netdev - Re: [Patch net] ipv4: restore rt->fi for reference counting

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Date:   Wed, 10 May 2017 09:32:12 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] ipv4: restore rt->fi for reference counting

On Tue, May 9, 2017 at 4:50 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-05-09 at 16:35 -0700, Cong Wang wrote:
>
>> All of them take RCU read lock, so, as I explained in the code comment,
>> they all should be fine because of synchronize_net() on unregister path.
>> Do you see anything otherwise?
>
> They might take rcu lock, but compiler is still allowed to read
> fi->fib_dev multiple times, and crashes might happen.
>
> You will need to audit all code and fix it, using proper
> rcu_dereference() or similar code ensuring compiler wont do stupid
> things.
>

Point taken. But without my patch, nh_dev is supposed to be protected
by RCU too, it is freed in a rcu callback and dereferenced like:

struct in_device *in_dev = __in_dev_get_rcu(nh->nh_dev);