linux-kernel - [PATCH 4.9 49/59] xen/manage: dont complain about an empty value in control/sysrq node

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Message-Id: <20181008175552.205117911@linuxfoundation.org>
Date:   Mon,  8 Oct 2018 20:31:56 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wei Liu <wei.liu2@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: [PATCH 4.9 49/59] xen/manage: dont complain about an empty value in control/sysrq node

4.9-stable review patch.  If anyone has any objections, please let me know.

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

From: Vitaly Kuznetsov <vkuznets@...hat.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@...hat.com>
Reviewed-by: Wei Liu <wei.liu2@...rix.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/xen/manage.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/drivers/xen/manage.c
+++ b/drivers/xen/manage.c
@@ -282,9 +282,11 @@ 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);