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    Subject[PATCH 4.18 140/168] xen/manage: dont complain about an empty value in control/sysrq node
    Date
    4.18-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Vitaly Kuznetsov <vkuznets@redhat.com>

    [ 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>
    ---
    drivers/xen/manage.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    --- a/drivers/xen/manage.c
    +++ b/drivers/xen/manage.c
    @@ -280,9 +280,11 @@ static void sysrq_handler(struct xenbus_
    /*
    * 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);

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