From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:38823)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <famz@redhat.com>) id 1YmbQH-0003SV-L2
	for qemu-devel@nongnu.org; Mon, 27 Apr 2015 01:17:38 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <famz@redhat.com>) id 1YmbQC-0000sD-M0
	for qemu-devel@nongnu.org; Mon, 27 Apr 2015 01:17:37 -0400
Date: Mon, 27 Apr 2015 13:17:28 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20150427051728.GE384@ad.nay.redhat.com>
References: <1429871600-10180-1-git-send-email-famz@redhat.com>
	<1429871600-10180-3-git-send-email-famz@redhat.com>
	<553A2245.9060304@redhat.com> <553A2E45.508@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <553A2E45.508@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v2 2/2] block: Fix NULL deference for
 unaligned write if qiov is NULL
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, qemu-stable@nongnu.org

On Fri, 04/24 13:51, Paolo Bonzini wrote:
> 
> 
> On 24/04/2015 13:00, Paolo Bonzini wrote:
> >> -        qemu_iovec_add(&local_qiov, head_buf, offset & (align - 1));
> >> -        qemu_iovec_concat(&local_qiov, qiov, 0, qiov->size);
> >> -        use_local_qiov = true;
> >> +        if (qiov) {
> >> +            qemu_iovec_init(&local_qiov, qiov ? qiov->niov + 2 : 1);
> >> +            qemu_iovec_add(&local_qiov, head_buf, offset & (align - 1));
> >> +            qemu_iovec_concat(&local_qiov, qiov, 0, qiov->size);
> >> +            use_local_qiov = true;
> >> +            bytes += offset & (align - 1);
> >> +            offset = offset & ~(align - 1);
> >> +        } else {
> >> +            memset(head_buf + (offset & (align - 1)), 0,
> >> +                   align - (offset & (align - 1)));
> 
> Actually, is the byte count correct if bytes < align?  In the case of
> your testcase, you'd destroy bytes 1536..4095.

Yes, good catch!

Fam

> 
> Same for the computation of bytes, below.  It could underflow.
> 
> Perhaps a qemu-iotests testcase, using qemu-io, is also necessary.
> 
> Paolo
> 
> >> +            ret = bdrv_aligned_pwritev(bs, &req, offset & ~(align - 1), align,
> >> +                                       &head_qiov, 0);
> >> +            if (ret < 0) {
> >> +                goto fail;
> >> +            }
> >> +            bytes -= align - (offset & (align - 1));
> >> +            offset = ROUND_UP(offset, align);
> >> +        }
> >> +    }