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

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Message-ID: <1dd57d9d-fed2-67b8-ac28-7ef3681eeed2@gmail.com>
Date:   Mon, 25 Feb 2019 08:39:27 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Hangbin Liu <liuhangbin@...il.com>
Cc:     netdev@...r.kernel.org, Stefano Brivio <sbrivio@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        David Ahern <dsa@...ulusnetworks.com>
Subject: Re: [PATCH net] sit: use ipv6_mod_enabled to check if ipv6 is
 disabled



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.