From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from zeniv.linux.org.uk ([195.92.253.2]:48204 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726748AbeLADSu (ORCPT ); Fri, 30 Nov 2018 22:18:50 -0500 Date: Fri, 30 Nov 2018 16:08:52 +0000 From: Al Viro To: "Eric W. Biederman" Cc: "gregkh@linuxfoundation.org" , Jan Glauber , Will Deacon , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "jslaby@suse.com" Subject: Re: dcache_readdir NULL inode oops Message-ID: <20181130160852.GN2217@ZenIV.linux.org.uk> References: <20181109155856.GC2091@brain-police> <20181110111656.GA16667@hc> <20181120182854.GC28838@arm.com> <20181120190317.GA29161@arm.com> <20181121131900.GA18931@hc> <20181123180525.GA21017@arm.com> <20181128200806.GC32668@arm.com> <20181129184950.GA7290@hc> <20181130104154.GA11991@kroah.com> <875zwe389q.fsf@xmission.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <875zwe389q.fsf@xmission.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Fri, Nov 30, 2018 at 09:16:49AM -0600, Eric W. Biederman wrote: > >> > + inode_lock(parent->d_inode); > >> > dentry->d_fsdata = NULL; > >> > drop_nlink(dentry->d_inode); > >> > d_delete(dentry); > >> > + inode_unlock(parent->d_inode); > >> > + > >> > dput(dentry); /* d_alloc_name() in devpts_pty_new() */ > >> > } > > > > This feels right but getting some feedback from others would be good. > > This is going to be special at least because we are not coming through > the normal unlink path and we are manipulating the dcache. > > This looks plausible. If this is whats going on then we have had this > bug for a very long time. I will see if I can make some time. > > It looks like in the general case everything is serialized by the > devpts_mutex. I wonder if just changing the order of operations > here would be enough. > > AKA: drop_nlink d_delete then dentry->d_fsdata. Ugh d_fsdata is not > implicated so that won't help here. It certainly won't. The thing is, this if (!dir_emit(ctx, next->d_name.name, next->d_name.len, d_inode(next)->i_ino, dt_type(d_inode(next)))) in dcache_readdir() obviously can block, so all we can hold over it is blocking locks. Which we do - specifically, ->i_rwsem on our directory. It's actually worse than missing inode_lock() - consider the effects of mount --bind /mnt/foo /dev/pts/42. What happens when that thing goes away? Right, a lost mount... I'll resurrect the "kernel-internal rm -rf done right" series and post it; devpts is not the only place suffering such problem (binfmt_misc, etc.)