xen/manage: don't complain about an empty value in control/sysrq node

[ Upstream commit 87dffe86d406bee8782cac2db035acb9a28620a7 ]

When guest receives a sysrq request from the host it acknowledges it by
writing '\0' to control/sysrq xenstore node. This, however, make xenstore
watch fire again but xenbus_scanf() fails to parse empty value with "%c"
format string:

 sysrq: SysRq : Emergency Sync
 Emergency Sync complete
 xen:manage: Error -34 reading sysrq code in control/sysrq

Ignore -ERANGE the same way we already ignore -ENOENT, empty value in
control/sysrq is totally legal.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/xen/manage.c b/drivers/xen/manage.c
index 587d12829925b4279ea4a28f24ece5f340f44569..0444ebdda3f01bc3dbb5177582cfea315548e013 100644 (file)
--- a/drivers/xen/manage.c
+++ b/drivers/xen/manage.c
@@ -283,9 +283,11 @@ static void sysrq_handler(struct xenbus_watch *watch, const char *path,
                /*
                 * The Xenstore watch fires directly after registering it and
                 * after a suspend/resume cycle. So ENOENT is no error but
-                * might happen in those cases.
+                * might happen in those cases. ERANGE is observed when we get
+                * an empty value (''), this happens when we acknowledge the
+                * request by writing '\0' below.
                 */
-               if (err != -ENOENT)
+               if (err != -ENOENT && err != -ERANGE)
                        pr_err("Error %d reading sysrq code in control/sysrq\n",
                               err);
                xenbus_transaction_end(xbt, 1);