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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT 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 CDE59C4360F for ; Thu, 4 Apr 2019 19:05:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9AE792082E for ; Thu, 4 Apr 2019 19:05:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729908AbfDDTFJ (ORCPT ); Thu, 4 Apr 2019 15:05:09 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:52520 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729818AbfDDTFJ (ORCPT ); Thu, 4 Apr 2019 15:05:09 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92 #3 (Red Hat Linux)) id 1hC7ff-0004e3-3u; Thu, 04 Apr 2019 19:05:07 +0000 Date: Thu, 4 Apr 2019 20:05:07 +0100 From: Al Viro To: Amir Goldstein Cc: Miklos Szeredi , Dmitry Vyukov , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com Subject: Re: [PATCH v2] acct: fix possible deadlock in acct_pin_kill Message-ID: <20190404190506.GE2217@ZenIV.linux.org.uk> References: <20190404105255.12189-1-amir73il@gmail.com> <20190404184448.GC2217@ZenIV.linux.org.uk> <20190404184944.GD2217@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190404184944.GD2217@ZenIV.linux.org.uk> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Apr 04, 2019 at 07:49:44PM +0100, Al Viro wrote: > On Thu, Apr 04, 2019 at 07:44:48PM +0100, Al Viro wrote: > > > Huh? sb_writers is taken when we *open* the new file. Then we replace > > its ->path.mnt with a clone and transfer the write count from the original > > to new one. And close the old file while we are at it. > > > > >From sb_writers POV mainline has > > sb_start_write(new_sb) // in open > > sb_start_write(new_sb) // mnt_want_write() on clone > > last write to old_sb, then sb_end_write(old_sb) // acct_pin_kill() > > sb_end_write(new_sb) // mnt_drop_write(mnt) > > and you flip the order of the last two lines. > > > > Could you explain how exactly does your patch help whatever > > problem overlayfs has? > > Ow... Sorry, -ENOCOFFEE... > > Let me wake up properly and look at that thing again. OK, so... My first reaction had been complete BS. However, the same goes for your analysis - it's not an ordering problem at all. What happens is that we are replacing file->path.mnt with a clone and we want the write count contribution (file is opened for write) to be transferred. That's it. We do *NOT* want any kind of freeze protection for the duration of switchover. IOW, the solution is to switch to __mnt_{want,drop}_write() for that switchover; we don't want to mess with freeze protection at all. Signed-off-by: Al Viro --- diff --git a/fs/internal.h b/fs/internal.h index 6a8b71643af4..2e7362837a6e 100644 --- a/fs/internal.h +++ b/fs/internal.h @@ -89,9 +89,7 @@ extern int sb_prepare_remount_readonly(struct super_block *); extern void __init mnt_init(void); -extern int __mnt_want_write(struct vfsmount *); extern int __mnt_want_write_file(struct file *); -extern void __mnt_drop_write(struct vfsmount *); extern void __mnt_drop_write_file(struct file *); /* diff --git a/include/linux/mount.h b/include/linux/mount.h index 9197ddbf35fb..bf8cc4108b8f 100644 --- a/include/linux/mount.h +++ b/include/linux/mount.h @@ -87,6 +87,8 @@ extern bool mnt_may_suid(struct vfsmount *mnt); struct path; extern struct vfsmount *clone_private_mount(const struct path *path); +extern int __mnt_want_write(struct vfsmount *); +extern void __mnt_drop_write(struct vfsmount *); struct file_system_type; extern struct vfsmount *fc_mount(struct fs_context *fc); diff --git a/kernel/acct.c b/kernel/acct.c index addf7732fb56..81f9831a7859 100644 --- a/kernel/acct.c +++ b/kernel/acct.c @@ -227,7 +227,7 @@ static int acct_on(struct filename *pathname) filp_close(file, NULL); return PTR_ERR(internal); } - err = mnt_want_write(internal); + err = __mnt_want_write(internal); if (err) { mntput(internal); kfree(acct); @@ -252,7 +252,7 @@ static int acct_on(struct filename *pathname) old = xchg(&ns->bacct, &acct->pin); mutex_unlock(&acct->lock); pin_kill(old); - mnt_drop_write(mnt); + __mnt_drop_write(mnt); mntput(mnt); return 0; }