Re: [PATCH v2 2/6] vhost-user-blk: Don't reconnect during initialisation

[Michael S. Tsirkin]

On Thu, Apr 29, 2021 at 07:13:12PM +0200, Kevin Wolf wrote:
> This is a partial revert of commits 77542d43149 and bc79c87bcde.
> 
> Usually, an error during initialisation means that the configuration was
> wrong. Reconnecting won't make the error go away, but just turn the
> error condition into an endless loop. Avoid this and return errors
> again.

So there are several possible reasons for an error:

1. remote restarted - we would like to reconnect,
   this was the original use-case for reconnect.

   I am not very happy that we are killing this usecase.

2. qemu detected an error and closed the connection
   looks like we try to handle that by reconnect,
   this is something we should address.
3. remote failed due to a bad command from qemu.
   this usecase isn't well supported at the moment.

   How about supporting it on the remote side? I think
   that if the data is well-formed just has a configuration
   remote can not support then instead of closing the connection, remote can 
wait
   for commands with need_reply set, and respond with
   an error. Or at least do it
   if VHOST_USER_PROTOCOL_F_REPLY_ACK has been negotiated.
   If VHOST_USER_SET_VRING_ERR is used then signalling that
   fd might also be reasonable.

   OTOH if qemu is buggy and sends malformed data and remote detects that then
   hacing qemu retry forever is ok, might actually be benefitial for
   debugging.



> Additionally, calling vhost_user_blk_disconnect() from the chardev event
> handler could result in use-after-free because none of the
> initialisation code expects that the device could just go away in the
> middle. So removing the call fixes crashes in several places.
> For example, using a num-queues setting that is incompatible with the
> backend would result in a crash like this (dereferencing dev->opaque,
> which is already NULL):
> 
>  #0  0x0000555555d0a4bd in vhost_user_read_cb (source=0x5555568f4690, 
> condition=(G_IO_IN | G_IO_HUP), opaque=0x7fffffffcbf0) at 
> ../hw/virtio/vhost-user.c:313
>  #1  0x0000555555d950d3 in qio_channel_fd_source_dispatch 
> (source=0x555557c3f750, callback=0x555555d0a478 <vhost_user_read_cb>, 
> user_data=0x7fffffffcbf0) at ../io/channel-watch.c:84
>  #2  0x00007ffff7b32a9f in g_main_context_dispatch () at 
> /lib64/libglib-2.0.so.0
>  #3  0x00007ffff7b84a98 in g_main_context_iterate.constprop () at 
> /lib64/libglib-2.0.so.0
>  #4  0x00007ffff7b32163 in g_main_loop_run () at /lib64/libglib-2.0.so.0
>  #5  0x0000555555d0a724 in vhost_user_read (dev=0x555557bc62f8, 
> msg=0x7fffffffcc50) at ../hw/virtio/vhost-user.c:402
>  #6  0x0000555555d0ee6b in vhost_user_get_config (dev=0x555557bc62f8, 
> config=0x555557bc62ac "", config_len=60) at ../hw/virtio/vhost-user.c:2133
>  #7  0x0000555555d56d46 in vhost_dev_get_config (hdev=0x555557bc62f8, 
> config=0x555557bc62ac "", config_len=60) at ../hw/virtio/vhost.c:1566
>  #8  0x0000555555cdd150 in vhost_user_blk_device_realize (dev=0x555557bc60b0, 
> errp=0x7fffffffcf90) at ../hw/block/vhost-user-blk.c:510
>  #9  0x0000555555d08f6d in virtio_device_realize (dev=0x555557bc60b0, 
> errp=0x7fffffffcff0) at ../hw/virtio/virtio.c:3660

Right. So that's definitely something to fix.

> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  hw/block/vhost-user-blk.c | 59 +++++++++++----------------------------
>  1 file changed, 17 insertions(+), 42 deletions(-)
> 
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index 7c85248a7b..c0b9958da1 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -50,6 +50,8 @@ static const int user_feature_bits[] = {
>      VHOST_INVALID_FEATURE_BIT
>  };
>  
> +static void vhost_user_blk_event(void *opaque, QEMUChrEvent event);
> +
>  static void vhost_user_blk_update_config(VirtIODevice *vdev, uint8_t *config)
>  {
>      VHostUserBlk *s = VHOST_USER_BLK(vdev);
> @@ -362,19 +364,6 @@ static void vhost_user_blk_disconnect(DeviceState *dev)
>      vhost_dev_cleanup(&s->dev);
>  }
>  
> -static void vhost_user_blk_event(void *opaque, QEMUChrEvent event,
> -                                 bool realized);
> -
> -static void vhost_user_blk_event_realize(void *opaque, QEMUChrEvent event)
> -{
> -    vhost_user_blk_event(opaque, event, false);
> -}
> -
> -static void vhost_user_blk_event_oper(void *opaque, QEMUChrEvent event)
> -{
> -    vhost_user_blk_event(opaque, event, true);
> -}
> -
>  static void vhost_user_blk_chr_closed_bh(void *opaque)
>  {
>      DeviceState *dev = opaque;
> @@ -382,12 +371,11 @@ static void vhost_user_blk_chr_closed_bh(void *opaque)
>      VHostUserBlk *s = VHOST_USER_BLK(vdev);
>  
>      vhost_user_blk_disconnect(dev);
> -    qemu_chr_fe_set_handlers(&s->chardev, NULL, NULL,
> -            vhost_user_blk_event_oper, NULL, opaque, NULL, true);
> +    qemu_chr_fe_set_handlers(&s->chardev, NULL, NULL, vhost_user_blk_event,
> +                             NULL, opaque, NULL, true);
>  }
>  
> -static void vhost_user_blk_event(void *opaque, QEMUChrEvent event,
> -                                 bool realized)
> +static void vhost_user_blk_event(void *opaque, QEMUChrEvent event)
>  {
>      DeviceState *dev = opaque;
>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> @@ -401,17 +389,7 @@ static void vhost_user_blk_event(void *opaque, 
> QEMUChrEvent event,
>          }
>          break;
>      case CHR_EVENT_CLOSED:
> -        /*
> -         * Closing the connection should happen differently on device
> -         * initialization and operation stages.
> -         * On initalization, we want to re-start vhost_dev initialization
> -         * from the very beginning right away when the connection is closed,
> -         * so we clean up vhost_dev on each connection closing.
> -         * On operation, we want to postpone vhost_dev cleanup to let the
> -         * other code perform its own cleanup sequence using vhost_dev data
> -         * (e.g. vhost_dev_set_log).
> -         */
> -        if (realized && !runstate_check(RUN_STATE_SHUTDOWN)) {
> +        if (!runstate_check(RUN_STATE_SHUTDOWN)) {
>              /*
>               * A close event may happen during a read/write, but vhost
>               * code assumes the vhost_dev remains setup, so delay the
> @@ -431,8 +409,6 @@ static void vhost_user_blk_event(void *opaque, 
> QEMUChrEvent event,
>               * knowing its type (in this case vhost-user).
>               */
>              s->dev.started = false;
> -        } else {
> -            vhost_user_blk_disconnect(dev);
>          }
>          break;
>      case CHR_EVENT_BREAK:
> @@ -489,33 +465,32 @@ static void vhost_user_blk_device_realize(DeviceState 
> *dev, Error **errp)
>      s->vhost_vqs = g_new0(struct vhost_virtqueue, s->num_queues);
>      s->connected = false;
>  
> -    qemu_chr_fe_set_handlers(&s->chardev,  NULL, NULL,
> -                             vhost_user_blk_event_realize, NULL, (void *)dev,
> -                             NULL, true);
> -
> -reconnect:
>      if (qemu_chr_fe_wait_connected(&s->chardev, errp) < 0) {
>          goto virtio_err;
>      }
>  
> -    /* check whether vhost_user_blk_connect() failed or not */
> -    if (!s->connected) {
> -        goto reconnect;
> +    if (vhost_user_blk_connect(dev) < 0) {
> +        error_setg(errp, "vhost-user-blk: could not connect");
> +        qemu_chr_fe_disconnect(&s->chardev);
> +        goto virtio_err;
>      }
> +    assert(s->connected);
>  
>      ret = vhost_dev_get_config(&s->dev, (uint8_t *)&s->blkcfg,
>                                 sizeof(struct virtio_blk_config));
>      if (ret < 0) {
> -        error_report("vhost-user-blk: get block config failed");
> -        goto reconnect;
> +        error_setg(errp, "vhost-user-blk: get block config failed");
> +        goto vhost_err;
>      }
>  
> -    /* we're fully initialized, now we can operate, so change the handler */
> +    /* we're fully initialized, now we can operate, so add the handler */
>      qemu_chr_fe_set_handlers(&s->chardev,  NULL, NULL,
> -                             vhost_user_blk_event_oper, NULL, (void *)dev,
> +                             vhost_user_blk_event, NULL, (void *)dev,
>                               NULL, true);
>      return;
>  
> +vhost_err:
> +    vhost_dev_cleanup(&s->dev);
>  virtio_err:
>      g_free(s->vhost_vqs);
>      s->vhost_vqs = NULL;
> -- 
> 2.30.2