tracing: trace_remove_event_call() should fail if call/file is in use

Change trace_remove_event_call(call) to return the error if this
call is active. This is what the callers assume but can't verify
outside of the tracing locks. Both trace_kprobe.c/trace_uprobe.c
need the additional changes, unregister_trace_probe() should abort
if trace_remove_event_call() fails.

The caller is going to free this call/file so we must ensure that
nobody can use them after trace_remove_event_call() succeeds.
debugfs should be fine after the previous changes and event_remove()
does TRACE_REG_UNREGISTER, but still there are 2 reasons why we need
the additional checks:

- There could be a perf_event(s) attached to this tp_event, so the
  patch checks ->perf_refcount.

- TRACE_REG_UNREGISTER can be suppressed by FTRACE_EVENT_FL_SOFT_MODE,
  so we simply check FTRACE_EVENT_FL_ENABLED protected by event_mutex.

Link: http://lkml.kernel.org/r/20130729175033.GB26284@redhat.com

Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 33 insertions, 2 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index a67c913e2f9f..ec04836273c0 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -1713,16 +1713,47 @@ static void __trace_remove_event_call(struct ftrace_event_call *call)
         destroy_preds(call);
 }
 
+static int probe_remove_event_call(struct ftrace_event_call *call)
+{
+        struct trace_array *tr;
+        struct ftrace_event_file *file;
+
+#ifdef CONFIG_PERF_EVENTS
+        if (call->perf_refcount)
+                return -EBUSY;
+#endif
+        do_for_each_event_file(tr, file) {
+                if (file->event_call != call)
+                        continue;
+                /*
+                 * We can't rely on ftrace_event_enable_disable(enable => 0)
+                 * we are going to do, FTRACE_EVENT_FL_SOFT_MODE can suppress
+                 * TRACE_REG_UNREGISTER.
+                 */
+                if (file->flags & FTRACE_EVENT_FL_ENABLED)
+                        return -EBUSY;
+                break;
+        } while_for_each_event_file();
+
+        __trace_remove_event_call(call);
+
+        return 0;
+}
+
 /* Remove an event_call */
-void trace_remove_event_call(struct ftrace_event_call *call)
+int trace_remove_event_call(struct ftrace_event_call *call)
 {
+        int ret;
+
         mutex_lock(&trace_types_lock);
         mutex_lock(&event_mutex);
         down_write(&trace_event_sem);
-        __trace_remove_event_call(call);
+        ret = probe_remove_event_call(call);
         up_write(&trace_event_sem);
         mutex_unlock(&event_mutex);
         mutex_unlock(&trace_types_lock);
+
+        return ret;
 }
 
 #define for_each_event(event, start, end)                       \