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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 05219C5B552 for ; Mon, 9 Jun 2025 14:46:18 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uOdkk-0003e3-PB; Mon, 09 Jun 2025 10:45:34 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uOdki-0003dY-NX for qemu-devel@nongnu.org; Mon, 09 Jun 2025 10:45:32 -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 1uOdkg-0007iG-Qg for qemu-devel@nongnu.org; Mon, 09 Jun 2025 10:45:32 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1749480330; 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=HVmTIGueD+jVw0J1onyWshU08PG/lW/jCzJxqrKQ0eQ=; b=B4G4QamBv9qHyLl1bnaRs5P5yefWPAI7r3ivLfyhGQSAb0Y0gSlvLogbecxe+QamjYRqNW WyHcEmqNe5wu6LTDs3/FHDDT+6/dfFytTHk210sxE+PIKdIUK+n+kZEE+Flc0+04rmVQSH UA5el6gOntFwdj0JhBFK+BKugr657dk= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-216-RJFE20AwMDStTN5uVC1ZAw-1; Mon, 09 Jun 2025 10:45:27 -0400 X-MC-Unique: RJFE20AwMDStTN5uVC1ZAw-1 X-Mimecast-MFC-AGG-ID: RJFE20AwMDStTN5uVC1ZAw_1749480325 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 9E4C01800287; Mon, 9 Jun 2025 14:45:25 +0000 (UTC) Received: from localhost (unknown [10.2.16.92]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id CDB3819560A3; Mon, 9 Jun 2025 14:45:24 +0000 (UTC) Date: Mon, 9 Jun 2025 10:45:23 -0400 From: Stefan Hajnoczi To: Hanna Czenczek Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, Kevin Wolf , Markus Armbruster , Brian Song Subject: Re: [PATCH v2 01/21] fuse: Copy write buffer content before polling Message-ID: <20250609144523.GD29452@fedora> References: <20250604132813.359438-1-hreitz@redhat.com> <20250604132813.359438-2-hreitz@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="hqmh3syZhVVH+iG3" Content-Disposition: inline In-Reply-To: <20250604132813.359438-2-hreitz@redhat.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --hqmh3syZhVVH+iG3 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Jun 04, 2025 at 03:27:53PM +0200, Hanna Czenczek wrote: > aio_poll() in I/O functions can lead to nested read_from_fuse_export() > calls, overwriting the request buffer's content. The only function > affected by this is fuse_write(), which therefore must use a bounce > buffer or corruption may occur. >=20 > Note that in addition we do not know whether libfuse-internal structures > can cope with this nesting, and even if we did, we probably cannot rely > on it in the future. This is the main reason why we want to remove > libfuse from the I/O path. >=20 > I do not have a good reproducer for this other than: >=20 > $ dd if=3D/dev/urandom of=3Dimage bs=3D1M count=3D4096 > $ dd if=3D/dev/zero of=3Dcopy bs=3D1M count=3D4096 > $ touch fuse-export > $ qemu-storage-daemon \ > --blockdev file,node-name=3Dfile,filename=3Dcopy \ > --export \ > fuse,id=3Dexp,node-name=3Dfile,mountpoint=3Dfuse-export,writable=3Dtr= ue \ > & >=20 > Other shell: > $ qemu-img convert -p -n -f raw -O raw -t none image fuse-export > $ killall -SIGINT qemu-storage-daemon > $ qemu-img compare image copy > Content mismatch at offset 0! >=20 > (The -t none in qemu-img convert is important.) >=20 > I tried reproducing this with throttle and small aio_write requests from > another qemu-io instance, but for some reason all requests are perfectly > serialized then. >=20 > I think in theory we should get parallel writes only if we set > fi->parallel_direct_writes in fuse_open(). In fact, I can confirm that > if we do that, that throttle-based reproducer works (i.e. does get > parallel (nested) write requests). I have no idea why we still get > parallel requests with qemu-img convert anyway. >=20 > Also, a later patch in this series will set fi->parallel_direct_writes > and note that it makes basically no difference when running fio on the > current libfuse-based version of our code. It does make a difference > without libfuse. So something quite fishy is going on. >=20 > I will try to investigate further what the root cause is, but I think > for now let's assume that calling blk_pwrite() can invalidate the buffer > contents through nested polling. >=20 > Cc: qemu-stable@nongnu.org > Signed-off-by: Hanna Czenczek > --- > block/export/fuse.c | 24 +++++++++++++++++++++--- > 1 file changed, 21 insertions(+), 3 deletions(-) Reviewed-by: Stefan Hajnoczi --hqmh3syZhVVH+iG3 Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQEzBAEBCgAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmhG84MACgkQnKSrs4Gr c8hQXQgAuksQqKt83+9A7jq/UzhrNkFAscpsmUjLojwkAmZH1Qg7CKVSOKvuxrwu AqYRTOrF6TTti8MmZ5m3XFKMpNXPcORaQKDTPVFZB8meH9Ek+prowszCuOjHjTt3 LPCzqdP+lM9vzgAaqUcIzsMCKlQMypA/mgWRn1ROamxkGLlY/GeZJQuOtNe3XBr1 o40NS6Zc19FpLHQaNalUp4puOHLT0C1SOinFQEQ2gOiDYzIOs2udbk8qV1iULpuY d/eX2xCYipxnEH9S5DSiIlGWxTaq2w09JUwoTtvmiAxh+4rkUsWHNfcgwB0qgCvR tS1zhhSvGFU2nClbwE2tnhjP4vIM+g== =Ybrd -----END PGP SIGNATURE----- --hqmh3syZhVVH+iG3--