_size, trace, thread);
+	fprintf(trace->output, "%.*s", (int)printed, msg);
 	err = 0;
 out_put:
 	thread__put(thread);
-- 
2.39.5