netdev - Re: [PATCH] ipv6: Fix the pmtu path for connected UDP socket

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Date:	Thu, 24 Mar 2016 12:23:18 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Wei Wang <tracywwnj@...il.com>, Martin KaFai Lau <kafai@...com>,
	Wei Wang <weiwan@...gle.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] ipv6: Fix the pmtu path for connected UDP socket

On Tue, Mar 22, 2016 at 4:36 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2016-03-22 at 13:13 -0700, Cong Wang wrote:
>> On Tue, Mar 22, 2016 at 11:03 AM, Wei Wang <tracywwnj@...il.com> wrote:
>> > Thanks Martin and Cong.
>> >
>> > I guess then we are going with the following fix in ip6_sk_update_pmtu():
>> > 1. call ip6_upate_pmtu() as it is
>> > 2. do a dst_check()
>> > 3. re-lookup() if it is invalid
>> > 4. and then do a ip6_dst_store()/dst_set
>>
>> Exactly, please try the attached patch. Note I did nothing more than a
>> compile test.
>>
>> Does it make sense to you now?
>
>
> Hard to reply on your patch as it was not inlined.

Sorry about that, it is still not ready to send out formally yet, because
I don't do any run-time tests yet.

>
> 1) Lot of code duplication, for some reason I do not yet understand.
>
> ip6_sk_update_pmtu() and ip6_update_pmtu() will basically do the same
> thing...

Hmm? After my patch ip6_sk_update_pmtu() will take care of sk_dst_cache,
but ip6_update_pmtu() doesn't do that since it has no 'sk' parameter.
It is pretty much similar to commit 9cb3a50c5f63.

>
> 2)
>
> +       if (sk->sk_state == TCP_ESTABLISHED)
> +               ip6_dst_store(sk, dst, &iph->daddr, &iph->saddr);
> +out:
>
>
> ip6_dst_store() will do :
>
> np->daddr_cache = daddr;  (&iph->daddr)
> np->saddr_cache = saddr;  (&iph->saddr)
>
> So when skb is freed, daddr_cache & saddr_cache point to freed data.

Ah, good catch, so ip6_dst_store() can only use the addr's stored
in socket or NULL.

I will send out 2 formal patches shortly.