linux-kernel - Re: [PATCH net] tcp: Export tcp_write_queue

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Message-ID: <be7ed9b0-e23d-e12b-4e6b-108853ec4776@gmail.com>
Date:   Thu, 30 Jul 2020 15:03:19 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] tcp: Export tcp_write_queue_purge()

On 7/30/20 2:32 PM, Eric Dumazet wrote:
> On Thu, Jul 30, 2020 at 2:24 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>>
>> On 7/30/20 2:16 PM, Eric Dumazet wrote:
>>> On Thu, Jul 30, 2020 at 2:07 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>>>>
>>>> After tcp_write_queue_purge() got uninlined with commit ac3f09ba3e49
>>>> ("tcp: uninline tcp_write_queue_purge()"), it became no longer possible
>>>> to reference this symbol from kernel modules.
>>>>
>>>> Fixes: ac3f09ba3e49 ("tcp: uninline tcp_write_queue_purge()")
>>>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>>>> ---
>>>>  net/ipv4/tcp.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
>>>> index 6f0caf9a866d..ea9d296a8380 100644
>>>> --- a/net/ipv4/tcp.c
>>>> +++ b/net/ipv4/tcp.c
>>>> @@ -2626,6 +2626,7 @@ void tcp_write_queue_purge(struct sock *sk)
>>>>         tcp_sk(sk)->packets_out = 0;
>>>>         inet_csk(sk)->icsk_backoff = 0;
>>>>  }
>>>> +EXPORT_SYMBOL(tcp_write_queue_purge);
>>>>
>>>>  int tcp_disconnect(struct sock *sk, int flags)
>>>>  {
>>>> --
>>>> 2.17.1
>>>>
>>>
>>> Hmmm.... which module would need this exactly ?
>>
>> None in tree unfortunately, and I doubt it would be published one day.
>> For consistency one could argue that given it used to be accessible, and
>> other symbols within net/ipv4/tcp.c are also exported, so this should
>> one be. Not going to hold that line of argumentation more than in this
>> email, if you object to it, that would be completely fine with me.
> 
> :)
> 
>>
>>>
>>> How come it took 3 years to discover this issue ?
>>
>> We just upgraded our downstream kernel from 4.9 to 5.4 and this is why
>> it took so long.
> 
> It is not because TCP used an inline function in the past that it
> means we have to keep
> the equivalent function available for all possible out-of-tree modules.
> 
> Sorry, we can not accept that out-of-tree modules use TCP stack like that.
> 
> You will have to carry this change locally. Or even better get rid of it.

Sure, that is completely fair, I had to try though :)
-- 
Florian