[PATCH 4.19 53/57] xen-netback: Check for hotplug-status existence before watching

From: Greg Kroah-Hartman
Date: Mon Apr 26 2021 - 03:46:05 EST

From: Michael Brown <mbrown@xxxxxxxxxxxxxxxx>

[ Upstream commit 2afeec08ab5c86ae21952151f726bfe184f6b23d ]

The logic in connect() is currently written with the assumption that
xenbus_watch_pathfmt() will return an error for a node that does not
exist. This assumption is incorrect: xenstore does allow a watch to
be registered for a nonexistent node (and will send notifications
should the node be subsequently created).

As of commit 1f2565780 ("xen-netback: remove 'hotplug-status' once it
has served its purpose"), this leads to a failure when a domU
transitions into XenbusStateConnected more than once. On the first
domU transition into Connected state, the "hotplug-status" node will
be deleted by the hotplug_status_changed() callback in dom0. On the
second or subsequent domU transition into Connected state, the
hotplug_status_changed() callback will therefore never be invoked, and
so the backend will remain stuck in InitWait.

This failure prevents scenarios such as reloading the xen-netfront
module within a domU, or booting a domU via iPXE. There is
unfortunately no way for the domU to work around this dom0 bug.

Fix by explicitly checking for existence of the "hotplug-status" node,
thereby creating the behaviour that was previously assumed to exist.

Signed-off-by: Michael Brown <mbrown@xxxxxxxxxxxxxxxx>
Reviewed-by: Paul Durrant <paul@xxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
drivers/net/xen-netback/xenbus.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
index 107bbd4ae825..78c56149559c 100644
--- a/drivers/net/xen-netback/xenbus.c
+++ b/drivers/net/xen-netback/xenbus.c
@@ -1043,11 +1043,15 @@ static void connect(struct backend_info *be)

- err = xenbus_watch_pathfmt(dev, &be->hotplug_status_watch, NULL,
- hotplug_status_changed,
- "%s/%s", dev->nodename, "hotplug-status");
- if (!err)
+ if (xenbus_exists(XBT_NIL, dev->nodename, "hotplug-status")) {
+ err = xenbus_watch_pathfmt(dev, &be->hotplug_status_watch,
+ NULL, hotplug_status_changed,
+ "%s/%s", dev->nodename,
+ "hotplug-status");
+ if (err)
+ goto err;
be->have_hotplug_status_watch = 1;
+ }