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

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Date:	Fri, 18 Mar 2016 10:19:31 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, weiwan@...gle.com,
	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 Wed, Mar 16, 2016 at 8:34 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2016-03-16 at 22:38 -0400, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Wed, 16 Mar 2016 17:22:07 -0700
>>
>> > One of the issue is that IPV6_MTU getsockopt() will not check the dst,
>> > but simply use __sk_dst_get() : It will then report old mtu.
>>
>> That's a bug.
>>
>> ipv4 does it right with a proper sk_dst_get() and so should
>> ipv6.
>
> Using rcu + __sk_dst_get() (in IPv6) is absolutely equivalent to
> sk_dst_get() + dst_release() (in IPv4), modulo atomic ops on dst
> refcnt...
>
> Presumably IPv6 implementation using rcu is slightly better if this
> getsockopt() is badly needed, but apparently nobody cares.
>
> sk_dst_check() is a different beast.
>
> The problem is that dst_mtu(dst) is not able to perform a route lookup
> by itself.
>
> Do we really want to use ip6_sk_dst_lookup_flow() and its associated
> setup for this IP6_MTU thing, and maybe other points we might use an
> obsolete dst ?
>
> Looking at the complexity of udpv6_sendmsg() and rawv6_sendmsg() I
> really wonder if it is worth extracting the route logic.

I think the reason why IPv4 does this correctly is ipv4_sk_update_pmtu()
takes care of the sk cached dst, while the similar part for IPv6,
ip6_sk_update_pmtu() does not that. As Martin already points it out
in the previous version.