From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:52084 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726521AbeK3Vus (ORCPT ); Fri, 30 Nov 2018 16:50:48 -0500 Date: Fri, 30 Nov 2018 11:41:54 +0100 From: "gregkh@linuxfoundation.org" To: Jan Glauber , "Eric W. Biederman" Cc: Will Deacon , Alexander Viro , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "jslaby@suse.com" Subject: Re: dcache_readdir NULL inode oops Message-ID: <20181130104154.GA11991@kroah.com> References: <20181109143744.GA12128@hc> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181129184950.GA7290@hc> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Adding Eric as he touched this code last :) On Thu, Nov 29, 2018 at 07:25:48PM +0000, Jan Glauber wrote: > On Wed, Nov 28, 2018 at 08:08:06PM +0000, Will Deacon wrote: > > I spent some more time looking at this today... > > > > On Fri, Nov 23, 2018 at 06:05:25PM +0000, Will Deacon wrote: > > > Doing some more debugging, it looks like the usual failure case is where > > > one CPU clears the inode field in the dentry via: > > > > > > devpts_pty_kill() > > > -> d_delete() // dentry->d_lockref.count == 1 > > > -> dentry_unlink_inode() > > > > > > whilst another CPU gets a pointer to the dentry via: > > > > > > sys_getdents64() > > > -> iterate_dir() > > > -> dcache_readdir() > > > -> next_positive() > > > > > > and explodes on the subsequent inode dereference when trying to pass the > > > inode number to dir_emit(): > > > > > > if (!dir_emit(..., d_inode(next)->i_ino, ...)) > > > > > > Indeed, the hack below triggers a warning, indicating that the inode > > > is being cleared concurrently. > > > > > > I can't work out whether the getdents64() path should hold a refcount > > > to stop d_delete() in its tracks, or whether devpts_pty_kill() shouldn't > > > be calling d_delete() like this at all. > > > > So the issue is that opening /dev/pts/ptmx creates a new pty in /dev/pts, > > which disappears when you close /dev/pts/ptmx. Consequently, when we tear > > down the dentry for the magic new file, we have to take the i_node rwsem of > > the *parent* so that concurrent path walkers don't trip over it whilst its > > being freed. I wrote a simple concurrent program to getdents(/dev/pts/) in > > one thread, whilst another opens and closes /dev/pts/ptmx: it crashes the > > kernel in seconds. > > I also made a testcase and verified that your fix is fine. I also tried > replacing open-close on /dev/ptmx with mkdir-rmdir but that does not > trigger the error. > > > Patch below, but I'd still like somebody else to look at this, please. > > I wonder why no inode_lock on parent is needed for devpts_pty_new(), but > I'm obviously not a VFS expert... So your patch looks good to me and > clearly solves the issue. > > thanks, > Jan > > > Will > > > > --->8 > > > > diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c > > index c53814539070..50ddb95ff84c 100644 > > --- a/fs/devpts/inode.c > > +++ b/fs/devpts/inode.c > > @@ -619,11 +619,17 @@ void *devpts_get_priv(struct dentry *dentry) > > */ > > void devpts_pty_kill(struct dentry *dentry) > > { > > - WARN_ON_ONCE(dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC); > > + struct super_block *sb = dentry->d_sb; > > + struct dentry *parent = sb->s_root; > > > > + WARN_ON_ONCE(sb->s_magic != DEVPTS_SUPER_MAGIC); Side note, I wonder if this is even needed anymore... > > + > > + 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. thanks, greg k-h