function_graph: Create function_graph_enter() to consolidate architecture code

Currently all the architectures do basically the same thing in preparing the
function graph tracer on entry to a function. This code can be pulled into a
generic location and then this will allow the function graph tracer to be
fixed, as well as extended.

Create a new function graph helper function_graph_enter() that will call the
hook function (ftrace_graph_entry) and the shadow stack operation
(ftrace_push_return_trace), and remove the need of the architecture code to
manage the shadow stack.

This is needed to prepare for a fix of a design bug on how the curr_ret_stack
is used.

Cc: stable@kernel.org
Fixes: 03274a3ffb ("tracing/fgraph: Adjust fgraph depth before calling trace return callback")
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/linux/ftrace.h

@@ -779,6 +779,9 @@ extern void return_to_handler(void);
 extern int
 ftrace_push_return_trace(unsigned long ret, unsigned long func, int *depth,
 			 unsigned long frame_pointer, unsigned long *retp);
+extern int
+function_graph_enter(unsigned long ret, unsigned long func,
+		     unsigned long frame_pointer, unsigned long *retp);
 
 unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
 				    unsigned long ret, unsigned long *retp);

kernel/trace/trace_functions_graph.c

@@ -182,6 +182,22 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func, int *depth,
 	return 0;
 }
 
+int function_graph_enter(unsigned long ret, unsigned long func,
+			 unsigned long frame_pointer, unsigned long *retp)
+{
+	struct ftrace_graph_ent trace;
+
+	trace.func = func;
+	trace.depth = current->curr_ret_stack + 1;
+
+	/* Only trace if the calling function expects to */
+	if (!ftrace_graph_entry(&trace))
+		return -EBUSY;
+
+	return ftrace_push_return_trace(ret, func, &trace.depth,
+					frame_pointer, retp);
+}
+
 /* Retrieve a function return address to the trace stack on thread info.*/
 static void
 ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret,