tools/oxenstored: Fix transaction handling in 32bit builds

In a 32bit build, the ocaml code 'proposed_id >= 0x7fffffff' compiles to:

  8055eac:       83 fb ff                cmp    $0xffffffff,%ebx
  8055eaf:       7d 0f                   jge    8055ec0 <...+0x20>

which in C is 'proposed_id >= INT_MIN', or in other words, tautologically
true.  As a result, 32bit builds of oxenstored always try to allocate the
transaction id 1, and fall into an infinite loop of trying the next id if
transaction 1 is already in use.

Restrict the range down to 1 billion, to sit in the positive half of a 31 bit
ocaml integer.  The compiled code is now:

  8055eac:       b9 ff ff ff 7f          mov    $0x7fffffff,%ecx
  8055eb1:       39 cb                   cmp    %ecx,%ebx
  8055eb3:       7d 0b                   jge    8055ec0 <...+0x20>

which (other than non-optimal code generation because of the unnecessary use
of %ecx), isn't unconditionally true.

In principle, the check could be changed to 'proposed_id == 0x7fffffff' which
would still allow for 2 billion transaction in 32bit builds.  However, in
64bit builds, this reintroduces a risk that if proposed_id is initially
greater than 0x7fffffff, it will not be clipped suitably into range.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: David Scott <dave@recoil.org>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>

tools/ocaml/xenstored/connection.ml

@@ -218,8 +218,16 @@ let fire_watch watch path =
 
 (* Search for a valid unused transaction id. *)
 let rec valid_transaction_id con proposed_id =
-	(* Clip proposed_id to the range [1, 0x7ffffffe] *)
-	let id = if proposed_id <= 0 || proposed_id >= 0x7fffffff then 1 else proposed_id in
+	(*
+	 * Clip proposed_id to the range [1, 0x3ffffffe]
+	 *
+	 * The chosen id must not trucate when written into the uint32_t tx_id
+	 * field, and needs to fit within the positive range of a 31 bit ocaml
+	 * integer to function when compiled as 32bit.
+	 *
+	 * Oxenstored therefore supports only 1 billion open transactions.
+	 *)
+	let id = if proposed_id <= 0 || proposed_id >= 0x3fffffff then 1 else proposed_id in
 
 	if Hashtbl.mem con.transactions id then (
 		(* Outstanding transaction with this id.  Try the next. *)