fgraph: Fix function type mismatches of ftrace_graph_return using ftrace_stub

The C compiler is allowing more checks to make sure that function pointers
are assigned to the correct prototype function. Unfortunately, the function
graph tracer uses a special name with its assigned ftrace_graph_return
function pointer that maps to a stub function used by the function tracer
(ftrace_stub). The ftrace_graph_return variable is compared to the
ftrace_stub in some archs to know if the function graph tracer is enabled or
not. This means we can not just simply create a new function stub that
compares it without modifying all the archs.

Instead, have the linker script create a function_graph_stub that maps to
ftrace_stub, and this way we can define the prototype for it to match the
prototype of ftrace_graph_return, and make the compiler checks all happy!

Link: http://lkml.kernel.org/r/20191015090055.789a0aed@gandalf.local.home

Cc: linux-sh@vger.kernel.org
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc:  Rich Felker <dalias@libc.org>
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

arch/sh/boot/compressed/misc.c

@@ -111,6 +111,11 @@ void __stack_chk_fail(void)
 	error("stack-protector: Kernel stack is corrupted\n");
 }
 
+/* Needed because vmlinux.lds.h references this */
+void ftrace_stub(void)
+{
+}
+
 #ifdef CONFIG_SUPERH64
 #define stackalign	8
 #else

include/asm-generic/vmlinux.lds.h

@@ -111,18 +111,29 @@
 
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
 #ifdef CC_USING_PATCHABLE_FUNCTION_ENTRY
+/*
+ * Need to also make ftrace_graph_stub point to ftrace_stub
+ * so that the same stub location may have different protocols
+ * and not mess up with C verifiers.
+ */
 #define MCOUNT_REC()	. = ALIGN(8);				\
 			__start_mcount_loc = .;			\
 			KEEP(*(__patchable_function_entries))	\
-			__stop_mcount_loc = .;
+			__stop_mcount_loc = .;			\
+			ftrace_graph_stub = ftrace_stub;
 #else
 #define MCOUNT_REC()	. = ALIGN(8);				\
 			__start_mcount_loc = .;			\
 			KEEP(*(__mcount_loc))			\
-			__stop_mcount_loc = .;
+			__stop_mcount_loc = .;			\
+			ftrace_graph_stub = ftrace_stub;
 #endif
 #else
-#define MCOUNT_REC()
+# ifdef CONFIG_FUNCTION_TRACER
+#  define MCOUNT_REC()	ftrace_graph_stub = ftrace_stub;
+# else
+#  define MCOUNT_REC()
+# endif
 #endif
 
 #ifdef CONFIG_TRACE_BRANCH_PROFILING

kernel/trace/fgraph.c

@@ -332,9 +332,14 @@ int ftrace_graph_entry_stub(struct ftrace_graph_ent *trace)
 	return 0;
 }
 
+/*
+ * 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 *);
+
 /* The callbacks that hook a function */
-trace_func_graph_ret_t ftrace_graph_return =
+trace_func_graph_ret_t ftrace_graph_return = ftrace_graph_stub;
-			(trace_func_graph_ret_t)ftrace_stub;
 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;
 
@@ -614,7 +619,7 @@ void unregister_ftrace_graph(struct fgraph_ops *gops)
 		goto out;
 
 	ftrace_graph_active--;
-	ftrace_graph_return = (trace_func_graph_ret_t)ftrace_stub;
+	ftrace_graph_return = ftrace_graph_stub;
 	ftrace_graph_entry = ftrace_graph_entry_stub;
 	__ftrace_graph_entry = ftrace_graph_entry_stub;
 	ftrace_shutdown(&graph_ops, FTRACE_STOP_FUNC_RET);