ftrace/kprobes: selftests: Check kretprobe maxactive is supported

Check the kretprobe maxactive is supported by kprobe_events
interface. To ensure the kernel feature, this changes ftrace
README to describe it.
Signed-off-by: NMasami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

kernel/trace/trace.c

@@ -4473,7 +4473,8 @@ static const char readme_msg[] =
 #endif
 #if defined(CONFIG_KPROBE_EVENTS) || defined(CONFIG_UPROBE_EVENTS)
 	"\t  accepts: event-definitions (one definition per line)\n"
-	"\t   Format: p|r[:[<group>/]<event>] <place> [<args>]\n"
+	"\t   Format: p[:[<group>/]<event>] <place> [<args>]\n"
+	"\t           r[maxactive][:[<group>/]<event>] <place> [<args>]\n"
 	"\t           -:[<group>/]<event>\n"
 #ifdef CONFIG_KPROBE_EVENTS
 	"\t    place: [<module>:]<symbol>[+<offset>]|<memaddr>\n"

tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_maxactive.tc

@@ -2,6 +2,7 @@
 # description: Kretprobe dynamic event with maxactive
 
 [ -f kprobe_events ] || exit_unsupported # this is configurable
+grep -q 'r\[maxactive\]' README || exit_unsupported # this is older version
 
 echo > kprobe_events