Re: [REGRESSION] 28676d869bbb (scsi: sg: check for valid direction before starting the request) breaks mtx tape library control

[Johannes Thumshirn]

Doug,

On Wed, Jul 19, 2017 at 10:36:54AM +0200, Johannes Thumshirn wrote:
> On Wed, Jul 19, 2017 at 03:13:34AM -0500, Jason L Tibbitts III wrote:
> > [   46.304530] sg_is_valid_dxfer: dxfer_direction: -2, dxfer_len: 0
> 
> Ahh now I see the -2 (SG_DXFER_TO_DEV) is the crucial point here. It is 0 in
> your case.
> 
> This would "fix" it but I'm not generally sure it is _the_ solution:
> 
> diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> index 1e82d4128a84..b421ec81d775 100644
> --- a/drivers/scsi/sg.c
> +++ b/drivers/scsi/sg.c
> @@ -764,7 +764,7 @@ static bool sg_is_valid_dxfer(sg_io_hdr_t *hp)
>  		return true;
>  	case SG_DXFER_TO_DEV:
>  	case SG_DXFER_TO_FROM_DEV:
> -		if (!hp->dxferp || hp->dxfer_len == 0)
> +		if (!hp->dxferp)
>  			return false;
>  		return true;
>  	case SG_DXFER_UNKNOWN:
> 
> Doug, what are the rules for SG_DXFER_TO_{FROM_}DEV? Apparently we can't
> be sure len > 0, can we rely on dxferp being present?

Any comments on this?

Jason, can you try the above? If it works and Doug doesn't respond, I'm
inclined yo submit this band aid.

Thanks,
	Johannes

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