From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:49838 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729250AbeK2HKh (ORCPT ); Thu, 29 Nov 2018 02:10:37 -0500 Date: Wed, 28 Nov 2018 20:08:06 +0000 From: Will Deacon To: Jan Glauber Cc: Alexander Viro , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , gregkh@linuxfoundation.org, jslaby@suse.com Subject: Re: dcache_readdir NULL inode oops Message-ID: <20181128200806.GC32668@arm.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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181123180525.GA21017@arm.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: 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. Patch below, but I'd still like somebody else to look at this, please. 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); + + 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() */ }