ftrace/samples: Add a sample module that implements modify_ftrace_direct()

Add a sample module that tests modify_ftrace_direct(), and this can be used
by the selftests as well.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

samples/ftrace/Makefile

@@ -2,3 +2,4 @@
 
 obj-$(CONFIG_SAMPLE_FTRACE_DIRECT) += ftrace-direct.o
 obj-$(CONFIG_SAMPLE_FTRACE_DIRECT) += ftrace-direct-too.o
+obj-$(CONFIG_SAMPLE_FTRACE_DIRECT) += ftrace-direct-modify.o

samples/ftrace/ftrace-direct-modify.c

@@ -0,0 +1,88 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/module.h>
+#include <linux/kthread.h>
+#include <linux/ftrace.h>
+
+void my_direct_func1(void)
+{
+	trace_printk("my direct func1\n");
+}
+
+void my_direct_func2(void)
+{
+	trace_printk("my direct func2\n");
+}
+
+extern void my_tramp1(void *);
+extern void my_tramp2(void *);
+
+static unsigned long my_ip = (unsigned long)schedule;
+
+asm (
+"	.pushsection    .text, \"ax\", @progbits\n"
+"   my_tramp1:"
+"	pushq %rbp\n"
+"	movq %rsp, %rbp\n"
+"	call my_direct_func1\n"
+"	leave\n"
+"	ret\n"
+"   my_tramp2:"
+"	pushq %rbp\n"
+"	movq %rsp, %rbp\n"
+"	call my_direct_func2\n"
+"	leave\n"
+"	ret\n"
+"	.popsection\n"
+);
+
+static unsigned long my_tramp = (unsigned long)my_tramp1;
+static unsigned long tramps[2] = {
+	(unsigned long)my_tramp1,
+	(unsigned long)my_tramp2,
+};
+
+static int simple_thread(void *arg)
+{
+	static int t;
+	int ret = 0;
+
+	while (!kthread_should_stop()) {
+		set_current_state(TASK_INTERRUPTIBLE);
+		schedule_timeout(2 * HZ);
+
+		if (ret)
+			continue;
+		t ^= 1;
+		ret = modify_ftrace_direct(my_ip, my_tramp, tramps[t]);
+		if (!ret)
+			my_tramp = tramps[t];
+		WARN_ON_ONCE(ret);
+	}
+
+	return 0;
+}
+
+static struct task_struct *simple_tsk;
+
+static int __init ftrace_direct_init(void)
+{
+	int ret;
+
+	ret = register_ftrace_direct(my_ip, my_tramp);
+	if (!ret)
+		simple_tsk = kthread_run(simple_thread, NULL, "event-sample-fn");
+	return ret;
+}
+
+static void __exit ftrace_direct_exit(void)
+{
+	kthread_stop(simple_tsk);
+	unregister_ftrace_direct(my_ip, my_tramp);
+}
+
+module_init(ftrace_direct_init);
+module_exit(ftrace_direct_exit);
+
+MODULE_AUTHOR("Steven Rostedt");
+MODULE_DESCRIPTION("Example use case of using modify_ftrace_direct()");
+MODULE_LICENSE("GPL");