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

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Message-ID: <df9f765f-2f73-d136-e020-de1b63963535@cumulusnetworks.com>
Date:   Mon, 25 Feb 2019 09:57:42 -0700
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Hangbin Liu <liuhangbin@...il.com>
Cc:     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 2/25/19 9:39 AM, Eric Dumazet wrote:
> 
> 
> On 02/25/2019 12:17 AM, Hangbin Liu wrote:
>> On Sun, Feb 24, 2019 at 08:24:51PM -0800, 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.