From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B9483C433ED for ; Tue, 6 Apr 2021 01:38:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8221C613AD for ; Tue, 6 Apr 2021 01:38:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232429AbhDFBiv (ORCPT ); Mon, 5 Apr 2021 21:38:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44658 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232367AbhDFBiv (ORCPT ); Mon, 5 Apr 2021 21:38:51 -0400 Received: from zeniv-ca.linux.org.uk (zeniv-ca.linux.org.uk [IPv6:2607:5300:60:148a::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9ABACC06174A; Mon, 5 Apr 2021 18:38:44 -0700 (PDT) Received: from viro by zeniv-ca.linux.org.uk with local (Exim 4.94 #2 (Red Hat Linux)) id 1lTafv-002se4-CA; Tue, 06 Apr 2021 01:38:39 +0000 Date: Tue, 6 Apr 2021 01:38:39 +0000 From: Al Viro To: Christian Brauner Cc: Jens Axboe , syzbot , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, io-uring@vger.kernel.org Subject: Re: [syzbot] WARNING in mntput_no_expire (2) Message-ID: References: <20210404170513.mfl5liccdaxjnpls@wittgenstein> <20210405114437.hjcojekyp5zt6huu@wittgenstein> <20210405170801.zrdhnon6g4ggb6c7@wittgenstein> <20210405200737.qurhkqitoxweousx@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210405200737.qurhkqitoxweousx@wittgenstein> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Mon, Apr 05, 2021 at 10:07:37PM +0200, Christian Brauner wrote: > > diff --git a/fs/namei.c b/fs/namei.c > > index 216f16e74351..82344f1139ff 100644 > > --- a/fs/namei.c > > +++ b/fs/namei.c > > @@ -2289,6 +2289,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags) > > int error; > > const char *s = nd->name->name; > > > > + nd->path.mnt = NULL; > > + nd->path.dentry = NULL; > > + > > /* LOOKUP_CACHED requires RCU, ask caller to retry */ > > if ((flags & (LOOKUP_RCU | LOOKUP_CACHED)) == LOOKUP_CACHED) > > return ERR_PTR(-EAGAIN); > > @@ -2322,8 +2325,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags) > > } > > > > nd->root.mnt = NULL; > > - nd->path.mnt = NULL; > > - nd->path.dentry = NULL; > > > > /* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */ > > if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) { > > Bingo. That fixes it. *grumble* OK, I suppose it'll do for backports, but longer term... I don't like how convoluted the rules for nameidata fields' validity are. In particular, for nd->path I would rather have it * cleared in set_nameidata() * cleared when it become invalid. That would be * places that drop rcu_read_lock() without having legitimized the sucker (already done, except for terminate_walk()) * terminate_walk() in non-RCU case after path_put(&nd->path) OTOH... wait a sec - the whole thing is this cycle regression, so... Could you verify that the variant below fixes that crap? Make sure nd->path.mnt and nd->path.dentry are always valid pointers Initialize them in set_nameidata() and make sure that terminate_walk() clears them once the pointers become potentially invalid (i.e. we leave RCU mode or drop them in non-RCU one). Currently we have "path_init() always initializes them and nobody accesses them outside of path_init()/terminate_walk() segments", which is asking for trouble. With that change we would have nd->path.{mnt,dentry} 1) always valid - NULL or pointing to currently allocated objects. 2) non-NULL while we are successfully walking 3) NULL when we are not walking at all 4) contributing to refcounts whenever non-NULL outside of RCU mode. Hopefully-fixes: 6c6ec2b0a3e0 ("fs: add support for LOOKUP_CACHED") Signed-off-by: Al Viro --- diff --git a/fs/namei.c b/fs/namei.c index 216f16e74351..fc8760d4314e 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -579,6 +579,8 @@ static void set_nameidata(struct nameidata *p, int dfd, struct filename *name) p->stack = p->internal; p->dfd = dfd; p->name = name; + p->path.mnt = NULL; + p->path.dentry = NULL; p->total_link_count = old ? old->total_link_count : 0; p->saved = old; current->nameidata = p; @@ -652,6 +654,8 @@ static void terminate_walk(struct nameidata *nd) rcu_read_unlock(); } nd->depth = 0; + nd->path.mnt = NULL; + nd->path.dentry = NULL; } /* path_put is needed afterwards regardless of success or failure */ @@ -2322,8 +2326,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags) } nd->root.mnt = NULL; - nd->path.mnt = NULL; - nd->path.dentry = NULL; /* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */ if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) {