Re: [PATCH v3 1/3] xen/blkfront: read response from backend only once

[Julien Grall]

Hi,

On 02/08/2021 15:06, Oleksandr Andrushchenko wrote:
> On 30.07.21 13:38, Juergen Gross wrote:
> > In order to avoid problems in case the backend is modifying a response
> > on the ring page while the frontend has already seen it, just read the
> > response into a local buffer in one go and then operate on that buffer
> > only.
> >
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> > Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> >    drivers/block/xen-blkfront.c | 35 ++++++++++++++++++-----------------
> >    1 file changed, 18 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> > index d83fee21f6c5..15e840287734 100644
> > --- a/drivers/block/xen-blkfront.c
> > +++ b/drivers/block/xen-blkfront.c
> > @@ -1496,7 +1496,7 @@ static bool blkif_completion(unsigned long *id,
> >    static irqreturn_t blkif_interrupt(int irq, void *dev_id)
> >    {
> >    	struct request *req;
> > -	struct blkif_response *bret;
> > +	struct blkif_response bret;
> >    	RING_IDX i, rp;
> >    	unsigned long flags;
> >    	struct blkfront_ring_info *rinfo = (struct blkfront_ring_info *)dev_id;
> > @@ -1513,8 +1513,9 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
> >    	for (i = rinfo->ring.rsp_cons; i != rp; i++) {
> >    		unsigned long id;
> >    
> > -		bret = RING_GET_RESPONSE(&rinfo->ring, i);
> > -		id   = bret->id;
> > +		RING_COPY_RESPONSE(&rinfo->ring, i, &bret);
>
> As per my understanding copying is still not an atomic operation as the request/response
>
> are multi-byte structures in general. IOW, what prevents the backend from modifying the ring while
>
> we are copying?

Nothing and, I believe, you are never going to be able to ensure 
atomicity with large structure (at least between entity that doesn't 
trust each other).

However, what you can do is copying the response once, check that it is 
consistent and then use it. If it is not consistent, then you can report 
an error.

This is better than what's currently in tree. IOW we may have multiple 
read so the code is prone to TOCTOU.

Cheers,

--
Julien Grall