netdev - Re: [PATCH] ipv6: ip6_append_data_mtu do not handle the mtu of the second fragment properly

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Message-ID: <CADvbK_fS0rhPodczcZ52YN9D7NBY_TG+6xXvt-shvvz+e3qLnw@mail.gmail.com>
Date:	Wed, 12 Mar 2014 10:40:50 +0800
From:	lucien xin <lucien.xin@...il.com>
To:	Xin Long <lucien.xin@...il.com>,
	network dev <netdev@...r.kernel.org>,
	Gao feng <gaofeng@...fujitsu.com>
Subject: Re: [PATCH] ipv6: ip6_append_data_mtu do not handle the mtu of the
 second fragment properly

On Tue, Mar 11, 2014 at 10:49 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
>
> Sorry, that is not correct:
>
> The min() protects the mtu going over np->frag_size (if set). In case we
> remove the min we would fallback to dev->mtu or dst_mtu and thus this could
> lead to a situation where the first fragment respects frag_size but second
> not. This confuses ip6_append_data and would lead to a crash.
>
yes, your analysis is quite right,  I ignore the code:
                if (np->frag_size < mtu) {
                        if (np->frag_size)
                                mtu = np->frag_size;
                }
> I am thinking about changing this to
>
> min(*mtu + rt->dst.header_len, pmtuprobe ? rt->dst.dev->mtu : dst_mtu(rt->dst.path))
>
> or we pass the np directly and test for frag_size again.

but I cannot understand the top half of ip6_append_data() has the code
to get mtu,
                if (rt->dst.flags & DST_XFRM_TUNNEL)
                        mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
                              rt->dst.dev->mtu : dst_mtu(&rt->dst);
                else
                        mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
                              rt->dst.dev->mtu : dst_mtu(rt->dst.path);

 why it need to calculate mtu again?  just "mtu=*mtu +
rt->dst.header_len", isn't it sufficient?
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