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=-10.0 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 B96DDC433DB for ; Fri, 5 Feb 2021 10:44:41 +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 0066A64DDC for ; Fri, 5 Feb 2021 10:44:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0066A64DDC 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]:55226 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l7ybP-00009S-KE for qemu-devel@archiver.kernel.org; Fri, 05 Feb 2021 05:44:39 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:51880) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l7yab-00084q-Gq for qemu-devel@nongnu.org; Fri, 05 Feb 2021 05:43:49 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:30652) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l7yaY-00040E-FN for qemu-devel@nongnu.org; Fri, 05 Feb 2021 05:43:48 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1612521824; 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: in-reply-to:in-reply-to:references:references; bh=WVbXENClknuo+rUYn1oIto34wAdR3aYEcF6u2XNibWM=; b=b0iRW19v5Wi5yP/HdP2U+Le85laUar41mblWT26S03T/zbuw5TnQFJ9Ozzz0zPlFtZ8zmh KOSK1n66wfNOOZRH5a5C1+bhudzo99n6dfzokB/lZkCgxHvvM8svdjh4NAOH3E0pTYHp2e CNXdNwJxcHq2TQ455vVEgaeEnXV7D7U= 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-227-WHv3nM7PMJ6wdWoDtHnouw-1; Fri, 05 Feb 2021 05:43:42 -0500 X-MC-Unique: WHv3nM7PMJ6wdWoDtHnouw-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 E50C8195D560; Fri, 5 Feb 2021 10:43:40 +0000 (UTC) Received: from localhost (ovpn-114-98.ams2.redhat.com [10.36.114.98]) by smtp.corp.redhat.com (Postfix) with ESMTP id 747605C5B5; Fri, 5 Feb 2021 10:43:40 +0000 (UTC) Date: Fri, 5 Feb 2021 10:43:39 +0000 From: "Richard W.M. Jones" To: Eric Blake Subject: Re: [PATCH] qemu-img: add seek and -n option to dd command Message-ID: <20210205104339.GD30079@redhat.com> References: <20210128140704.6547-1-pl@kamp.de> <99982043-ed89-5fbe-afe2-691a9c19280d@redhat.com> <567ab8bb-b452-d6c2-dec7-bc2cf1e34ec8@kamp.de> <3dff97ae-5172-d9ca-509f-2a520c2841b5@redhat.com> MIME-Version: 1.0 In-Reply-To: <3dff97ae-5172-d9ca-509f-2a520c2841b5@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=rjones@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=rjones@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.351, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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: kwolf@redhat.com, Peter Lieven , qemu-devel@nongnu.org, qemu-block@nongnu.org, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Feb 04, 2021 at 02:44:03PM -0600, Eric Blake wrote: > On 2/4/21 2:09 PM, Peter Lieven wrote: > > Am 02.02.21 um 16:51 schrieb Eric Blake: > >> On 1/28/21 8:07 AM, Peter Lieven wrote: > >>> Signed-off-by: Peter Lieven > >> Your commit message says 'what', but not 'why'. Generally, the one-line > >> 'what' works well as the subject line, but you want the commit body to > >> give an argument why your patch should be applied, rather than blank. > >> > >> Here's the last time we tried to improve qemu-img dd: > >> https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg02618.html > > > > > > I was not aware of that story. My use case is that I want to be > > > > able to "patch" an image that Qemu is able to handle by overwriting > > > > certain sectors. And I especially do not want to "mount" that image > > > > via qemu-nbd because I might not trust it. I totally want to avoid that the host > > > > system tries to analyse that image in terms of scanning the bootsector, partprobe, > > > > lvm etc. pp. > > qemu-nbd does not have to mount an image (yes, one use of qemu-nbd is to > use -c /dev/nbdX to get the kernel to mount it; but other uses are to > expose the NBD image in user-space only with no kernel involvement, and > therefore no system mount efforts). I agree, there's nothing unsafe about qemu-nbd (provided you don't use the -c option). > Another thing you might try is libnbd, which now includes a utility > nbdcopy. It should make it easier to overwrite a portion of an NBD > image using only user-space actions. I'm not sure if Rich has got it > doing partial file overwrites yet (.../me goes and compiles the latest > git checkout... nope, still a TODO item to implement subsetting), but it > may be possible to combine nbdkit's --filter=offset with the full NBD > image in order to then easily point nbdcopy to only the subset you care > about. Definitely some ideas worthy of implementation. TBH I would use nbdsh. For example to overwrite the sector at 1M in a qcow2 image with "1"s: $ qemu-img create -f qcow2 test.qcow2 10M $ nbdsh -c 'h.connect_systemd_socket_activation(["qemu-nbd","-t","-f","qcow2","test.qcow2"])' \ -c 'h.pwrite(b"1"*512, 1024*1024)' and to show it was really overwritten: $ nbdcopy -- [ qemu-nbd -f qcow2 test.qcow2 ] - | hexdump -C 00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00100000 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 |1111111111111111| * 00100200 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00a00000 Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/