oxenstored: keep track of each transaction's operations

A list of (request, response) pairs from the operations performed within the
transaction will be useful to support transaction replay.

Since this consumes memory, the number of requests per transaction must not be
left unbounded. Hence a new quota for this is introduced. This quota, configured
via the configuration key 'quota-maxrequests', limits the size of transactions
initiated by domUs.

After the maximum number of requests has been exhausted, any further requests
will result in EQUOTA errors. The client may then choose to end the transaction;
a successful commit will result in the retention of only the prior requests.

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Reviewed-by: Euan Harris <euan.harris@citrix.com>
Acked-by: David Scott <dave@recoil.org>

tools/ocaml/xenstored/define.ml

@@ -27,6 +27,7 @@ let default_config_dir = "/etc/xen"
 
 let maxwatch = ref (50)
 let maxtransaction = ref (20)
+let maxrequests = ref (-1)   (* maximum requests per transaction *)
 
 let domid_self = 0x7FF0
 

tools/ocaml/xenstored/oxenstored.conf

@@ -18,6 +18,7 @@ quota-maxentity = 1000
 quota-maxsize = 2048
 quota-maxwatch = 100
 quota-transaction = 10
+quota-maxrequests = 1024
 
 # Activate filed base backend
 persistent = false

tools/ocaml/xenstored/process.ml

@@ -155,7 +155,7 @@ let do_transaction_end con t domains cons data =
 	if not success then
 		raise Transaction_again;
 	if commit then
-		process_watch (List.rev (Transaction.get_ops t)) cons
+		process_watch (List.rev (Transaction.get_paths t)) cons
 
 let do_introduce con t domains cons data =
 	if not (Connection.is_dom0 con)
@@ -303,7 +303,7 @@ let reply_ack fct con t doms cons data =
 	fct con t doms cons data;
 	Packet.Ack (fun () ->
 		if Transaction.get_id t = Transaction.none then
-			process_watch (Transaction.get_ops t) cons
+			process_watch (Transaction.get_paths t) cons
 	)
 
 let reply_data fct con t doms cons data =
@@ -384,6 +384,15 @@ let process_packet ~store ~cons ~doms ~con ~req =
 			in
 		let response = input_handle_error ~cons ~doms ~fct ~con ~t ~req in
 
+		let response = try
+			if tid <> Transaction.none then
+				(* Remember the request and response for this operation in case we need to replay the transaction *)
+				Transaction.add_operation ~perm:(Connection.get_perm con) t req response;
+			response
+		with Quota.Limit_reached ->
+			Packet.Error "EQUOTA"
+		in
+
 		(* Put the response on the wire *)
 		send_response ty con t rid response
 	with exn ->

tools/ocaml/xenstored/transaction.ml

@@ -75,7 +75,8 @@ type t = {
 	ty: ty;
 	store: Store.t;
 	quota: Quota.t;
-	mutable ops: (Xenbus.Xb.Op.operation * Store.Path.t) list;
+	mutable paths: (Xenbus.Xb.Op.operation * Store.Path.t) list;
+	mutable operations: (Packet.request * Packet.response) list;
 	mutable read_lowpath: Store.Path.t option;
 	mutable write_lowpath: Store.Path.t option;
 }
@@ -86,16 +87,24 @@ let make id store =
 		ty = ty;
 		store = if id = none then store else Store.copy store;
 		quota = Quota.copy store.Store.quota;
-		ops = [];
+		paths = [];
+		operations = [];
 		read_lowpath = None;
 		write_lowpath = None;
 	}
 
 let get_id t = match t.ty with No -> none | Full (id, _, _) -> id
 let get_store t = t.store
-let get_ops t = t.ops
+let get_paths t = t.paths
 
-let add_wop t ty path = t.ops <- (ty, path) :: t.ops
+let add_wop t ty path = t.paths <- (ty, path) :: t.paths
+let add_operation ~perm t request response =
+	if !Define.maxrequests >= 0
+		&& not (Perms.Connection.is_dom0 perm)
+		&& List.length t.operations >= !Define.maxrequests
+		then raise Quota.Limit_reached;
+	t.operations <- (request, response) :: t.operations
+let get_operations t = List.rev t.operations
 let set_read_lowpath t path = t.read_lowpath <- get_lowest path t.read_lowpath
 let set_write_lowpath t path = t.write_lowpath <- get_lowest path t.write_lowpath
 
@@ -141,7 +150,7 @@ let getperms t perm path =
 	r
 
 let commit ~con t =
-	let has_write_ops = List.length t.ops > 0 in
+	let has_write_ops = List.length t.paths > 0 in
 	let has_coalesced = ref false in
 	let has_commited =
 	match t.ty with

tools/ocaml/xenstored/xenstored.ml

@@ -95,6 +95,7 @@ let parse_config filename =
 		("quota-transaction", Config.Set_int Define.maxtransaction);
 		("quota-maxentity", Config.Set_int Quota.maxent);
 		("quota-maxsize", Config.Set_int Quota.maxsize);
+		("quota-maxrequests", Config.Set_int Define.maxrequests);
 		("test-eagain", Config.Set_bool Transaction.test_eagain);
 		("persistent", Config.Set_bool Disk.enable);
 		("xenstored-log-file", Config.String Logging.set_xenstored_log_destination);