perf trace: Avoid garbage when not printing a syscall's arguments

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: a98392b ("perf trace: Use beautifiers on syscalls:sys_enter_ handlers")
Signed-off-by: Benjamin Peterson <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Tested-by: Howard Chu <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: benjaminp

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);