Avoid race in nscd

2012-05-15  Jeff Law  <law@redhat.com>
            Andreas Jaeger  <aj@suse.de>

        [BZ #13594]
        * nscd/nscd-client.h (__nscd_acquire_maplock): New function, split
        out from...
        * nscd/nscd_helper.c (__nscd_get_map_ref): ... here.
        * nscd/nscd-client.h: Add __nscd_acquire_maplock.
        * nscd/nscd_gethst_r.c (__nscd_get_nl_timestamp): Add locking to
        code changing __hst_map_handle.map.

1 files changed, 19 insertions, 2 deletions

diff --git a/nscd/nscd-client.h b/nscd/nscd-client.h
index e57a23c057..325368e73b 100644
--- a/nscd/nscd-client.h
+++ b/nscd/nscd-client.h
@@ -1,5 +1,4 @@
-/* Copyright (c) 1998, 1999, 2000, 2003, 2004, 2005, 2006, 2007, 2009, 2011
-   Free Software Foundation, Inc.
+/* Copyright (c) 1998-2012 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Thorsten Kukuk <kukuk@suse.de>, 1998.
 
@@ -322,6 +321,24 @@ struct locked_map_ptr
 };
 #define libc_locked_map_ptr(class, name) class struct locked_map_ptr name
 
+/* Try acquiring lock for mapptr, returns true if it succeeds, false
+   if not.  */
+static inline bool __nscd_acquire_maplock (volatile struct locked_map_ptr *mapptr)
+{
+  int cnt = 0;
+  while (__builtin_expect (atomic_compare_and_exchange_val_acq (&mapptr->lock,
+								1, 0) != 0, 0))
+    {
+      // XXX Best number of rounds?
+      if (__builtin_expect (++cnt > 5, 0))
+	return false;
+
+      atomic_delay ();
+    }
+
+  return true;
+}
+
 
 /* Open socket connection to nscd server.  */
 extern int __nscd_open_socket (const char *key, size_t keylen,