[Xen-devel] [PATCH] xen/manage: don't complain about an empty value in control/sysrq node

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
This is a follow-up to my Xen toolstack patch:
https://lists.xenproject.org/archives/html/xen-devel/2018-09/msg00266.html
without it we're seeing -EPERM on write and the issue I'm trying to address
here stays hidden.
---
 drivers/xen/manage.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/manage.c b/drivers/xen/manage.c
index c93d8ef8df34..5bb01a62f214 100644
--- a/drivers/xen/manage.c
+++ b/drivers/xen/manage.c
@@ -280,9 +280,11 @@ 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);
-- 
2.14.4


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