Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964901AbcJTOYm (ORCPT ); Thu, 20 Oct 2016 10:24:42 -0400 Received: from bender.open-source.co.uk ([83.170.98.46]:49708 "EHLO mail.open-source.co.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964843AbcJTOYj (ORCPT ); Thu, 20 Oct 2016 10:24:39 -0400 Date: Thu, 20 Oct 2016 15:24:37 +0100 From: Darren Austin To: linux-kernel@vger.kernel.org Subject: BUG: User triggerable kernel panic in 4.8 (possibly 4.9) Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2534 Lines: 50 [ Please CC me with any replies as I will be leaving the list shortly ] Hi, I'm not sure if this is the best place to report an issue i've discovered with the kernel and the 'fsc' mount option - please let me know if there is some other mailing list or person I should be notifying about this. The bug appears (at least for me) when using an NFS server, and a client which mounts an export from that server with the 'fsc' option (whether or not the fscache daemon is running or not). It also seems easiest to trigger using the 'nano' editor, but other commands will trigger it randomly. I've tested this bug on the Ubuntu 1610 kernel (4.8.0) and with the 4.8.2 kernel from http://kernel.ubuntu.com/~kernel-ppa/mainline/. The latter purports to be a kernel built from the unmodified kernel source, and simply packaged in a .deb, so this isn't a Ubuntu kernel problem. Unless the problem has been corrected in the 4.9 series, I suspect the bug may also persist there - i've not had the opportunity to check 4.9.x kernels as yet. I can repeatidly reproduce this bug on my system, so it's definitely not a one off - it causes a kernel panic and complete lock-up every time. The NFS share I tested with is exported with options: rw,async,insecure,insecure_locks,no_root_squash,anongid=99,anonuid=99,no_subtree_check (but the export options don't seem to matter to trigger the bug) and mounted on the client with options: vers=4,hard,intr,acl,rw,fsc via the Linux automounter (but the issue persists on mounts from fstab or when mounted manually). To reproduce the bug is quite simple... 1) Set up the server export and client mount as detailed above. 2) From the console (or in a terminal; but I only tested this once) in the directory where you've mounted the NFS share, run: nano testfile.txt and add write some text to the file. 2) Save the file and exit (Ctl+X is how I did it). 3) When back at the prompt, immediately hit the up arrow on the keyboard (to load the last typed command into the buffer) and hit enter. 4) Watch as the pretty text of the panic scrolls by :) With the help of people on the nano-dev mailing list, I figured out that it's the 'fsc' option which causes the panic - repeated tests without that option active do not trigger it. However, this is /not/ a nano specific bug - it can be triggered by any command used on the mount. And besides, nano shouldn't be able to take down the system :) If any further information is required, please don't hesitate to reply. Darren.