netdev - Re: [PATCH net-next] macvlan: fix failure during registration v2

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Date:	Thu, 21 Apr 2016 12:44:02 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Francesco Ruggeri <fruggeri@...sta.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Mahesh Bandewar <maheshb@...gle.com>
Subject: Re: [PATCH net-next] macvlan: fix failure during registration v2

Francesco Ruggeri <fruggeri@...sta.com> writes:

> If macvlan_common_newlink fails in register_netdevice after macvlan_init
> then it decrements port->count twice, first in macvlan_uninit (from
> register_netdevice or rollback_registered) and then again in
> macvlan_common_newlink.
> A similar problem may exist in the ipvlan driver.
> This patch consolidates modifications to port->count into macvlan_init
> and macvlan_uninit (thanks to Eric Biederman for suggesting this approach).
> In macvtap_device_event it also avoids cleaning up in NETDEV_UNREGISTER
> if NETDEV_REGISTER had previously failed.
>
> Signed-off-by: Francesco Ruggeri <fruggeri@...sta.com>

The macvlan_init bits obviously corect.

The macvtap bits I don't really understand what is going on.


> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index 95394ed..e770221 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -1303,6 +1303,8 @@ static int macvtap_device_event(struct notifier_block *unused,
>  		}
>  		break;
>  	case NETDEV_UNREGISTER:
> +		if (vlan->minor == 0)
> +			break;

I don't understand this bit.  A minor of 0 is never assigned.  That is
clear from the code.  On what code path can you get here without
assigning a minor?

My gut says this either needs a big fat comment explaining what is going
on, or a slight refactoring of the code (like moving port count
increments into macvlan_init) that makes it so this code path can't
happen.

>  		devt = MKDEV(MAJOR(macvtap_major), vlan->minor);
>  		device_destroy(macvtap_class, devt);
>  		macvtap_free_minor(vlan);

Eric