[tip:tracing/urgent] tracing/events: Don't increment @pos in s_start()

[tip-bot for Li Zefan]

Commit-ID:  e1c7e2a6e67fe9db19dd15e71614526a31b5fdb1
Gitweb:     http://git.kernel.org/tip/e1c7e2a6e67fe9db19dd15e71614526a31b5fdb1
Author:     Li Zefan <lizf@xxxxxxxxxxxxxx>
AuthorDate: Wed, 24 Jun 2009 09:52:29 +0800
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Wed, 24 Jun 2009 11:02:49 +0200

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@xxxxxxxxxxxxx>
Signed-off-by: Li Zefan <lizf@xxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
LKML-Reference: <4A4186DD.6090405@xxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>


---
 kernel/trace/trace_events.c |   28 ++++++++++++++++++++++------
 1 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index aa08be6..53c8fd3 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)
--
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