Re: [Xen-devel] [PATCH] oxenstored: fix short-write issue

[Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>]

David Scott, le Wed 28 Oct 2015 13:34:10 +0000, a Ãcrit :
> However I believe the âsuspiciousâ OCaml patch fixes the specific issue (â or 
> have I missed something?).

It does.

> Does anyone have time to prototype what a C-level fix would look like?

Here is a try, untested though.

Signed-off-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>

diff --git a/tools/ocaml/libs/xb/xs_ring_stubs.c 
b/tools/ocaml/libs/xb/xs_ring_stubs.c
index fd561a2..5c771d6 100644
--- a/tools/ocaml/libs/xb/xs_ring_stubs.c
+++ b/tools/ocaml/libs/xb/xs_ring_stubs.c
@@ -99,12 +99,12 @@ CAMLprim value ml_interface_write(value ml_interface,
        int result;

        struct xenstore_domain_interface *intf = interface->addr;
-       XENSTORE_RING_IDX cons, prod;
+       XENSTORE_RING_IDX cons, old_prod, prod;
        int can_write;
        uint32_t connection;

        cons = *(volatile uint32_t*)&intf->rsp_cons;
-       prod = *(volatile uint32_t*)&intf->rsp_prod;
+       old_prod = prod = *(volatile uint32_t*)&intf->rsp_prod;
        connection = *(volatile uint32_t*)&intf->connection;

        if (connection != XENSTORE_CONNECTED)
@@ -116,15 +116,30 @@ CAMLprim value ml_interface_write(value ml_interface,
                goto exit;
        }
        if (MASK_XENSTORE_IDX(prod) >= MASK_XENSTORE_IDX(cons))
+       {
                can_write = XENSTORE_RING_SIZE - MASK_XENSTORE_IDX(prod);
-       else
+               if (can_write < len)
+               {
+                       /* Not enough room at end of ring.  Copy what we can 
first.  */
+                       memcpy(intf->rsp + MASK_XENSTORE_IDX(prod), buffer, 
can_write);
+                       prod += can_write;
+                       len -= can_write;
+                       buffer += can_write;
+
+                       /* We are now back at beginning of ring.  */
+                       can_write = MASK_XENSTORE_IDX(cons) - 
MASK_XENSTORE_IDX(prod);
+               }
+       }
+       else
                can_write = MASK_XENSTORE_IDX(cons) - MASK_XENSTORE_IDX(prod);
        if (can_write < len)
+               /* Real short write: not enough room.  */
                len = can_write;
        memcpy(intf->rsp + MASK_XENSTORE_IDX(prod), buffer, len);
+       prod += len;
        xen_mb();
-       intf->rsp_prod += len;
-       result = len;
+       intf->rsp_prod = prod;
+       result = prod - old_prod;
 exit:
        ml_result = Val_int(result);
        CAMLreturn(ml_result);

Attachment: patch
Description: Text document

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