libceph: add a compatibility check interface

An upcoming change implements semantic change that could lead to
a crash if an old version of the libceph kernel module is used with
a new version of the rbd kernel module.

In order to preclude that possibility, this adds a compatibilty
check interface.  If this interface doesn't exist, the modules are
obviously not compatible.  But if it does exist, this provides a way
of letting the caller know whether it will operate properly with
this libceph module.

Perhaps confusingly, it returns false right now.  The semantic
change mentioned above will make it return true.

This resolves:
    http://tracker.ceph.com/issues/3800

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/net/ceph/ceph_common.c b/net/ceph/ceph_common.c
index ee71ea26777a..a98c03ff853f 100644
--- a/net/ceph/ceph_common.c
+++ b/net/ceph/ceph_common.c
@@ -26,6 +26,22 @@
 #include "crypto.h"
 
 
+/*
+ * Module compatibility interface.  For now it doesn't do anything,
+ * but its existence signals a certain level of functionality.
+ *
+ * The data buffer is used to pass information both to and from
+ * libceph.  The return value indicates whether libceph determines
+ * it is compatible with the caller (from another kernel module),
+ * given the provided data.
+ *
+ * The data pointer can be null.
+ */
+bool libceph_compatible(void *data)
+{
+	return false;
+}
+EXPORT_SYMBOL(libceph_compatible);
 
 /*
  * find filename portion of a path (/foo/bar/baz -> baz)