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

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Date:	Tue, 11 Mar 2014 15:49:06 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Xin Long <lucien.xin@...il.com>
Cc:	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 12:31:49PM +0800, Xin Long wrote:
> -			*mtu = min(*mtu, pmtuprobe ?
> -				   rt->dst.dev->mtu :
> -				   dst_mtu(rt->dst.path));
> +			*mtu = pmtuprobe ? rt->dst.dev->mtu :
> +				   dst_mtu(rt->dst.path);

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.

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.

Good catch which should be fixed. Thanks!

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