tracing/probes: cleanup: Set trace_probe::nr_args at trace_probe_init

BugLink: https://bugs.launchpad.net/bugs/2097575

[ Upstream commit 035ba76014c096316fa809a46ce0a1b9af1cde0d ]

Instead of incrementing the trace_probe::nr_args, init it at
trace_probe_init(). Without this change, there is no way to get the number
of trace_probe arguments while parsing it.
This is a cleanup, so the behavior is not changed.

Link: https://lore.kernel.org/all/170952363585.229804.13060759900346411951.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Stable-dep-of: 373b9338c972 ("uprobe: avoid out-of-bounds memory access of fetching args")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

kernel/trace/trace_probe.c

@@ -1383,9 +1383,6 @@ int traceprobe_parse_probe_arg(struct trace_probe *tp, int i, const char *arg,
 	struct probe_arg *parg = &tp->args[i];
 	const char *body;
 
-	/* Increment count for freeing args in error case */
-	tp->nr_args++;
-
 	body = strchr(arg, '=');
 	if (body) {
 		if (body - arg > MAX_ARG_NAME_LEN) {
@@ -1770,7 +1767,7 @@ void trace_probe_cleanup(struct trace_probe *tp)
 }
 
 int trace_probe_init(struct trace_probe *tp, const char *event,
-		     const char *group, bool alloc_filter)
+		     const char *group, bool alloc_filter, int nargs)
 {
 	struct trace_event_call *call;
 	size_t size = sizeof(struct trace_probe_event);
@@ -1806,6 +1803,11 @@ int trace_probe_init(struct trace_probe *tp, const char *event,
 		goto error;
 	}
 
+	tp->nr_args = nargs;
+	/* Make sure pointers in args[] are NULL */
+	if (nargs)
+		memset(tp->args, 0, sizeof(tp->args[0]) * nargs);
+
 	return 0;
 
 error: