tracing/user_events: Fix the incorrect trace record for empty arguments events

The user_events support events that has empty arguments. But the trace event
is discarded and not really committed when the arguments is empty. Fix this
by not attempting to copy in zero-length data.

Link: https://lkml.kernel.org/r/20230606062027.1008398-2-sunliming@kylinos.cn

Acked-by: Beau Belgrave <beaub@linux.microsoft.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: sunliming <sunliming@kylinos.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: sunliming

kernel/trace/trace_events_user.c

@@ -1432,7 +1432,7 @@ static void user_event_ftrace(struct user_event *user, struct iov_iter *i,
 	if (unlikely(!entry))
 		return;
 
-	if (unlikely(!copy_nofault(entry + 1, i->count, i)))
+	if (unlikely(i->count != 0 && !copy_nofault(entry + 1, i->count, i)))
 		goto discard;
 
 	if (!list_empty(&user->validators) &&
@@ -1473,7 +1473,7 @@ static void user_event_perf(struct user_event *user, struct iov_iter *i,
 
 		perf_fetch_caller_regs(regs);
 
-		if (unlikely(!copy_nofault(perf_entry + 1, i->count, i)))
+		if (unlikely(i->count != 0 && !copy_nofault(perf_entry + 1, i->count, i)))
 			goto discard;
 
 		if (!list_empty(&user->validators) &&