[Xen-changelog] [xen master] tools/oxenstored: Avoid allocating invalid transaction ids

[patchbot@xxxxxxx]

commit 0897514b4b376a167f968f79c6ea0dee1061458e
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Oct 26 10:34:21 2016 +0100
Commit:     Wei Liu <wei.liu2@xxxxxxxxxx>
CommitDate: Wed Oct 26 12:37:45 2016 +0100

    tools/oxenstored: Avoid allocating invalid transaction ids
    
    The transaction id of 0 is reserved, meaning "not in a transaction".  It is 
up
    to the xenstored server to allocate transaction ids.  While oxenstored 
starts
    its ids at 1, but insufficient care is taken with truncation cases.
    
    A 32bit oxenstored has an int with 31 bits of width, meaning that the
    transaction id will wrap around to 0 after 2 billion transactions.
    
    A 64bit oxenstored has an int with 63 bits of width, meaning that once 4
    billion transactions are used, the allocated id will be truncated when 
written
    into the uin32_t field in the ring.  This causes the client to reply with 
the
    truncated id, breaking any further attempt to use any transactions.
    
    Limit all transaction ids to the range between 1 and 0x7ffffffe.  This is 
the
    best which can be done without making oxenstored depend on Stdint or 
Cstruct,
    yet still work for 32bit builds.
    
    Also check that the proposed new transaction id isn't currently in use.  For
    the first 2 billion transactions there is no chance of a collision, and 
after
    that, the chance is at most 20 (the default open transaction quota) in 2
    billion.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: David Scott <dave@xxxxxxxxxx>
    Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
---
 tools/ocaml/xenstored/connection.ml | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/tools/ocaml/xenstored/connection.ml 
b/tools/ocaml/xenstored/connection.ml
index b18336f..0b47009 100644
--- a/tools/ocaml/xenstored/connection.ml
+++ b/tools/ocaml/xenstored/connection.ml
@@ -216,14 +216,23 @@ let fire_watch watch path =
        let data = Utils.join_by_null [ new_path; watch.token; "" ] in
        send_reply watch.con Transaction.none 0 Xenbus.Xb.Op.Watchevent data
 
-let find_next_tid con =
-       let ret = con.next_tid in con.next_tid <- con.next_tid + 1; ret
+(* 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
+
+       if Hashtbl.mem con.transactions id then (
+               (* Outstanding transaction with this id.  Try the next. *)
+               valid_transaction_id con (id + 1)
+       ) else
+               id
 
 let start_transaction con store =
        if !Define.maxtransaction > 0 && not (is_dom0 con)
        && Hashtbl.length con.transactions > !Define.maxtransaction then
                raise Quota.Transaction_opened;
-       let id = find_next_tid con in
+       let id = valid_transaction_id con con.next_tid in
+       con.next_tid <- id + 1;
        let ntrans = Transaction.make id store in
        Hashtbl.add con.transactions id ntrans;
        Logging.start_transaction ~tid:id ~con:(get_domstr con);
--
generated by git-patchbot for /home/xen/git/xen.git#master

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