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Date: Fri, 24 Apr 2015 13:51:33 +0200
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Subject: Re: [Qemu-devel] [PATCH v2 2/2] block: Fix NULL deference for
 unaligned write if qiov is NULL
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To: Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org
Cc: qemu-block@nongnu.org, qemu-stable@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>



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.

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);
>> +        }
>> +    }