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.

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

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 hw/block/vhost-user-blk.c | 54 ++++++++++-----------------------------
 1 file changed, 13 insertions(+), 41 deletions(-)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index f5e9682703..e824b0a759 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:
@@ -490,31 +466,27 @@ 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, &err) < 0) {
         error_report_err(err);
         goto virtio_err;
     }
 
-    /* check whether vhost_user_blk_connect() failed or not */
-    if (!s->connected) {
-        goto reconnect;
+    if (vhost_user_blk_connect(dev) < 0) {
+        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;
+        goto virtio_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;
 
-- 
2.30.2

Reviewed-by: Denis Plotnikov <den-plotnikov@yandex-team.ru>

On 22.04.2021 20:02, 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.
>
> 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
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   hw/block/vhost-user-blk.c | 54 ++++++++++-----------------------------
>   1 file changed, 13 insertions(+), 41 deletions(-)
>
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index f5e9682703..e824b0a759 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:
> @@ -490,31 +466,27 @@ 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, &err) < 0) {
>           error_report_err(err);
>           goto virtio_err;
>       }
>   
> -    /* check whether vhost_user_blk_connect() failed or not */
> -    if (!s->connected) {
> -        goto reconnect;
> +    if (vhost_user_blk_connect(dev) < 0) {
> +        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;
> +        goto virtio_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;
>   

Given what you've shown with the use-after-free, I agree the changes
need to be reverted. I'm a little uneasy about removing the reconnect
logic from the device realization completely though.

On Thu, Apr 22, 2021 at 07:02:17PM +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.
> 

Is that nessesarily true? As I understand it the main usecases for
device reconnect are to allow a device backend to be restarted after a
failure or to allow the backend to be upgraded without restarting the
guest. I agree - misconfiguration could be a common cause of a device
backend crashing at realize time, but couldn't there be others? Maybe
transient memory pressure?

Especially in the case where one process is connecting to many different
vhost-user-blk instances, I could imagine power-ons and incoming
migrations racing with backend restarts quite frequently. Should
these cases cause failures?

We can still hit the infinite looping case you describe post-realize.
Why should we treat pre-realize differently?

> 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
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  hw/block/vhost-user-blk.c | 54 ++++++++++-----------------------------
>  1 file changed, 13 insertions(+), 41 deletions(-)
> 
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index f5e9682703..e824b0a759 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:
> @@ -490,31 +466,27 @@ 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, &err) < 0) {
>          error_report_err(err);
>          goto virtio_err;
>      }
>  
> -    /* check whether vhost_user_blk_connect() failed or not */
> -    if (!s->connected) {
> -        goto reconnect;
> +    if (vhost_user_blk_connect(dev) < 0) {
> +        qemu_chr_fe_disconnect(&s->chardev);
> +        goto virtio_err;
>      }
> +    assert(s->connected);

Maybe a good compromise here would be to retry some small number of
times (or even just once) so that cases like daemon upgrades and
recoverable crashes racing with power-ons and incoming migrations
don't result in failures? 

As a more general solution, we could have a user defined parameter to
specify a number of repeated connection failures to allow both pre and
post realize before bringing QEMU down. Thoughts?

>  
>      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;
> +        goto virtio_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;
>  
> -- 
> 2.30.2
> 

On 4/22/21 7:02 PM, 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.
> 
> 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

TIL initialisation wording.

> 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
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  hw/block/vhost-user-blk.c | 54 ++++++++++-----------------------------
>  1 file changed, 13 insertions(+), 41 deletions(-)