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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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.


> +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?


>  static int xlbd_reserve_minors(unsigned int minor, unsigned int nr)
>  {
>       unsigned int end = minor + nr;
> @@ -746,6 +757,18 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>  
>               bret = RING_GET_RESPONSE(&info->ring, i);
>               id   = bret->id;
> +             /*
> +              * The backend has messed up and given us an id that we would
> +              * never have given to it (we stamp it up to BLK_RING_SIZE -
> +              * look in get_id_from_freelist.
> +              */
> +             if (id >= BLK_RING_SIZE) {
> +                     /* We can't safely get the 'struct request' as
> +                      * the id is busted. So limp along. */
> +                     WARN(1, "%s: response to %s has incorrect id (%d)!\n",
> +                          info->gd->disk_name, op_name(bret->operation), id);
> +                     continue;
> +             }
>               req  = info->shadow[id].request;

Do you think it would be better to goto error_out, instead of continue?
I guess that depends on whether we expect the other requests to be in a
good shape or not...

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