ftrace: Rename ftrace_graph_stub to ftrace_stub_graph

The ftrace_graph_stub was created and points to ftrace_stub as a way to
assign the functon graph tracer function pointer to a stub function with a
different prototype than what ftrace_stub has and not trigger the C
verifier. The ftrace_graph_stub was created via the linker script
vmlinux.lds.h. Unfortunately, powerpc already uses the name
ftrace_graph_stub for its internal implementation of the function graph
tracer, and even though powerpc would still build, the change via the linker
script broke function tracer on powerpc from working.

By using the name ftrace_stub_graph, which does not exist anywhere else in
the kernel, this should not be a problem.

Link: https://lore.kernel.org/r/1573849732.5937.136.camel@lca.pw

Fixes: b83b43ffc6 ("fgraph: Fix function type mismatches of ftrace_graph_return using ftrace_stub")
Reorted-by: Qian Cai <cai@lca.pw>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/asm-generic/vmlinux.lds.h

@@ -112,7 +112,7 @@
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
 #ifdef CC_USING_PATCHABLE_FUNCTION_ENTRY
 /*
- * Need to also make ftrace_graph_stub point to ftrace_stub
+ * Need to also make ftrace_stub_graph point to ftrace_stub
  * so that the same stub location may have different protocols
  * and not mess up with C verifiers.
  */
@@ -120,17 +120,17 @@
 			__start_mcount_loc = .;			\
 			KEEP(*(__patchable_function_entries))	\
 			__stop_mcount_loc = .;			\
-			ftrace_graph_stub = ftrace_stub;
+			ftrace_stub_graph = ftrace_stub;
 #else
 #define MCOUNT_REC()	. = ALIGN(8);				\
 			__start_mcount_loc = .;			\
 			KEEP(*(__mcount_loc))			\
 			__stop_mcount_loc = .;			\
-			ftrace_graph_stub = ftrace_stub;
+			ftrace_stub_graph = ftrace_stub;
 #endif
 #else
 # ifdef CONFIG_FUNCTION_TRACER
-#  define MCOUNT_REC()	ftrace_graph_stub = ftrace_stub;
+#  define MCOUNT_REC()	ftrace_stub_graph = ftrace_stub;
 # else
 #  define MCOUNT_REC()
 # endif

kernel/trace/fgraph.c

@@ -336,10 +336,10 @@ int ftrace_graph_entry_stub(struct ftrace_graph_ent *trace)
  * Simply points to ftrace_stub, but with the proper protocol.
  * Defined by the linker script in linux/vmlinux.lds.h
  */
-extern void ftrace_graph_stub(struct ftrace_graph_ret *);
+extern void ftrace_stub_graph(struct ftrace_graph_ret *);
 
 /* The callbacks that hook a function */
-trace_func_graph_ret_t ftrace_graph_return = ftrace_graph_stub;
+trace_func_graph_ret_t ftrace_graph_return = ftrace_stub_graph;
 trace_func_graph_ent_t ftrace_graph_entry = ftrace_graph_entry_stub;
 static trace_func_graph_ent_t __ftrace_graph_entry = ftrace_graph_entry_stub;
 
@@ -619,7 +619,7 @@ void unregister_ftrace_graph(struct fgraph_ops *gops)
 		goto out;
 
 	ftrace_graph_active--;
-	ftrace_graph_return = ftrace_graph_stub;
+	ftrace_graph_return = ftrace_stub_graph;
 	ftrace_graph_entry = ftrace_graph_entry_stub;
 	__ftrace_graph_entry = ftrace_graph_entry_stub;
 	ftrace_shutdown(&graph_ops, FTRACE_STOP_FUNC_RET);