Re: [Qemu-devel] [PATCH] vhost-user: fix qemu crash caused by failed backend

[Marc-André Lureau]

Hi

On Thu, Sep 27, 2018 at 7:37 PM Liang Li <address@hidden> wrote:
>
> During live migration, when stopping vhost-user device, 'vhost_dev_stop'
> will be called, 'vhost_dev_stop' will call a batch of 'vhost_user_read'
> and 'vhost_user_write'. If a previous 'vhost_user_read' or 'vhost_user_write'
> failed because the vhost user backend failed, the 'CHR_EVENT_CLOSED' event
> will be triggerd, followed by the call chain 
> chr_closed_bh()->vhost_user_stop()->
> vhost_net_cleanup()->vhost_dev_cleanup()
>
> vhost_dev_cleanup will clear vhost_dev struct, so the later 'vhost_user_read'
> or 'vhost_user_read' will reference null pointer and cause qemu crash

Do you have a backtrace to help understand the issue?
thanks

>
> Signed-off-by: Liang Li <address@hidden>
> ---
>  hw/net/vhost_net.c        |  6 ++++++
>  hw/virtio/vhost-user.c    | 15 +++++++++++++--
>  include/hw/virtio/vhost.h |  1 +
>  include/net/vhost_net.h   |  1 +
>  net/vhost-user.c          |  3 +++
>  5 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index e037db6..77994e9 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -113,6 +113,11 @@ uint64_t vhost_net_get_features(struct vhost_net *net, 
> uint64_t features)
>              features);
>  }
>
> +void vhost_net_mark_break_down(struct vhost_net *net)
> +{
> +    net->dev.break_down = true;
> +}
> +
>  void vhost_net_ack_features(struct vhost_net *net, uint64_t features)
>  {
>      net->dev.acked_features = net->dev.backend_features;
> @@ -156,6 +161,7 @@ struct vhost_net *vhost_net_init(VhostNetOptions *options)
>      net->dev.max_queues = 1;
>      net->dev.nvqs = 2;
>      net->dev.vqs = net->vqs;
> +    net->dev.break_down = false;
>
>      if (backend_kernel) {
>          r = vhost_net_get_fd(options->net_backend);
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index b041343..1394719 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -213,14 +213,20 @@ static bool ioeventfd_enabled(void)
>  static int vhost_user_read(struct vhost_dev *dev, VhostUserMsg *msg)
>  {
>      struct vhost_user *u = dev->opaque;
> -    CharBackend *chr = u->user->chr;
> +    CharBackend *chr;
>      uint8_t *p = (uint8_t *) msg;
>      int r, size = VHOST_USER_HDR_SIZE;
>
> +    if (dev->break_down) {
> +        goto fail;
> +    }
> +
> +    chr = u->user->chr;
>      r = qemu_chr_fe_read_all(chr, p, size);
>      if (r != size) {
>          error_report("Failed to read msg header. Read %d instead of %d."
>                       " Original request %d.", r, size, msg->hdr.request);
> +        dev->break_down = true;
>          goto fail;
>      }
>
> @@ -299,9 +305,12 @@ static int vhost_user_write(struct vhost_dev *dev, 
> VhostUserMsg *msg,
>                              int *fds, int fd_num)
>  {
>      struct vhost_user *u = dev->opaque;
> -    CharBackend *chr = u->user->chr;
> +    CharBackend *chr;
>      int ret, size = VHOST_USER_HDR_SIZE + msg->hdr.size;
>
> +    if (dev->break_down) {
> +        return -1;
> +    }
>      /*
>       * For non-vring specific requests, like VHOST_USER_SET_MEM_TABLE,
>       * we just need send it once in the first time. For later such
> @@ -312,6 +321,7 @@ static int vhost_user_write(struct vhost_dev *dev, 
> VhostUserMsg *msg,
>          return 0;
>      }
>
> +    chr = u->user->chr;
>      if (qemu_chr_fe_set_msgfds(chr, fds, fd_num) < 0) {
>          error_report("Failed to set msg fds.");
>          return -1;
> @@ -319,6 +329,7 @@ static int vhost_user_write(struct vhost_dev *dev, 
> VhostUserMsg *msg,
>
>      ret = qemu_chr_fe_write_all(chr, (const uint8_t *) msg, size);
>      if (ret != size) {
> +        dev->break_down = true;
>          error_report("Failed to write msg."
>                       " Wrote %d instead of %d.", ret, size);
>          return -1;
> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index a7f449f..86d0dc5 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -74,6 +74,7 @@ struct vhost_dev {
>      bool started;
>      bool log_enabled;
>      uint64_t log_size;
> +    bool break_down;
>      Error *migration_blocker;
>      const VhostOps *vhost_ops;
>      void *opaque;
> diff --git a/include/net/vhost_net.h b/include/net/vhost_net.h
> index 77e4739..06f2c08 100644
> --- a/include/net/vhost_net.h
> +++ b/include/net/vhost_net.h
> @@ -27,6 +27,7 @@ void vhost_net_cleanup(VHostNetState *net);
>
>  uint64_t vhost_net_get_features(VHostNetState *net, uint64_t features);
>  void vhost_net_ack_features(VHostNetState *net, uint64_t features);
> +void vhost_net_mark_break_down(VHostNetState *net);
>
>  bool vhost_net_virtqueue_pending(VHostNetState *net, int n);
>  void vhost_net_virtqueue_mask(VHostNetState *net, VirtIODevice *dev,
> diff --git a/net/vhost-user.c b/net/vhost-user.c
> index a39f9c9..b99e20b 100644
> --- a/net/vhost-user.c
> +++ b/net/vhost-user.c
> @@ -270,6 +270,9 @@ static void net_vhost_user_event(void *opaque, int event)
>          if (s->watch) {
>              AioContext *ctx = qemu_get_current_aio_context();
>
> +            if (s->vhost_net) {
> +                vhost_net_mark_break_down(s->vhost_net);
> +            }
>              g_source_remove(s->watch);
>              s->watch = 0;
>              qemu_chr_fe_set_handlers(&s->chr, NULL, NULL, NULL, NULL,
> --
> 1.8.3.1
>
>


-- 
Marc-André Lureau