ARM: ftrace: Only set kernel memory back to read-only after boot

Dynamic ftrace requires modifying the code segments that are usually
set to read-only. To do this, a per arch function is called both before
and after the ftrace modifications are performed. The "before" function
will set kernel code text to read-write to allow for ftrace to make the
modifications, and the "after" function will set the kernel code text
back to "read-only" to keep the kernel code text protected.

The issue happens when dynamic ftrace is tested at boot up. The test is
done before the kernel code text has been set to read-only. But the
"before" and "after" calls are still performed. The "after" call will
change the kernel code text to read-only prematurely, and other boot
code that expects this code to be read-write will fail.

The solution is to add a variable that is set when the kernel code text
is expected to be converted to read-only, and make the ftrace "before"
and "after" calls do nothing if that variable is not yet set. This is
similar to the x86 solution from commit 1623963 ("ftrace, x86:
make kernel text writable only for conversions").

Reported-by: Stefan Agner <stefan@agner.ch>
Tested-by: Stefan Agner <stefan@agner.ch>
Link: http://lkml.kernel.org/r/20180620212906.24b7b66e@vmware.local.home
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

arch/arm/mm/init.c

@@ -736,20 +736,29 @@ static int __mark_rodata_ro(void *unused)
 	return 0;
 }
 
+static int kernel_set_to_readonly __read_mostly;
+
 void mark_rodata_ro(void)
 {
+	kernel_set_to_readonly = 1;
 	stop_machine(__mark_rodata_ro, NULL, NULL);
 	debug_checkwx();
 }
 
 void set_kernel_text_rw(void)
 {
+	if (!kernel_set_to_readonly)
+		return;
+
 	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), false,
 				current->active_mm);
 }
 
 void set_kernel_text_ro(void)
 {
+	if (!kernel_set_to_readonly)
+		return;
+
 	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,
 				current->active_mm);
 }