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dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:52772 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kX6tf-00085S-F7 for qemu-devel@archiver.kernel.org; Mon, 26 Oct 2020 14:07:07 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:59210) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kX6iS-0002jT-DE for qemu-devel@nongnu.org; Mon, 26 Oct 2020 13:55:32 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:51710) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kX6iO-0004yg-55 for qemu-devel@nongnu.org; Mon, 26 Oct 2020 13:55:31 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1603734927; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=1f0xSmS9ka5VP6euVxr+4iJi6AIc3Maa8rn7FL7Oq9U=; b=FNJnlpk3VctCnq4mTw0VBbRId+91+WFU81A1zrId1icmicoBvJ/a/ssaKnMsFIkNu7PCqh poluWiTcovgqx6/FtS0j0ZhTKt2ZAnyopQ308mm0oDyVAjxSh5SGbXkoa9W6oDviLPdarn xKaxVGdTjx1pDTU00xPGKYx+wUXoikk= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-215-YFbQYHJ5P1-cefGH1AaiDw-1; Mon, 26 Oct 2020 13:55:12 -0400 X-MC-Unique: YFbQYHJ5P1-cefGH1AaiDw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id EAFAA1016CEC for ; Mon, 26 Oct 2020 17:55:11 +0000 (UTC) Received: from work-vm (ovpn-112-208.ams2.redhat.com [10.36.112.208]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5AE045C1DC; Mon, 26 Oct 2020 17:55:00 +0000 (UTC) Date: Mon, 26 Oct 2020 17:54:57 +0000 From: "Dr. David Alan Gilbert" To: Max Reitz Subject: Re: [PATCH 0/8] virtiofsd: Announce submounts to the guest Message-ID: <20201026175457.GF152260@work-vm> References: <20200909184028.262297-1-mreitz@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200909184028.262297-1-mreitz@redhat.com> User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/26 02:39:09 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: virtio-fs@redhat.com, Miklos Szeredi , qemu-devel@nongnu.org, Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Max Reitz (mreitz@redhat.com) wrote: > RFC: https://www.redhat.com/archives/virtio-fs/2020-May/msg00024.html > > Branch: https://github.com/XanClic/qemu.git virtiofs-submounts-v2 > Branch: https://git.xanclic.moe/XanClic/qemu.git virtiofs-submounts-v2 Queued > > > (Note that there is an accompanying Linux (kernel) series > “fuse: Mirror virtio-fs submounts”.) > > > Hi, > > We want to (be able to) announce the host mount structure of the shared > directory to the guest so it can replicate that structure. This ensures > that whenever the combination of st_dev and st_ino is unique on the > host, it will be unique in the guest as well. > > This feature is optional and needs to be enabled explicitly, so that the > mount structure isn’t leaked to the guest if the user doesn’t want it to > be. > > The last patch in this series adds a test script. For it to pass, you > need to compile a kernel with the accompanying “fuse: Mirror virtio-fs > submounts” patch series, and provide it to the test (as described in the > test patch). > > > Known caveats: > - stat(2) doesn’t trigger auto-mounting. Therefore, issuing a stat() on > a sub-mountpoint before it’s been auto-mounted will show its parent’s > st_dev together with the st_ino it has in the sub-mounted filesystem. > > For example, imagine you want to share a whole filesystem with the > guest, which on the host first looks like this: > > root/ (st_dev=64, st_ino=128) > sub_fs/ (st_dev=64, st_ino=234) > > And then you mount another filesystem under sub_fs, so it looks like > this: > > root/ (st_dev=64, st_ino=128) > sub_fs/ (st_dev=96, st_ino=128) > ... > > As you can see, sub_fs becomes a mount point, so its st_dev and st_ino > change from what they were on root’s filesystem to what they are in > the sub-filesystem. In fact, root and sub_fs now have the same > st_ino, which is not unlikely given that both are root nodes in their > respective filesystems. > > Now, this filesystem is shared with the guest through virtiofsd. > There is no way for virtiofsd to uncover sub_fs’s original st_ino > value of 234, so it will always provide st_ino=128 to the guest. > However, virtiofsd does notice that sub_fs is a mount point and > announces this fact to the guest. > > We want this to result in something like the following tree in the > guest: > > root/ (st_dev=32, st_ino=128) > sub_fs/ (st_dev=33, st_ino=128) > ... > > That is, sub_fs should be a different filesystem that’s auto-mounted. > However, as stated above, stat(2) doesn’t trigger auto-mounting, so > before it happens, the following structure will be visible: > > root/ (st_dev=32, st_ino=128) > sub_fs/ (st_dev=32, st_ino=128) > > That is, sub_fs and root will have the same st_dev/st_ino combination. > > This can easily be seen by executing find(1) on root in the guest, > which will subsequently complain about an alleged filesystem loop. > > To properly fix this problem, we probably would have to be able to > uncover sub_fs’s original st_ino value (i.e. 234) and let the guest > use that until the auto-mount happens. However, there is no way to > get that value (from userspace at least). > > Note that NFS with crossmnt has the exact same issue. > > > - You can unmount auto-mounted submounts in the guest, but then you > still cannot unmount them on the host. The guest still holds a > reference to the submount’s root directory, because that’s just a > normal entry in its parent directory (on the submount’s parent > filesystem). > > This is kind of related to the issue noted above: When the submount is > unmounted, the guest shouldn’t have a reference to sub_fs as the > submount’s root directory (host’s st_dev=96, st_ino=128), but to it as > a normal entry in its parent filesystem (st_dev=64, st_ino=234). > > (When you have multiple nesting levels, you can unmount inner mounts > when the outer ones have been unmounted in the guest. For example, > say you have a structure A/B/C/D, where each is a mount point, then > unmounting D, C, and B in the guest will allow the host to unmount D > and C.) > > > Max Reitz (8): > linux/fuse.h: Pull in from Linux > virtiofsd: Announce FUSE_ATTR_FLAGS > virtiofsd: Add attr_flags to fuse_entry_param > virtiofsd: Add fuse_reply_attr_with_flags() > virtiofsd: Store every lo_inode's parent_dev > virtiofsd: Announce sub-mount points > tests/acceptance/boot_linux: Accept SSH pubkey > tests/acceptance: Add virtiofs_submounts.py > > include/standard-headers/linux/fuse.h | 11 +- > tools/virtiofsd/fuse_common.h | 8 + > tools/virtiofsd/fuse_lowlevel.h | 20 ++ > tools/virtiofsd/fuse_lowlevel.c | 34 ++- > tools/virtiofsd/helper.c | 1 + > tools/virtiofsd/passthrough_ll.c | 84 ++++- > tests/acceptance/boot_linux.py | 13 +- > tests/acceptance/virtiofs_submounts.py | 289 ++++++++++++++++++ > .../virtiofs_submounts.py.data/cleanup.sh | 46 +++ > .../guest-cleanup.sh | 30 ++ > .../virtiofs_submounts.py.data/guest.sh | 138 +++++++++ > .../virtiofs_submounts.py.data/host.sh | 127 ++++++++ > 12 files changed, 780 insertions(+), 21 deletions(-) > create mode 100644 tests/acceptance/virtiofs_submounts.py > create mode 100644 tests/acceptance/virtiofs_submounts.py.data/cleanup.sh > create mode 100644 tests/acceptance/virtiofs_submounts.py.data/guest-cleanup.sh > create mode 100644 tests/acceptance/virtiofs_submounts.py.data/guest.sh > create mode 100644 tests/acceptance/virtiofs_submounts.py.data/host.sh > > -- > 2.26.2 > > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK