rxrpc: Allow the reply time to be obtained on a client call

Allow the epoch value to be queried on a server connection.  This is in the
rxrpc header of every packet for use in routing and is derived from the
client's state.  It's also not supposed to change unless the client gets
restarted.

AFS can make use of this information to deduce whether a fileserver has
been restarted because the fileserver makes client calls to the filesystem
driver's cache manager to send notifications (ie. callback breaks) about
conflicting changes from other clients.  These convey the fileserver's own
epoch value back to the filesystem.
Signed-off-by: David Howells <dhowells@redhat.com>

Documentation/networking/rxrpc.txt

@@ -1080,6 +1080,20 @@ The kernel interface functions are as follows:
      successful, the timestamp will be stored into *_ts and true will be
      returned; false will be returned otherwise.
 
+ (*) Get remote client epoch.
+
+	u32 rxrpc_kernel_get_epoch(struct socket *sock,
+				   struct rxrpc_call *call)
+
+     This allows the epoch that's contained in packets of an incoming client
+     call to be queried.  This value is returned.  The function always
+     successful if the call is still in progress.  It shouldn't be called once
+     the call has expired.  Note that calling this on a local client call only
+     returns the local epoch.
+
+     This value can be used to determine if the remote client has been
+     restarted as it shouldn't change otherwise.
+
 
 =======================
 CONFIGURABLE PARAMETERS

include/net/af_rxrpc.h

@@ -78,6 +78,7 @@ int rxrpc_kernel_retry_call(struct socket *, struct rxrpc_call *,
 int rxrpc_kernel_check_call(struct socket *, struct rxrpc_call *,
 			    enum rxrpc_call_completion *, u32 *);
 u32 rxrpc_kernel_check_life(struct socket *, struct rxrpc_call *);
+u32 rxrpc_kernel_get_epoch(struct socket *, struct rxrpc_call *);
 bool rxrpc_kernel_get_reply_time(struct socket *, struct rxrpc_call *,
 				 ktime_t *);
 

net/rxrpc/af_rxrpc.c

@@ -385,6 +385,20 @@ u32 rxrpc_kernel_check_life(struct socket *sock, struct rxrpc_call *call)
 }
 EXPORT_SYMBOL(rxrpc_kernel_check_life);
 
+/**
+ * rxrpc_kernel_get_epoch - Retrieve the epoch value from a call.
+ * @sock: The socket the call is on
+ * @call: The call to query
+ *
+ * Allow a kernel service to retrieve the epoch value from a service call to
+ * see if the client at the other end rebooted.
+ */
+u32 rxrpc_kernel_get_epoch(struct socket *sock, struct rxrpc_call *call)
+{
+	return call->conn->proto.epoch;
+}
+EXPORT_SYMBOL(rxrpc_kernel_get_epoch);
+
 /**
  * rxrpc_kernel_check_call - Check a call's state
  * @sock: The socket the call is on