nbd/server: Reject 0-length block status request

The NBD spec says that behavior is unspecified if the client
requests 0 length for block status; but since the structured
reply is documenting as returning a non-zero length, it's
easier to just diagnose this with an EINVAL error than to
figure out what to return.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180621124937.166549-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

nbd/server.c

@@ -2007,6 +2007,10 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
                                       "discard failed", errp);
 
     case NBD_CMD_BLOCK_STATUS:
+        if (!request->len) {
+            return nbd_send_generic_reply(client, request->handle, -EINVAL,
+                                          "need non-zero length", errp);
+        }
         if (client->export_meta.valid && client->export_meta.base_allocation) {
             return nbd_co_send_block_status(client, request->handle,
                                             blk_bs(exp->blk), request->from,