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 5C97CC433B4 for ; Sun, 4 Apr 2021 15:56:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1A2C06137B for ; Sun, 4 Apr 2021 15:56:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231152AbhDDP4N (ORCPT ); Sun, 4 Apr 2021 11:56:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60150 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230495AbhDDP4L (ORCPT ); Sun, 4 Apr 2021 11:56:11 -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 61B70C061756; Sun, 4 Apr 2021 08:56:06 -0700 (PDT) Received: from viro by zeniv-ca.linux.org.uk with local (Exim 4.94 #2 (Red Hat Linux)) id 1lT56Y-002UUF-A9; Sun, 04 Apr 2021 15:56:02 +0000 Date: Sun, 4 Apr 2021 15:56:02 +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> <20210404113445.xo6ntgfpxigcb3x6@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210404113445.xo6ntgfpxigcb3x6@wittgenstein> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Sun, Apr 04, 2021 at 01:34:45PM +0200, Christian Brauner wrote: > Sorry for not replying to your earlier mail but I've been debugging this > too. My current theory is that it's related to LOOKUP_ROOT_GRABBED when > LOOKUP_CACHED is specified _possibly_ with an interaction how > create_io_thread() is created with CLONE_FS. The reproducer requires you > either have called pivot_root() or chroot() in order for the failure to > happen. So I think the fact that we skip legitimize_root() when > LOOKUP_CACHED is set might figure into this. I can keep digging. > > Funny enough I already placed a printk statement into the place you > wanted one too so I just amended mine. Here's what you get: > > If pivot pivot_root() is used before the chroot() you get: > > [ 637.464555] AAAA: count(-1) | mnt_mntpoint(/) | mnt->mnt.mnt_root(/) | id(579) | dev(tmpfs) > > if you only call chroot, i.e. make the pivot_root() branch a simple > if (true) you get: > > [ 955.206117] AAAA: count(-2) | mnt_mntpoint(/) | mnt->mnt.mnt_root(/) | id(580) | dev(tmpfs) Very interesting. What happens if you call loop() twice? And now I wonder whether it's root or cwd, actually... Hmm... How about this: fd = open("/proc/self/mountinfo", 0); mkdir("./newroot/foo", 0777); mount("./newroot/foo", "./newroot/foo", 0, MS_BIND, NULL); chroot("./newroot"); chdir("/foo"); while (1) { static char buf[4096]; int n = read(fd, buf, 4096); if (n <= 0) break; write(1, buf, n); } close(fd); drop_caps(); loop(); as the end of namespace_sandbox_proc(), instead of chroot("./newroot"); chdir("/"); drop_caps(); loop(); sequence we have there? > The cat /proc/self/mountinfo is for the id(579) below: ... and it misses the damn thing, since we call it before the mount in question had been created ;-/ So we'd probably be better off not trying to be clever and just doing that as explicit (and completely untested) read-write loop above.