tracing: Define MCOUNT_INSN_SIZE when not defined without direct calls

In order to handle direct calls along side of function graph tracer, a check
is made to see if the address being traced by the function graph tracer is a
direct call or not. To get the address used by direct callers, the return
address is subtracted by MCOUNT_INSN_SIZE.

For some archs with certain configurations, MCOUNT_INSN_SIZE is undefined
here. But these should not be using direct calls anyway. Just define
MCOUNT_INSN_SIZE to zero in this case.

Link: https://lore.kernel.org/r/202001020219.zvE3vsty%lkp@intel.com

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: ff20576 ("ftrace: Fix function_graph tracer interaction with BPF trampoline")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/fgraph.c

@@ -96,6 +96,20 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func,
 	return 0;
 }
 
+/*
+ * Not all archs define MCOUNT_INSN_SIZE which is used to look for direct
+ * functions. But those archs currently don't support direct functions
+ * anyway, and ftrace_find_rec_direct() is just a stub for them.
+ * Define MCOUNT_INSN_SIZE to keep those archs compiling.
+ */
+#ifndef MCOUNT_INSN_SIZE
+/* Make sure this only works without direct calls */
+# ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+#  error MCOUNT_INSN_SIZE not defined with direct calls enabled
+# endif
+# define MCOUNT_INSN_SIZE 0
+#endif
+
 int function_graph_enter(unsigned long ret, unsigned long func,
 			 unsigned long frame_pointer, unsigned long *retp)
 {