Re: [PATCH v2 3/4] Ovmfpkg/VirtioScsiDxe: map virtio-scsi request and response buffers

[Brijesh Singh <brijesh.singh@amd.com>]

Thanks for quick feedback Laszlo.

I am addressing all your review comments and submitting the v3 soon

-Brijesh

On 08/31/2017 08:49 AM, Laszlo Ersek wrote:
> On 08/31/17 15:23, Laszlo Ersek wrote:
>> On 08/30/17 22:45, Brijesh Singh wrote:
> 
>>> @@ -492,10 +645,50 @@ VirtioScsiPassThru (
>>>     //
>>>     if (VirtioFlush (Dev->VirtIo, VIRTIO_SCSI_REQUEST_QUEUE, &Dev->Ring,
>>>           &Indices, NULL) != EFI_SUCCESS) {
>>> -    return ReportHostAdapterError (Packet);
>>> +    Status = ReportHostAdapterError (Packet);
>>> +    goto UnmapResponseBuffer;
>>>     }
>>>
>>> -  return ParseResponse (Packet, &Response);
>>> +  Status = ParseResponse (Packet, Response);
>>> +
>>> +  //
>>> +  // If virtio request was successful and it was a CPU read request then we
>>> +  // have used an intermediate buffer. Copy the data from intermediate buffer
>>> +  // to the final buffer.
>>> +  //
>>> +  if (!EFI_ERROR (Status) && (Packet->InTransferLength > 0)) {
>>> +    CopyMem (Packet->InDataBuffer, InDataBuffer, Packet->InTransferLength);
>>> +  }
>>
>> (7) The comment is exactly right, but the condition that you check
>> after is incorrect.
>>
>> The right thing to do is to call CopyMem() *unconditionally*.
>>
>> Namely, at this point we are past ParseResponse(). As I wrote before,
>> ParseResponse() updates the Packet->... fields in every case, even if
>> it reports an EFI_STATUS that is different from EFI_SUCCESS. And
>> whatever we expose to the caller through "Packet->InTransferLength"
>> *must* be reflected in "Packet->InDataBuffer" regardless of return
>> status.
>>
>> Therefore the Status check must be dropped. And then we need not check
>> (Packet->InTransferLength>0) either, because the CopyMem() will deal
>> with it internally.
>>
>> Think of it like this: the "worst" that can happen, on error, is that
>> "Packet->InTransferLength" is unchanged from its "input" value, and we
>> overwrite the caller's "Packet->InDataBuffer" entirely. What is the
>> data we are going to put there? It's all zeroes, from your
>>
>>    ZeroMem (InDataBuffer, Packet->InTransferLength);
>>
>> higher up.
>>
>> So, again, this CopyMem() needs to be unconditional -- as the comment
>> says, if the *virtio* request was successful (== we talked to the
>> virtio-scsi adapter), then we have to copy the data, even if the
>> *SCSI* request produced an error status in ParseResponse.
> 
> I have to correct myself a little bit -- although I think you would have
> caught me anyway :) --, namely we should keep the "if", but the
> condition should be:
> 
>    InDataBuffer != NULL
> 
> Admittedly, it is likely that none of the CopyMem() implementations
> would have problems with a NULL "SourceBuffer", if "Length" was zero.
> 
> Nonetheless, the interface contract in
> 
>    MdePkg/Include/Library/BaseMemoryLib.h
> 
> does not mark SourceBuffer OPTIONAL -- neither does the UEFI spec, for
> the similar gBS->CopyMem() boot service --, for the case when Length==0,
> so we should do an explicit check:
> 
>    if (InDataBuffer != NULL) {
>      CopyMem (Packet->InDataBuffer, InDataBuffer, Packet->InTransferLength);
>    }
> 
> Thank you,
> Laszlo
>