From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39215) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WNRIq-0002wr-Qn for qemu-devel@nongnu.org; Tue, 11 Mar 2014 14:21:30 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WNRIk-0001t9-Me for qemu-devel@nongnu.org; Tue, 11 Mar 2014 14:21:24 -0400 Received: from mx1.redhat.com ([209.132.183.28]:63119) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WNRIk-0001st-EH for qemu-devel@nongnu.org; Tue, 11 Mar 2014 14:21:18 -0400 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id s2BILBuY024028 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Tue, 11 Mar 2014 14:21:17 -0400 Message-ID: <531F5176.3000803@redhat.com> Date: Tue, 11 Mar 2014 19:09:58 +0100 From: Max Reitz MIME-Version: 1.0 References: <1394491449-10897-1-git-send-email-mreitz@redhat.com> <1394491449-10897-2-git-send-email-mreitz@redhat.com> <531E47BF.8080709@redhat.com> <20140311101604.GB3215@dhcp-200-207.str.redhat.com> In-Reply-To: <20140311101604.GB3215@dhcp-200-207.str.redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 1/3] qcow2: Check bs->drv in copy_sectors() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf , Laszlo Ersek Cc: qemu-devel@nongnu.org, Stefan Hajnoczi On 11.03.2014 11:16, Kevin Wolf wrote: > Am 11.03.2014 um 00:16 hat Laszlo Ersek geschrieben: >> On 03/10/14 23:44, Max Reitz wrote: >>> Before dereferencing bs->drv for a call to its member bdrv_co_readv(), >>> copy_sectors() should check whether that pointer is indeed valid, since >>> it may have been set to NULL by e.g. a concurrent write triggering the >>> corruption prevention mechanism. >>> >>> Signed-off-by: Max Reitz >>> --- >>> To be precise, this still is a race condition. If bs->drv is set to NULL >>> after the check and before the call to bdrv_co_readv(), QEMU will >>> obviously still crash. However, in order to circumvent this behavior, we >>> would probably have to re-lock s->lock, check bs->drv, take the function >>> pointer to bdrv_co_readv() and then unlock s->lock before the function >>> is called. I found this rather ugly and therefore this still has a very >>> small chance of running into a race condition. >>> Therefore, I'm asking for your opinion on this, whether we can really >>> take this chance or should rather "do it right". In fact, if I were a >>> reviewer, I'd probably reject this patch and request the solution with >>> the function pointer (if there is no better solution), but I was afraid >>> to send such an ugly patch. > No, the code is fine. Remember that qcow2 is not threaded, we're talking > about coroutines here. There is no way for the code to yield between > your check and the protected place. Ah, okay, that makes sense. Thank you. :-) Max >>> block/qcow2-cluster.c | 4 ++++ >>> 1 file changed, 4 insertions(+) >>> >>> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c >>> index 36c1bed..9499df9 100644 >>> --- a/block/qcow2-cluster.c >>> +++ b/block/qcow2-cluster.c >>> @@ -380,6 +380,10 @@ static int coroutine_fn copy_sectors(BlockDriverState *bs, >>> >>> BLKDBG_EVENT(bs->file, BLKDBG_COW_READ); >>> >>> + if (!bs->drv) { >>> + return -ENOMEDIUM; >>> + } >>> + >>> /* Call .bdrv_co_readv() directly instead of using the public block-layer >>> * interface. This avoids double I/O throttling and request tracking, >>> * which can lead to deadlock when block layer copy-on-read is enabled. >>> >> I can't answer your question nor review this patch -- instead, I have a >> question of my own: when you say "set to NULL by [...] the corruption >> prevention mechanism", do you mean qcow2_pre_write_overlap_check(): >> >> bs->drv = NULL; /* make BDS unusable */ > Yes, this is the place. > >> If so: I thought that it was quite a bold move, but also that we'd find >> the SIGSEGVs sooner or later... :) > In fact, if you use the block layer API, most functions check for > bs->drv and return -ENOMEDIUM if it is NULL. The problem here is that we > directly dereference the pointer without going through block.c (there's > a good reason for this, see the comment, but it still makes it somewhat > special). > > Kevin