[syslog-ng] [PATCH 7/7] [logreader]: Proper wakeup function handling

Gergely Nagy algernon at balabit.hu
Mon May 23 11:56:15 CEST 2011

From: Juhasz Viktor <jviktor at balabit.hu>

Rearrange how self->super.wakeup is handled, in order to stop a race
condition where it might be called earlier than it is set, or after it
should have been cleared.

If the reader is uninitialized, but the writer tries to flush the
queue, and call log_msg_refcache_stop(), that in turn will call the
reader's wakeup function. Since the reader is uninitialized, this can
easily corrupt the ivykis event list.

To avoid this scenario, the wakeup callback should be set differently,
and cleared early.

Signed-off-by: Viktor Juhasz <jviktor at balabit.hu>
 lib/logreader.c |   16 ++++++++++++----
 1 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/lib/logreader.c b/lib/logreader.c
index 268be6e..4e56d05 100644
--- a/lib/logreader.c
+++ b/lib/logreader.c
@@ -577,7 +577,8 @@ static gboolean
 log_reader_init(LogPipe *s)
   LogReader *self = (LogReader *) s;
+  /* If reader is initialized than log_msg_ref_cache_stop can call the wakeup function */
+  self->super.wakeup = log_reader_wakeup;
   if (!log_source_init(s))
     return FALSE;
   /* check for new data */
@@ -612,7 +613,6 @@ log_reader_init(LogPipe *s)
   if (!log_reader_start_watches(self))
     return FALSE;
   return TRUE;
@@ -622,7 +622,14 @@ log_reader_deinit(LogPipe *s)
   LogReader *self = (LogReader *) s;
+  /*
+    the wakeup function must be set to NULL, because it can be occured than
+    the reader is uninitialized but the writer flush the queue and call the log_msg_refcache_stop and
+    log_msg_refcache_stop call the wakeup function if it is not null, but in this case the reader
+    is uninitialized and the call of iv_event_post for the unregistered event can cause an ivykis list corruption.
+    IF logreader is uninitialized than the wakeup function is NULL so the iv_event_post isn't called.
+  */
+  self->super.wakeup = NULL;
   if (!log_source_deinit(s))
@@ -684,7 +691,8 @@ log_reader_new(LogProto *proto)
   self->super.super.init = log_reader_init;
   self->super.super.deinit = log_reader_deinit;
   self->super.super.free_fn = log_reader_free;
-  self->super.wakeup = log_reader_wakeup;
+  /* Set wakeup function to NULL because this is set in the log_reader_init and unset in the log_reader_deinit */
+  self->super.wakeup = NULL;
   self->proto = proto;
   self->immediate_check = FALSE;
   self->pollable_state = -1;

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