tracing/events: Don't increment @pos in s_start()

While testing syscall tracepoints posted by Jason, I found 3 entries
were missing when reading available_events. The output size of
available_events is < 4 pages, which means we lost 1 entry per page.

The cause is, it's wrong to increment @pos in s_start().

Actually there's another bug here -- reading avaiable_events/set_events
can race with module unload:

  # cat available_events               |
      s_start()                        |
      s_stop()                         |
                                       | # rmmod foo.ko
      s_start()                        |
        call = list_entry(m->private)  |

@call might be freed and accessing it will lead to crash.

Reviewed-by: Liming Wang <liming.wang@windriver.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <4A4186DD.6090405@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 22 insertions, 6 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index aa08be69a1b6..53c8fd376a88 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -300,10 +300,18 @@ t_next(struct seq_file *m, void *v, loff_t *pos)
 
 static void *t_start(struct seq_file *m, loff_t *pos)
 {
+        struct ftrace_event_call *call = NULL;
+        loff_t l;
+
         mutex_lock(&event_mutex);
-        if (*pos == 0)
-                m->private = ftrace_events.next;
-        return t_next(m, NULL, pos);
+
+        m->private = ftrace_events.next;
+        for (l = 0; l <= *pos; ) {
+                call = t_next(m, NULL, &l);
+                if (!call)
+                        break;
+        }
+        return call;
 }
 
 static void *
@@ -332,10 +340,18 @@ s_next(struct seq_file *m, void *v, loff_t *pos)
 
 static void *s_start(struct seq_file *m, loff_t *pos)
 {
+        struct ftrace_event_call *call = NULL;
+        loff_t l;
+
         mutex_lock(&event_mutex);
-        if (*pos == 0)
-                m->private = ftrace_events.next;
-        return s_next(m, NULL, pos);
+
+        m->private = ftrace_events.next;
+        for (l = 0; l <= *pos; ) {
+                call = s_next(m, NULL, &l);
+                if (!call)
+                        break;
+        }
+        return call;
 }
 
 static int t_show(struct seq_file *m, void *v)