Re: [Xen-devel] [PATCH 2/2] xen/blkfront: Add BUG_ON to deal withmisbehaving backends.

[Konrad Rzeszutek Wilk]

On Fri, Jun 01, 2012 at 11:16:29AM +0100, Stefano Stabellini wrote:
> On Thu, 31 May 2012, Konrad Rzeszutek Wilk wrote:
> > The blkfront_remove part is .. that is going to take some surgery to do
> > and I don't think I am going to be able to attempt that within the next couple
> > of weeks. So lets put that on the TODO list and just do this one?
> 
> OK
> 
> > >From 4aabb5b44778fc0c0b8d4f5a2e2cd8e8490064d7 Mon Sep 17 00:00:00 2001
> > From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Date: Fri, 25 May 2012 17:34:51 -0400
> > Subject: [PATCH] xen/blkfront: Add WARN to deal with misbehaving backends.
> > 
> > Part of the ring structure is the 'id' field which is under
> > control of the frontend. The frontend stamps it with "some"
> > value (this some in this implementation being a value less
> > than BLK_RING_SIZE), and when it gets a response expects
> > said value to be in the response structure. We have a check
> > for the id field when spolling new requests but not when
> > de-spolling responses.
> > 
> > We also add an extra check in add_id_to_freelist to make
> > sure that the 'struct request' was not NULL - as we cannot
> > pass a NULL to __blk_end_request_all, otherwise that crashes
> > (and all the operations that the response is dealing with
> > end up with __blk_end_request_all).
> > 
> > Lastly we also print the name of the operation that failed.
> > 
> > [v1: s/BUG/WARN/ suggested by Stefano]
> > [v2: Add extra check in add_id_to_freelist]
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > ---
> >  drivers/block/xen-blkfront.c |   39 +++++++++++++++++++++++++++++----------
> >  1 files changed, 29 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> > index 60eed4b..c7ef8a4 100644
> > --- a/drivers/block/xen-blkfront.c
> > +++ b/drivers/block/xen-blkfront.c
> > @@ -144,11 +144,22 @@ static int get_id_from_freelist(struct blkfront_info *info)
> >  static void add_id_to_freelist(struct blkfront_info *info,
> >  			       unsigned long id)
> >  {
> > +	BUG_ON(info->shadow[id].req.u.rw.id != id);
> >  	info->shadow[id].req.u.rw.id  = info->shadow_free;
> > +	BUG_ON(info->shadow[id].request == NULL);
> >  	info->shadow[id].request = NULL;
> >  	info->shadow_free = id;
> >  }
> 
> Like Jan said, it would be best to change the two BUG_ON into WARN_ON
> and return an error.

Yes. I missed that.
> 
> 
> > +static const char *op_name(int op)
> > +{
> > +	const char *names[BLKIF_OP_DISCARD+1] = {
> > +		"read" , "write", "barrier", "flush", "reserved", "discard"};
> > +
> > +	if (op > BLKIF_OP_DISCARD)
> > +		return "unknown";
> > +	return names[op];
> > +}
> 
> Considering that op is an int, shoudn't we check for negative values
> too?

Yes! I also converted this per Jan's excellent idea.

Please see: