xenbus: track caller request id

Web:        https://git.kernel.org/torvalds/c/29fee6eed2811ff1089b30fc579a2d19d78016ab
Commit:     29fee6eed2811ff1089b30fc579a2d19d78016ab
Parent:     1e3510b2b053b8253a99511efb668fcc7ae8fcd7
Refname:    refs/heads/master
Author:     Joao Martins <[email protected]>
AuthorDate: Fri Feb 2 17:42:33 2018 +0000
Committer:  Juergen Gross <[email protected]>
CommitDate: Sat Feb 17 09:40:33 2018 +0100

    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: <[email protected]> # 4.11
    Fixes: fd8aa9095a ("xen: optimize xenbus driver for multiple concurrent xenstore accesses")
    Reported-by: Bhavesh Davda <[email protected]>
    Signed-off-by: Joao Martins <[email protected]>
    Reviewed-by: Juergen Gross <[email protected]>
    Signed-off-by: Juergen Gross <[email protected]>
---
 drivers/xen/xenbus/xenbus.h       | 1 +
 drivers/xen/xenbus/xenbus_comms.c | 1 +
 drivers/xen/xenbus/xenbus_xs.c    | 3 +++
 3 files changed, 5 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus.h b/drivers/xen/xenbus/xenbus.h
index 149c5e7efc89..092981171df1 100644
--- 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 5b081a01779d..d239fc3c5e3d 100644
--- 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 3e59590c7254..3f3b29398ab8 100644
--- 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;
--
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