tracing, x86: fix fixup section to return to original code

Impact: fix to prevent a kernel crash on fault

If for some reason the pointer to the parent function on the
stack takes a fault, the fix up code will not return back to
the original faulting code. This can lead to unpredictable
results and perhaps even a kernel panic.

A fault should not happen, but if it does, we should simply
disable the tracer, warn, and continue running the kernel.
It should not lead to a kernel crash.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

arch/x86/kernel/ftrace.c

@@ -491,13 +491,15 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
 		"1: " _ASM_MOV " (%[parent_old]), %[old]\n"
 		"2: " _ASM_MOV " %[return_hooker], (%[parent_replaced])\n"
 		"   movl $0, %[faulted]\n"
+		"3:\n"
 
 		".section .fixup, \"ax\"\n"
-		"3: movl $1, %[faulted]\n"
+		"4: movl $1, %[faulted]\n"
+		"   jmp 3b\n"
 		".previous\n"
 
-		_ASM_EXTABLE(1b, 3b)
-		_ASM_EXTABLE(2b, 3b)
+		_ASM_EXTABLE(1b, 4b)
+		_ASM_EXTABLE(2b, 4b)
 
 		: [parent_replaced] "=r" (parent), [old] "=r" (old),
 		  [faulted] "=r" (faulted)