tracing: fprobe-event: Fix to check tracepoint event and return

Fix to check the tracepoint event is not valid with $retval.
The commit 08c9306f ("tracing/fprobe-event: Assume fprobe is
a return event by $retval") introduced automatic return probe
conversion with $retval. But since tracepoint event does not
support return probe, $retval is not acceptable.

Without this fix, ftracetest, tprobe_syntax_errors.tc fails;

[22] Tracepoint probe event parser error log check      [FAIL]
 ----
 # tail 22-tprobe_syntax_errors.tc-log.mRKroL
 + ftrace_errlog_check trace_fprobe t kfree ^$retval dynamic_events
 + printf %s t kfree
 + wc -c
 + pos=8
 + printf %s t kfree ^$retval
 + tr -d ^
 + command=t kfree $retval
 + echo Test command: t kfree $retval
 Test command: t kfree $retval
 + echo
 ----

So 't kfree $retval' should fail (tracepoint doesn't support
return probe) but passed it.

Link: https://lore.kernel.org/all/169944555933.45057.12831706585287704173.stgit@devnote2/

Fixes: 08c9306f ("tracing/fprobe-event: Assume fprobe is a return event by $retval")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

kernel/trace/trace_fprobe.c

@@ -927,11 +927,12 @@ static int parse_symbol_and_return(int argc, const char *argv[],
 	for (i = 2; i < argc; i++) {
 		tmp = strstr(argv[i], "$retval");
 		if (tmp && !isalnum(tmp[7]) && tmp[7] != '_') {
+			if (is_tracepoint) {
+				trace_probe_log_set_index(i);
+				trace_probe_log_err(tmp - argv[i], RETVAL_ON_PROBE);
+				return -EINVAL;
+			}
 			*is_return = true;
-			/*
-			 * NOTE: Don't check is_tracepoint here, because it will
-			 * be checked when the argument is parsed.
-			 */
 			break;
 		}
 	}