ftrace: Add another example of register_ftrace_direct() use case

Add another module sample that registers a direct trampoline to a function
via register_ftrace_direct(). Having another module that does this allows to
test the use case of multiple direct callers registered, as more than one
direct caller goes into another path, and is needed to perform proper
testing of the register_ftrace_direct() call.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

samples/ftrace/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
 
 obj-$(CONFIG_SAMPLE_FTRACE_DIRECT) += ftrace-direct.o
+obj-$(CONFIG_SAMPLE_FTRACE_DIRECT) += ftrace-direct-too.o

samples/ftrace/ftrace-direct-too.c

+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/module.h>
+
+#include <linux/mm.h> /* for handle_mm_fault() */
+#include <linux/ftrace.h>
+
+void my_direct_func(struct vm_area_struct *vma,
+			unsigned long address, unsigned int flags)
+{
+	trace_printk("handle mm fault vma=%p address=%lx flags=%x\n",
+		     vma, address, flags);
+}
+
+extern void my_tramp(void *);
+
+asm (
+"	.pushsection    .text, \"ax\", @progbits\n"
+"   my_tramp:"
+"	pushq %rbp\n"
+"	movq %rsp, %rbp\n"
+"	pushq %rdi\n"
+"	pushq %rsi\n"
+"	pushq %rdx\n"
+"	call my_direct_func\n"
+"	popq %rdx\n"
+"	popq %rsi\n"
+"	popq %rdi\n"
+"	leave\n"
+"	ret\n"
+"	.popsection\n"
+);
+
+
+static int __init ftrace_direct_init(void)
+{
+	return register_ftrace_direct((unsigned long)handle_mm_fault,
+				     (unsigned long)my_tramp);
+}
+
+static void __exit ftrace_direct_exit(void)
+{
+	unregister_ftrace_direct((unsigned long)handle_mm_fault,
+				 (unsigned long)my_tramp);
+}
+
+module_init(ftrace_direct_init);
+module_exit(ftrace_direct_exit);
+
+MODULE_AUTHOR("Steven Rostedt");
+MODULE_DESCRIPTION("Another example use case of using register_ftrace_direct()");
+MODULE_LICENSE("GPL");