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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=ham 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 BB399C433E0 for ; Mon, 21 Dec 2020 12:12:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6E38D2253A for ; Mon, 21 Dec 2020 12:12:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726226AbgLUMMr (ORCPT ); Mon, 21 Dec 2020 07:12:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50136 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726027AbgLUMMq (ORCPT ); Mon, 21 Dec 2020 07:12:46 -0500 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [IPv6:2002:c35c:fd02::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 61CBCC0613D3; Mon, 21 Dec 2020 04:12:06 -0800 (PST) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1krK2W-002uPH-LQ; Mon, 21 Dec 2020 12:11:48 +0000 Date: Mon, 21 Dec 2020 12:11:48 +0000 From: Al Viro To: Joseph Qi Cc: Liangyan , Miklos Szeredi , linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] ovl: fix dentry leak in ovl_get_redirect Message-ID: <20201221121148.GP3579531@ZenIV.linux.org.uk> References: <20201220120927.115232-1-liangyan.peng@linux.alibaba.com> <20201221062653.GO3579531@ZenIV.linux.org.uk> <52a76e73-d46b-d0fd-a75a-76b4a86149b3@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <52a76e73-d46b-d0fd-a75a-76b4a86149b3@linux.alibaba.com> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-unionfs@vger.kernel.org On Mon, Dec 21, 2020 at 07:14:44PM +0800, Joseph Qi wrote: > Hi Viro, > > On 12/21/20 2:26 PM, Al Viro wrote: > > On Sun, Dec 20, 2020 at 08:09:27PM +0800, Liangyan wrote: > > > >> +++ b/fs/overlayfs/dir.c > >> @@ -973,6 +973,7 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect) > >> for (d = dget(dentry); !IS_ROOT(d);) { > >> const char *name; > >> int thislen; > >> + struct dentry *parent = NULL; > >> > >> spin_lock(&d->d_lock); > >> name = ovl_dentry_get_redirect(d); > >> @@ -992,7 +993,22 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect) > >> > >> buflen -= thislen; > >> memcpy(&buf[buflen], name, thislen); > >> - tmp = dget_dlock(d->d_parent); > >> + parent = d->d_parent; > >> + if (unlikely(!spin_trylock(&parent->d_lock))) { > >> + rcu_read_lock(); > >> + spin_unlock(&d->d_lock); > >> +again: > >> + parent = READ_ONCE(d->d_parent); > >> + spin_lock(&parent->d_lock); > >> + if (unlikely(parent != d->d_parent)) { > >> + spin_unlock(&parent->d_lock); > >> + goto again; > >> + } > >> + rcu_read_unlock(); > >> + spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED); > >> + } > >> + tmp = dget_dlock(parent); > >> + spin_unlock(&parent->d_lock); > >> spin_unlock(&d->d_lock); > > > > Yecchhhh.... What's wrong with just doing > > spin_unlock(&d->d_lock); > > parent = dget_parent(d); > > dput(d); > > d = parent; > > instead of that? > > > > Now race happens on non-RCU path in lookup_fast(), I'm afraid d_seq can > not close the race window. Explain, please. What exactly are you observing?