Re: [PATCH] net/vhost-net: do not assert on null pointer return from tap_get_vhost_net()

[Michael S. Tsirkin]

On Wed, Jun 28, 2023 at 04:58:04PM +0530, Ani Sinha wrote:
> When 'vhost=off' or no vhost specific options at all are passed for the tap
> net-device backend, tap_get_vhost_net() can return NULL. The function
> net_init_tap_one() does not call vhost_net_init() on such cases and therefore
> vhost_net pointer within the tap device state structure remains NULL. Hence,
> assertion here on a NULL pointer return from tap_get_vhost_net() would not be
> correct. Remove it and fix the crash generated by qemu upon initialization in
> the following call chain :
> 
> qdev_realize() -> pci_qdev_realize() -> virtio_device_realize() ->
> virtio_bus_device_plugged() -> virtio_net_get_features() -> get_vhost_net()
> 
> fixes: 0e994668d00c9c ("vhost_net: add an assertion for TAP client backends")
> Signed-off-by: Ani Sinha <anisinha@redhat.com>

I added a bunch of tags and sent it upstream. Take a look
at the pull request so you can do it yourself going
forward, pls.

> ---
>  hw/net/vhost_net.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index 6db23ca323..6b958d6363 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -507,7 +507,12 @@ VHostNetState *get_vhost_net(NetClientState *nc)
>      switch (nc->info->type) {
>      case NET_CLIENT_DRIVER_TAP:
>          vhost_net = tap_get_vhost_net(nc);
> -        assert(vhost_net);
> +        /*
> +         * tap_get_vhost_net() can return NULL if a tap net-device backend is
> +         * created with 'vhost=off' option, 'vhostforce=off' or no vhost or
> +         * vhostforce or vhostfd options at all. Please see 
> net_init_tap_one().
> +         * Hence, we omit the assertion here.
> +         */
>          break;
>  #ifdef CONFIG_VHOST_NET_USER
>      case NET_CLIENT_DRIVER_VHOST_USER:
> -- 
> 2.39.1