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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 98104C433B4 for ; Thu, 1 Apr 2021 19:28:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 65F60610CF for ; Thu, 1 Apr 2021 19:28:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235192AbhDAT2P (ORCPT ); Thu, 1 Apr 2021 15:28:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52366 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235032AbhDAT2K (ORCPT ); Thu, 1 Apr 2021 15:28:10 -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 6BB39C05BD24; Thu, 1 Apr 2021 12:11:17 -0700 (PDT) Received: from viro by zeniv-ca.linux.org.uk with local (Exim 4.94 #2 (Red Hat Linux)) id 1lS2im-001iJP-Kk; Thu, 01 Apr 2021 19:11:12 +0000 Date: Thu, 1 Apr 2021 19:11:12 +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: <0000000000003a565e05bee596f2@google.com> <20210401154515.k24qdd2lzhtneu47@wittgenstein> <90e7e339-eaec-adb2-cfed-6dc058a117a3@kernel.dk> <20210401174613.vymhhrfsemypougv@wittgenstein> <20210401175919.jpiylhfrlb4xb67u@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210401175919.jpiylhfrlb4xb67u@wittgenstein> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Thu, Apr 01, 2021 at 07:59:19PM +0200, Christian Brauner wrote: > I _think_ I see what the issue is. It seems that an assumption made in > this commit might be wrong and we're missing a mnt_add_count() bump that > we would otherwise have gotten if we've moved the failure handling into > the unlazy helpers themselves. > > Al, does that sound plausible? mnt_add_count() on _what_? Failure in legitimize_links() ends up with nd->path.mnt zeroed, in both callers. So which vfsmount would be affected? Rules: in RCU mode: no mounts pinned out of RCU mode: nd->path.mnt and all nd->stack[i].link.mnt for i below nd->depth are either NULL or pinned Transition from RCU to non-RCU mode happens in try_to_unlazy() and try_to_unlazy_next(). References (if any) are dropped by eventual terminate_walk() (after that the contents of nameidata is junk). __legitimize_mnt() is the primitive for pinning. Return values: 0 -- successfully pinned (or given NULL as an argument) 1 -- failed, refcount not affected -1 -- failed, refcount bumped. It stays in RCU mode in all cases. One user is __legitimize_path(); it also stays in RCU mode. If it fails to legitimize path->mnt, it will zero it *IF* __legitimize_mnt() reports that refcount hadn't been taken. In all other cases, path->mnt is pinned. IOW, the caller is responsible for path_put() regardless of the outcome. Another user is legitimize_mnt(). _That_ will make sure that refcount is unaffected in case of failure (IOW, if __legitimize_mnt() reports failure with refcount bumped, we drop out of RCU mode, do mntput() and go back). On failure in legitimize_links() we either leave nd->depth equal to zero (in which case all nd->stack[...].link.mnt are to be ignored) or we set it one higher than the last attempted legitimize_path() in there. In the latter case, all entries in nd->stack below the value we put into nd->depth had legitimize_path() called (and thus have ->mnt either NULL or pinned) and everything starting from nd->depth is to be ignored. nd->path handling: 1) Callers of legitimize_links() are responsible for zeroing nd->path.mnt on legitimize_links() failure. Both do that, AFAICS. 2) in try_to_unlazy() we proceed to call legitimize_path() on nd->path. Once that call is done, we have nd->path.mnt pinned or NULL, so nothing further is needed with it. 3) in try_to_unlazy_next() we use legitimize_mnt() instead. Failure of that is handled by zeroing nd->path.mnt; success means that nd->path.mnt is pinned and should be left alone. We could use __legitimize_mnt() in try_to_unlazy_next() (basically, substitute the body of legitimize_mnt() there and massage it a bit), but that ends up being harder to follow: res = __legitimize_mnt(nd->path.mnt, nd->m_seq); if (unlikely(res)) { if (res < 0) // pinned, leave it there goto out1; else // not pinned, zero it goto out2; } instead of if (unlikely(!legitimize_mnt(nd->path.mnt, nd->m_seq))) goto out2; we have now.