netdev - Re: [PATCH net] sit: use ipv6_mod_enabled to check if ipv6 is disabled

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Message-ID: <20190226015533.GR10051@dhcp-12-139.nay.redhat.com>
Date:   Tue, 26 Feb 2019 09:55:33 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     David Ahern <dsa@...ulusnetworks.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
        Stefano Brivio <sbrivio@...hat.com>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] sit: use ipv6_mod_enabled to check if ipv6 is
 disabled

On Mon, Feb 25, 2019 at 09:57:42AM -0700, David Ahern wrote:
> On 2/25/19 9:39 AM, Eric Dumazet wrote:
> >>> On 02/24/2019 08:12 PM, Hangbin Liu wrote:
> >>>> ipv6_mod_enabled() is more safe and gentle to check if ipv6 is disabled
> >>>> at running time.
> >>>
> >>> Why is it better exactly ?
> >>>
> >>> IPv6 can be enabled on the host, but disabled per device
> >>>
> >>> /proc/sys/net/ipv6/conf/{name}/disable_ipv6
> >>
> >> Sorry, it looks I didn't make it clear in the commit description.
> >> This issue only occurs when IPv6 is disabled at boot time as there is
> >> no IPv6 route entry. Disable ipv6 on specific interface is not affected.
> >> So check ipv6_mod_enabled() is enough and we don't need to worry about
> >> the rcu_read_lock or the dev status.
> >>
> >> Should I update the commit description?
> > 
> > Certainly. Are you telling us skb->dev could be NULL here ?
> > 
> > Because rcu_read_lock() should already be asserted.
> > 
> 
> Same response as geneve. The existing check is more appropriate and
> relevant for the code path: is ipv6 enabled on this device versus is
> ipv6 enabled at all.

Hi David,

Just as I said, this issue only occurs when IPv6 is disabled at boot time
as there is no IPv6 route entry. Disable ipv6 on specific interface should
not be affected(IPv6 route/fib has inited). So I think use ipv6_mod_enabled()
would be more suitable in this scenario. Did I miss something?

Thanks
Hangbin