linux-kernel - Re: [Xen-devel] [PATCH 3/3] xen/blkback: Check for insane amounts of request on the ring.

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Message-ID: <20130128161356.GN4838@konrad-lan.dumpdata.com>
Date:	Mon, 28 Jan 2013 11:13:57 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH 3/3] xen/blkback: Check for insane amounts of
 request on the ring.

On Mon, Jan 28, 2013 at 04:03:49PM +0000, David Vrabel wrote:
> On 28/01/13 15:44, Konrad Rzeszutek Wilk wrote:
> > On Mon, Jan 28, 2013 at 11:38:50AM +0000, David Vrabel wrote:
> >> On 25/01/13 18:43, Konrad Rzeszutek Wilk wrote:
> >>>
> >>> Check that the ring does not have an insane amount of requests
> >>> (more than there could fit on the ring).
> >> [...]
> >>> --- a/drivers/block/xen-blkback/blkback.c
> >>> +++ b/drivers/block/xen-blkback/blkback.c
> >> [...]
> >>> @@ -415,8 +415,12 @@ int xen_blkif_schedule(void *arg)
> >>>  		blkif->waiting_reqs = 0;
> >>>  		smp_mb(); /* clear flag *before* checking for work */
> >>>  
> >>> -		if (do_block_io_op(blkif))
> >>> +		rc = do_block_io_op(blkif);
> >>> +		if (rc > 0)
> >>>  			blkif->waiting_reqs = 1;
> >>> +		if (rc == -EACCES)
> >>> +			wait_event_interruptible(blkif->shutdown_wq,
> >>> +						 kthread_should_stop());
> >>
> >> So if this happens then the backend silently stops processing any
> >> further requests?  I think it should print an error and disconnect from
> >> the front end.
> > 
> > I was not sure what it should print?
> > 
> > "Frontend provided bogus ring requests. Halting ring processing"?
> 
> That would be fine but you may want to print the bad values, in case
> they're useful for tracking down bugs in frontends.

OK.
> 
> > It does eventually disconnect from the front-end as the XenBus enters
> > the Disconnect phase. It seemed the strike the balance - so that if
> > the frontend negotiates again the ring can start again and penalize
> > the frontend for giving bogus data.
> 
> Ok.
> 
> David
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