linux-kernel - Re: [Xen-devel] [PATCH 6/6] xen-blkfront: prepare request locally, only then put it on the shared ring

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Date:   Tue, 1 May 2018 10:15:26 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Roger Pau Monné <roger.pau@...rix.com>
CC:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, Juergen Gross <jgross@...e.com>,
        Jens Axboe <axboe@...nel.dk>,
        open list <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>,
        "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
        <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH 6/6] xen-blkfront: prepare request locally,
 only then put it on the shared ring

On Tue, May 01, 2018 at 09:22:31AM +0100, Roger Pau Monné wrote:
> On Mon, Apr 30, 2018 at 11:01:50PM +0200, Marek Marczykowski-Górecki wrote:
> >  					    struct request *req,
> > -					    struct blkif_request **ring_req)
> > +					    struct blkif_request *ring_req)
> >  {
> >  	unsigned long id;
> >  
> > -	*ring_req = RING_GET_REQUEST(&rinfo->ring, rinfo->ring.req_prod_pvt);
> > -	rinfo->ring.req_prod_pvt++;
> > -
> >  	id = get_id_from_freelist(rinfo);
> >  	rinfo->shadow[id].request = req;
> >  	rinfo->shadow[id].status = REQ_WAITING;
> >  	rinfo->shadow[id].associated_id = NO_ASSOCIATED_ID;
> >  
> > -	(*ring_req)->u.rw.id = id;
> > +	ring_req->u.rw.id = id;
> >  
> >  	return id;
> >  }
> > @@ -545,23 +542,28 @@ static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo,
> >  static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_info *rinfo)
> >  {
> >  	struct blkfront_info *info = rinfo->dev_info;
> > -	struct blkif_request *ring_req;
> > +	struct blkif_request ring_req = { 0 };
> >  	unsigned long id;
> >  
> >  	/* Fill out a communications ring structure. */
> >  	id = blkif_ring_get_request(rinfo, req, &ring_req);
> 
> Maybe I'm missing something obvious here, but you are adding a struct
> allocated on the stack to the shadow ring copy, isn't this dangerous?

The above comment is wrong, you are storing a pointer to 'req' in the
shadow ring copy, which is fine and is not the ring request.

Roger.