netdev - ESP trailer_len calculation

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Message-Id: <20100924.144044.179940003.davem@davemloft.net>
Date:	Fri, 24 Sep 2010 14:40:44 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	kaber@...sh.net
CC:	herbert@...dor.apana.org.au, eric.dumazet@...il.com,
	netdev@...r.kernel.org
Subject: ESP trailer_len calculation


Eric Dumazet and I recently were looking into a strange artifact in
ESP ->trailer_len calculations.

Eric was seeing values like "17" which looked strange.

He foudn that it's because of this line in esp4.c:esp_init_state()

	x->props.trailer_len = align + 1 + crypto_aead_authsize(esp->aead);

which comes from commit:

commit c5c2523893747f88a83376abad310c8ad13f7197
Author: Patrick McHardy <kaber@...sh.net>
Date:   Mon Apr 9 11:47:18 2007 -0700

    [XFRM]: Optimize MTU calculation

which is based upon discussion threads:

http://marc.info/?l=linux-netdev&m=115468159401118&w=2

and

http://marc.info/?l=linux-netdev&m=117561805827241&w=2

Even more strange, in the orignal version of this patch the
calcaluation is actually:

	x->props.trailer_len = align - 1 + esp->auth.icv_trunc_len;

(ie. 'align - 1' instead of 'align + 1')

It seems that this "- 1 " or "+ 1" term can be completely eliminated,
unless there are some funny semantics wrt. the padding area of ESP.

Patrick and Herbert, what do you guys think?
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