s3: Fix a long-standing problem with recycled PIDs

When a samba server process dies hard, it has no chance to clean up its entries
in locking.tdb, brlock.tdb, connections.tdb and sessionid.tdb.

For locking.tdb and brlock.tdb Samba is robust by checking every time we read
an entry from the database if the corresponding process still exists. If it
does not exist anymore, the entry is deleted. This is not 100% failsafe though:
On systems with a limited PID space there is a non-zero chance that between the
smbd's death and the fresh access, the PID is recycled by another long-running
process. This renders all files that had been locked by the killed smbd
potentially unusable until the new process also dies.

This patch is supposed to fix the problem the following way: Every process ID
in every database is augmented by a random 64-bit number that is stored in a
serverid.tdb. Whenever we need to check if a process still exists we know its
PID and the 64-bit number. We look up the PID in serverid.tdb and compare the
64-bit number. If it's the same, the process still is a valid smbd holding the
lock. If it is different, a new smbd has taken over.

I believe this is safe against an smbd that has died hard and the PID has been
taken over by a non-samba process. This process would not have registered
itself with a fresh 64-bit number in serverid.tdb, so the old one still exists
in serverid.tdb. We protect against this case by the parent smbd taking care of
deregistering PIDs from serverid.tdb and the fact that serverid.tdb is
CLEAR_IF_FIRST.

CLEAR_IF_FIRST does not work in a cluster, so the automatic cleanup does not
work when all smbds are restarted. For this, "net serverid wipe" has to be run
before smbd starts up. As a convenience, "net serverid wipedbs" also cleans up
sessionid.tdb and connections.tdb.

While there, this also cleans up overloading connections.tdb with all the
process entries just for messaging_send_all().

Volker

1 files changed, 66 insertions, 0 deletions

diff --git a/source3/include/serverid.h b/source3/include/serverid.h
new file mode 100644
index 00000000000..9ef778cdc0c
--- /dev/null
+++ b/source3/include/serverid.h
@@ -0,0 +1,66 @@
+/*
+   Unix SMB/CIFS implementation.
+   Implementation of a reliable server_exists()
+   Copyright (C) Volker Lendecke 2010
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef __SERVERID_H__
+#define __SERVERID_H__
+
+#include "includes.h"
+
+/*
+ * Register a server with its unique id
+ */
+bool serverid_register(const struct server_id *id, uint32_t msg_flags);
+
+/*
+ * Register ourselves with a unique id
+ */
+bool serverid_register_self(uint32_t msg_flags);
+
+/*
+ * De-register a server with its unique id
+ */
+bool serverid_deregister(const struct server_id *id);
+
+/*
+ * De-register ourself
+ */
+bool serverid_deregister_self(void);
+
+/*
+ * Check existence of a server id
+ */
+bool serverid_exists(const struct server_id *id);
+
+/*
+ * Walk the list of server_ids registered
+ */
+bool serverid_traverse(int (*fn)(struct db_record *rec,
+				 const struct server_id *id,
+				 uint32_t msg_flags,
+				 void *private_data),
+		       void *private_data);
+
+/*
+ * Walk the list of server_ids registered read-only
+ */
+bool serverid_traverse_read(int (*fn)(const struct server_id *id,
+				      uint32_t msg_flags,
+				      void *private_data),
+			    void *private_data);
+#endif