Re: [PATCH] xen-netback: Check for hotplug-status existence before watching

[Paul Durrant <xadimgnik@gmail.com>]

On 13/04/2021 16:25, Michael Brown wrote:
> 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@fensystems.co.uk>

Reviewed-by: Paul Durrant <paul@xen.org>

> ---
>   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 a5439c130130..d24b7a7993aa 100644
> --- a/drivers/net/xen-netback/xenbus.c
> +++ b/drivers/net/xen-netback/xenbus.c
> @@ -824,11 +824,15 @@ static void connect(struct backend_info *be)
>   	xenvif_carrier_on(be->vif);
>   
>   	unregister_hotplug_status_watch(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;
> +	}
>   
>   	netif_tx_wake_all_queues(be->vif->dev);
>   
>