Re: [Xen-devel] [PATCH 2/3] xen: return xenstore command failures via response instead of rc

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/22/2016 02:19 AM, Juergen Gross wrote:
> When the xenbus driver does some special handling for a Xenstore
> command any error condition related to the command should be returned
> via an error response instead of letting the related write operation
> fail. Otherwise the user land handler might take wrong decisions
> assuming the connection to Xenstore is broken.

Do we expect the user to always read the reply if no error code was
returned?

-boris

>
> While at it try to return the same error values xenstored would
> return for those cases.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  drivers/xen/xenbus/xenbus_dev_frontend.c | 47 
> ++++++++++++++++++--------------
>  1 file changed, 27 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/xen/xenbus/xenbus_dev_frontend.c 
> b/drivers/xen/xenbus/xenbus_dev_frontend.c
> index a068281..79130b3 100644
> --- a/drivers/xen/xenbus/xenbus_dev_frontend.c
> +++ b/drivers/xen/xenbus/xenbus_dev_frontend.c
> @@ -302,6 +302,29 @@ static void watch_fired(struct xenbus_watch *watch,
>       mutex_unlock(&adap->dev_data->reply_mutex);
>  }
>  
> +static int xenbus_command_reply(struct xenbus_file_priv *u,
> +                             unsigned int msg_type, const char *reply)
> +{
> +     struct {
> +             struct xsd_sockmsg hdr;
> +             const char body[16];
> +     } msg;
> +     int rc;
> +
> +     msg.hdr = u->u.msg;
> +     msg.hdr.type = msg_type;
> +     msg.hdr.len = strlen(reply) + 1;
> +     if (msg.hdr.len > sizeof(msg.body))
> +             return -E2BIG;
> +
> +     mutex_lock(&u->reply_mutex);
> +     rc = queue_reply(&u->read_buffers, &msg, sizeof(msg.hdr) + msg.hdr.len);
> +     wake_up(&u->read_waitq);
> +     mutex_unlock(&u->reply_mutex);
> +
> +     return rc;
> +}
> +
>  static int xenbus_write_transaction(unsigned msg_type,
>                                   struct xenbus_file_priv *u)
>  {
> @@ -321,7 +344,7 @@ static int xenbus_write_transaction(unsigned msg_type,
>                       if (trans->handle.id == u->u.msg.tx_id)
>                               break;
>               if (&trans->list == &u->transactions)
> -                     return -ESRCH;
> +                     return xenbus_command_reply(u, XS_ERROR, "ENOENT");
>       }
>  
>       reply = xenbus_dev_request_and_reply(&u->u.msg);
> @@ -372,12 +395,12 @@ static int xenbus_write_watch(unsigned msg_type, struct 
> xenbus_file_priv *u)
>       path = u->u.buffer + sizeof(u->u.msg);
>       token = memchr(path, 0, u->u.msg.len);
>       if (token == NULL) {
> -             rc = -EILSEQ;
> +             rc = xenbus_command_reply(u, XS_ERROR, "EINVAL");
>               goto out;
>       }
>       token++;
>       if (memchr(token, 0, u->u.msg.len - (token - path)) == NULL) {
> -             rc = -EILSEQ;
> +             rc = xenbus_command_reply(u, XS_ERROR, "EINVAL");
>               goto out;
>       }
>  
> @@ -411,23 +434,7 @@ static int xenbus_write_watch(unsigned msg_type, struct 
> xenbus_file_priv *u)
>       }
>  
>       /* Success.  Synthesize a reply to say all is OK. */
> -     {
> -             struct {
> -                     struct xsd_sockmsg hdr;
> -                     char body[3];
> -             } __packed reply = {
> -                     {
> -                             .type = msg_type,
> -                             .len = sizeof(reply.body)
> -                     },
> -                     "OK"
> -             };
> -
> -             mutex_lock(&u->reply_mutex);
> -             rc = queue_reply(&u->read_buffers, &reply, sizeof(reply));
> -             wake_up(&u->read_waitq);
> -             mutex_unlock(&u->reply_mutex);
> -     }
> +     rc = xenbus_command_reply(u, msg_type, "OK");
>  
>  out:
>       return rc;


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