Re: [PATCH 1/5] vhost-user-blk: Don't reconnect during initialisation

[Raphael Norwitz]

On Wed, Apr 28, 2021 at 07:31:13PM +0200, Kevin Wolf wrote:
> Am 28.04.2021 um 18:52 hat Raphael Norwitz geschrieben:
> > 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?
> 
> I think there is one main difference between realize() and later
> operation, which is that we can actually deliver an error to the user
> during realize(). When we're just starting QEMU and processing the CLI
> arguments, failure is very obvious, and in the context of QMP
> device-add, the client is actively waiting for a result, too.
> 
> Later on, there is no good way to communicate an error (writes to stderr
> just end up in some logfile at best, QAPI events can be missed), and
> even if there were, we would have to do something with the guest until
> the user/management application actually reacts to the error. The latter
> is not a problem during realize() because the device wasn't even plugged
> in yet.
> 
> So I think there are good reasons why it could make sense to distinguish
> initialisation and operation.
>

Fair enough. I'm happy in this case, provided we remain resiliant
against one-off daemon restarts during realize.

> > > 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?
> 
> Retrying once or even a small number of times sounds reasonable enough.
> At first I thought it wouldn't help because restarting the daemon might
> take some time, but with qemu_chr_fe_wait_connected() we already wait
> until we can successfully connect in each iteration, and we would only
> retry for errors that happen afterwards.
> 
> But I think what we really want to do before retrying is distinguishing
> errors that are actually related to the connection itself from errors
> that relate to the content of the communication (i.e. invalid requests
> or configuration).
> In fact, I think the error I hit wasn't even produced on the remote
> side, it came from QEMU itself. Making vhost_user_blk_connect() return
> -EAGAIN in the right cases and reconnecting only there sounds much
> better than just blindly retrying.
> 

Agreed - we should definitely identify any invalid requests or
configuration and treat them differently to legitimate cases.

> Of course, adjusting error reporting so that we can distinguish these
> cases will probably touch much more places than would be appropriate for
> this patch, so I'd suggest that we indeed just revert the reconnection
> during realize() in this series, but then try to add some more selective
> retry logic back on top of it (without using the event handler, which
> only made everything more complicated in a function that waits
> synchronously anyway).

Sounds good to me. I don't see a good reason to use an event handler in
realize since it is synchronous.

> 
> Kevin
>