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

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Date:	Wed, 16 Mar 2016 20:34:12 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	weiwan@...gle.com, edumazet@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH] ipv6: Fix the pmtu path for connected UDP socket

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.