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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 24FF2C43331 for ; Fri, 27 Mar 2020 17:53:28 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id E4AEA2071B for ; Fri, 27 Mar 2020 17:53:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="KSwektmy" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E4AEA2071B Authentication-Results: mail.kernel.org; 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]:45066 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jHtAd-0005KP-2k for qemu-devel@archiver.kernel.org; Fri, 27 Mar 2020 13:53:27 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:35218) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jHt5G-0003jI-PN for qemu-devel@nongnu.org; Fri, 27 Mar 2020 13:47:56 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jHt5F-0005Ow-0h for qemu-devel@nongnu.org; Fri, 27 Mar 2020 13:47:54 -0400 Received: from us-smtp-delivery-74.mimecast.com ([216.205.24.74]:56444) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jHt5E-0005ME-So for qemu-devel@nongnu.org; Fri, 27 Mar 2020 13:47:52 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1585331272; h=from:from:reply-to: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=jyga5obXmtc3I50IueSDa8L2rkgqVdsgrs+uCszs/QE=; b=KSwektmyDK6C+se39HAZLu1YQtQIMXk0koPAT+KVU/9J/SM6qeqwtM2FqjGD/MjKoe6Crp nI4dg/pTHlxf3CavLBsVAjgm+zJnrK4jntqGkNRtChLLV/w7xEd3TqJJpARME/yAtRttCc x4eZnnQ1iACb9i2fkwPg69m//6trtmI= 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-319-tFWlEfo0Ov2eE7vxybtTcQ-1; Fri, 27 Mar 2020 13:47:50 -0400 X-MC-Unique: tFWlEfo0Ov2eE7vxybtTcQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A0CD6149C0; Fri, 27 Mar 2020 17:47:49 +0000 (UTC) Received: from redhat.com (unknown [10.36.110.20]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5425D60BF3; Fri, 27 Mar 2020 17:47:45 +0000 (UTC) Date: Fri, 27 Mar 2020 17:47:42 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eric Blake Subject: Re: [PATCH 3/3] nbd: Use shutdown(SHUT_WR) after last item sent Message-ID: <20200327174742.GU1619@redhat.com> References: <20200327161936.2225989-1-eblake@redhat.com> <20200327161936.2225989-4-eblake@redhat.com> <20200327163548.GP1619@redhat.com> <4a56f56e-60b8-6b1f-f805-31a192eb6148@redhat.com> MIME-Version: 1.0 In-Reply-To: <4a56f56e-60b8-6b1f-f805-31a192eb6148@redhat.com> User-Agent: Mutt/1.13.3 (2020-01-12) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 216.205.24.74 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Kevin Wolf , qemu-devel@nongnu.org, qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Mar 27, 2020 at 12:42:21PM -0500, Eric Blake wrote: > On 3/27/20 11:35 AM, Daniel P. Berrang=C3=A9 wrote: > > On Fri, Mar 27, 2020 at 11:19:36AM -0500, Eric Blake wrote: > > > Although the remote end should always be tolerant of a socket being > > > arbitrarily closed, there are situations where it is a lot easier if > > > the remote end can be guaranteed to read EOF even before the socket > > > has closed. In particular, when using gnutls, if we fail to inform > > > the remote end about an impending teardown, the remote end cannot > > > distinguish between our closing the socket as intended vs. a maliciou= s > > > intermediary interrupting things, and may result in spurious error > > > messages. > >=20 > > Does this actually matter in the NBD case ? > >=20 > > It has an explicit NBD command for requesting shutdown, and once > > that's processed, it is fine to just close the socket abruptly - I > > don't see a benefit to a TLS shutdown sequence on top. >=20 > You're right that the NBD protocol has ways for the client to advertise i= t > will be shutting down, AND documents that the server must be robust to > clients that just abruptly disconnect after that point. But we don't hav= e > control over all such servers, and there may very well be a server that l= ogs > an error on abrupt closure, where it would be silent if we did a proper > gnutls_bye. Which is more important: maximum speed in disconnecting afte= r > we expressed intent, or maximum attempt at catering to all sorts of remot= e > implementations that might not be as tolerant as qemu is of an abrupt > termination? It is the cost / benefit tradeoff here that matters. Correctly using gnutls_bye(), in contexts which aren't expected to block is non-trivial bringing notable extra code complexity. It isn't an obvious win to me for something that just changes an error message for a scenario that can already be cleanly handled at the application protocol level. >=20 > > AFAIK, the TLS level clean shutdown is only required if the > > application protocol does not have any way to determine an > > unexpected shutdown itself. >=20 > 'man gnutls_bye' states: >=20 > Note that not all implementations will properly terminate a TLS > connec=E2=80=90 > tion. Some of them, usually for performance reasons, will > terminate > only the underlying transport layer, and thus not > distinguishing > between a malicious party prematurely terminating the connectio= n > and > normal termination. >=20 > You're right that because the protocol has an explicit message, we can > reliably distinguish any early termination prior to > NBD_OPT_ABORT/NBD_CMD_DISC as being malicious, so the only case where it > matters is if we have a premature termination after we asked for clean > shutdown, at which point a malicious termination didn't lose any data. So= on > that front, I guess you are right that not using gnutls_bye isn't going t= o > have much impact. >=20 > >=20 > > This is relevant for HTTP where the connection data stream may not > > have a well defined end condition. > >=20 > > In the NBD case though, we have an explicit NBD_CMD_DISC to trigger > > the disconnect. After processing that message, an EOF is acceptable > > regardless of whether , > > before processing that message, any EOF is a unexpected. > >=20 > > > Or, we can end up with a deadlock where both ends are stuc= k > > > on a read() from the other end but neither gets an EOF. > >=20 > > If the socket has been closed abruptly why would it get stuck in > > read() - it should see EOF surely ? >=20 > That's what I'm trying to figure out: the nbdkit testsuite definitely hun= g > even though 'qemu-nbd --list' exited, but I haven't yet figured out wheth= er > the bug lies in nbdkit proper or in libnbd, nor whether a cleaner tls > shutdown would have prevented the hang in a more reliable manner. > https://www.redhat.com/archives/libguestfs/2020-March/msg00191.html Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberrange= :| |: https://libvirt.org -o- https://fstop138.berrange.com= :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange= :|