x86/ftrace: Remove fault protection code in prepare_ftrace_return

Removing the fault protection code when writing return_hooker
to stack. As Steven noted:

> That protection was there from the beginning due to being "paranoid",
> considering ftrace was bricking network cards. But that protection
> would not have even protected against that.

Link: https://lkml.kernel.org/r/20211008091336.33616-3-jolsa@kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 23d221a..7f12eac 100644 (file)
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -620,12 +620,10 @@ int ftrace_disable_ftrace_graph_caller(void)
  * Hook the return address and push it in the stack of return addrs
  * in current thread info.
  */
-void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
+void prepare_ftrace_return(unsigned long ip, unsigned long *parent,
                           unsigned long frame_pointer)
 {
        unsigned long return_hooker = (unsigned long)&return_to_handler;
-       unsigned long old;
-       int faulted;
 
        /*
         * When resuming from suspend-to-ram, this function can be indirectly
@@ -645,37 +643,7 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
        if (unlikely(atomic_read(&current->tracing_graph_pause)))
                return;
 
-       /*
-        * Protect against fault, even if it shouldn't
-        * happen. This tool is too much intrusive to
-        * ignore such a protection.
-        */
-       asm volatile(
-               "1: " _ASM_MOV " (%[parent]), %[old]\n"
-               "2: " _ASM_MOV " %[return_hooker], (%[parent])\n"
-               "   movl $0, %[faulted]\n"
-               "3:\n"
-
-               ".section .fixup, \"ax\"\n"
-               "4: movl $1, %[faulted]\n"
-               "   jmp 3b\n"
-               ".previous\n"
-
-               _ASM_EXTABLE(1b, 4b)
-               _ASM_EXTABLE(2b, 4b)
-
-               : [old] "=&r" (old), [faulted] "=r" (faulted)
-               : [parent] "r" (parent), [return_hooker] "r" (return_hooker)
-               : "memory"
-       );
-
-       if (unlikely(faulted)) {
-               ftrace_graph_stop();
-               WARN_ON(1);
-               return;
-       }
-
-       if (function_graph_enter(old, self_addr, frame_pointer, parent))
-               *parent = old;
+       if (!function_graph_enter(*parent, ip, frame_pointer, parent))
+               *parent = return_hooker;
 }
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */