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 Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 51EBBCA6009 for ; Wed, 7 Oct 2026 08:39:34 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1xEN8f-0005OA-1f; Wed, 07 Oct 2026 04:36:37 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xEI57-0001IB-0S for qemu-devel@nongnu.org; Tue, 06 Oct 2026 23:12:49 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xEI4z-0001C6-BM for qemu-devel@nongnu.org; Tue, 06 Oct 2026 23:12:34 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1791342722; 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=UUpuq8kGZVhpW7rdFIIKKgacW+psg4/ICzoFgyy6TBA=; b=gHNOSTd89O3TCGeuNlTqUEfd6Sp/vEEaWsnKvxuaVanP1RGMZE5Q4ZbZfYyEy2R1o2qzto VgOBy+0ZWazijbcc/HUM5/IIV7KDGjZyadmCbaUpstE8X4qzttZhP0aOqT7sg2ABncZ07J EA3Oudxpa/2BfOCwXiOZTJ+xy87gRUw= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-113-0_gdHdaPNq6b3JDfHOFTJA-1; Tue, 06 Oct 2026 16:56:47 -0400 X-MC-Unique: 0_gdHdaPNq6b3JDfHOFTJA-1 X-Mimecast-MFC-AGG-ID: 0_gdHdaPNq6b3JDfHOFTJA_1791320207 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id A7BC6195E909; Tue, 6 Oct 2026 20:56:46 +0000 (UTC) Received: from redhat.com (headnet05.pony-001.prod.iad2.dc.redhat.com [10.2.32.117]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0B1C03001D31; Tue, 6 Oct 2026 20:56:44 +0000 (UTC) Date: Tue, 6 Oct 2026 21:56:41 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: qemu-devel@nongnu.org, Fabiano Rosas , Akash Kulhalli , Juraj Marcin Subject: Re: [PATCH v2 3/7] migration: Fix socket listen backlog Message-ID: References: <20261006182046.3767283-1-peterx@redhat.com> <20261006182046.3767283-4-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20261006182046.3767283-4-peterx@redhat.com> User-Agent: Mutt/2.4.2 (2026-08-30) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 10 X-Spam_score: 1.0 X-Spam_bar: + X-Spam_report: (1.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.24, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Oct 06, 2026 at 02:20:42PM -0400, Peter Xu wrote: > QEMU incoming side may have set a wrong listen() backlog. After the > support of multifd + postcopy now we have four possibilities on setting the > backlog, some of them are wrong: > > Vanilla: 1 (correct) > Multifd: N (wrong, should be N+1) > Preempt: 2 (correct) > Multifd+Preempt: N (wrong, should be N+2) > > No real report for this, found by code observation only. Logically the > backlog (pending accept requests on destination on the listening sockets) > needs to be the maximum of possible channels. Otherwise if the accept > queue full on destination, source QEMU may hit connection errors. >From a robustness POV, we might also consider retrying connections if we see a transient failure. Though I'm not sure offhand what errrno would result on the client when the accept() queue is full, hopefully there's something distinct. This would mitigate a risk of DoS from an unauthorized client making its own incoming connections concurrently. Such a client is not necessarily malicious, as there are monitoring tools or port scanners that randomly open connections to TCP listener sockets. > Cc: Daniel P. Berrangé > Fixes: 0705e56496 ("multifd: Use number of channels as listen backlog") > Fixes: e274188612 ("migration: enable multifd and postcopy together") > Signed-off-by: Peter Xu > --- > migration/socket.c | 25 ++++++++++++++++++------- > 1 file changed, 18 insertions(+), 7 deletions(-) Reviewed-by: Daniel P. Berrangé With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|