xenbus: track caller request id

Commit fd8aa9095a95 ("xen: optimize xenbus driver for multiple concurrent
xenstore accesses") optimized xenbus concurrent accesses but in doing so
broke UABI of /dev/xen/xenbus. Through /dev/xen/xenbus applications are in
charge of xenbus message exchange with the correct header and body. Now,
after the mentioned commit the replies received by application will no
longer have the header req_id echoed back as it was on request (see
specification below for reference), because that particular field is being
overwritten by kernel.

struct xsd_sockmsg
{
  uint32_t type;  /* XS_??? */
  uint32_t req_id;/* Request identifier, echoed in daemon's response.  */
  uint32_t tx_id; /* Transaction id (0 if not related to a transaction). */
  uint32_t len;   /* Length of data following this. */

  /* Generally followed by nul-terminated string(s). */
};

Before there was only one request at a time so req_id could simply be
forwarded back and forth. To allow simultaneous requests we need a
different req_id for each message thus kernel keeps a monotonic increasing
counter for this field and is written on every request irrespective of
userspace value.

Forwarding again the req_id on userspace requests is not a solution because
we would open the possibility of userspace-generated req_id colliding with
kernel ones. So this patch instead takes another route which is to
artificially keep user req_id while keeping the xenbus logic as is. We do
that by saving the original req_id before xs_send(), use the private kernel
counter as req_id and then once reply comes and was validated, we restore
back the original req_id.

Cc: <stable@vger.kernel.org> # 4.11
Fixes: fd8aa9095a ("xen: optimize xenbus driver for multiple concurrent xenstore accesses")
Reported-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

diff --git a/drivers/xen/xenbus/xenbus.h b/drivers/xen/xenbus/xenbus.h
index 149c5e7efc89e75f0fd56a01b40f0f15b462af04..092981171df17ffe076dadd0582566902c03feda 100644 (file)
--- a/drivers/xen/xenbus/xenbus.h
+++ b/drivers/xen/xenbus/xenbus.h
@@ -76,6 +76,7 @@ struct xb_req_data {
        struct list_head list;
        wait_queue_head_t wq;
        struct xsd_sockmsg msg;
+       uint32_t caller_req_id;
        enum xsd_sockmsg_type type;
        char *body;
        const struct kvec *vec;

diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
index 5b081a01779de8001e48c1c0e2e0f9dea6697332..d239fc3c5e3ded66ad28194fcdc7d09a6df801da 100644 (file)
--- a/drivers/xen/xenbus/xenbus_comms.c
+++ b/drivers/xen/xenbus/xenbus_comms.c
@@ -309,6 +309,7 @@ static int process_msg(void)
                        goto out;
 
                if (req->state == xb_req_state_wait_reply) {
+                       req->msg.req_id = req->caller_req_id;
                        req->msg.type = state.msg.type;
                        req->msg.len = state.msg.len;
                        req->body = state.body;

diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index 3e59590c7254ddc8f1a08f4232262a74a29e3711..3f3b29398ab8e2b711ce724cf756ea8c241433cb 100644 (file)
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -227,6 +227,8 @@ static void xs_send(struct xb_req_data *req, struct xsd_sockmsg *msg)
        req->state = xb_req_state_queued;
        init_waitqueue_head(&req->wq);
 
+       /* Save the caller req_id and restore it later in the reply */
+       req->caller_req_id = req->msg.req_id;
        req->msg.req_id = xs_request_enter(req);
 
        mutex_lock(&xb_write_mutex);
@@ -310,6 +312,7 @@ static void *xs_talkv(struct xenbus_transaction t,
        req->num_vecs = num_vecs;
        req->cb = xs_wake_up;
 
+       msg.req_id = 0;
        msg.tx_id = t.id;
        msg.type = type;
        msg.len = 0;