ftrace: Add perf text poke events for ftrace trampolines

Add perf text poke events for ftrace trampolines when created and when
freed.

There can be 3 text_poke events for ftrace trampolines:

1. NULL -> trampoline
   By ftrace_update_trampoline() when !ops->trampoline
   Trampoline created

2. [e.g. on x86] CALL rel32 -> CALL rel32
   By arch_ftrace_update_trampoline() when ops->trampoline and
                        ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP
   [e.g. on x86] via text_poke_bp() which generates text poke events
   Trampoline-called function target updated

3. trampoline -> NULL
   By ftrace_trampoline_free() when ops->trampoline and
                 ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP
   Trampoline freed
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200512121922.8997-9-adrian.hunter@intel.com

kernel/trace/ftrace.c

@@ -2791,6 +2791,13 @@ static void ftrace_trampoline_free(struct ftrace_ops *ops)
 {
 	if (ops && (ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP) &&
 	    ops->trampoline) {
+		/*
+		 * Record the text poke event before the ksymbol unregister
+		 * event.
+		 */
+		perf_event_text_poke((void *)ops->trampoline,
+				     (void *)ops->trampoline,
+				     ops->trampoline_size, NULL, 0);
 		perf_event_ksymbol(PERF_RECORD_KSYMBOL_TYPE_OOL,
 				   ops->trampoline, ops->trampoline_size,
 				   true, FTRACE_TRAMPOLINE_SYM);
@@ -6816,6 +6823,13 @@ static void ftrace_update_trampoline(struct ftrace_ops *ops)
 		perf_event_ksymbol(PERF_RECORD_KSYMBOL_TYPE_OOL,
 				   ops->trampoline, ops->trampoline_size, false,
 				   FTRACE_TRAMPOLINE_SYM);
+		/*
+		 * Record the perf text poke event after the ksymbol register
+		 * event.
+		 */
+		perf_event_text_poke((void *)ops->trampoline, NULL, 0,
+				     (void *)ops->trampoline,
+				     ops->trampoline_size);
 	}
 }