netdev - net/ipv4/route.c: Routing anomaly introduced by 89aef89 (ipv4: Delete routing cache.)

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Message-ID: <20130708113457.4a0d6023@pluto>
Date:	Mon, 8 Jul 2013 11:34:57 +0300
From:	Shmulik Ladkani <shmulik.ladkani@...il.com>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	Alexey Kuznetsov <kuznet@....inr.ac.ru>, netdev@...r.kernel.org,
	eyal.birger@...il.com
Subject: net/ipv4/route.c: Routing anomaly introduced by 89aef89 (ipv4:
 Delete routing cache.)

Hi,

In 89aef89 (ipv4: Delete routing cache) the following was removed from
ip_route_input_common (nowadays named ip_route_input_noref):

-       tos &= IPTOS_RT_MASK;

As a side effect, the 'tos' argument passed to 'ip_route_input_slow' is
the "raw" u8, as present in iph->tos (opposed to the formerly "masked"
value).

This may affect the route calculation, as this value is later placed in
fl4.flowi4_tos for matching purposes (using equality to the tos stored
in the FIB Rule and/or Route).

(BTW, the question whether applying the historical IPTOS_RT_MASK
prior route lookup is interesting on its own...)

However, 89aef89 created an anomaly - since in the output path, routing
*still* uses the IPTOS_RT_MASK (see __ip_route_output_key for example).

It seems the omission of 'tos &= IPTOS_RT_MASK' from
'ip_route_input_noref' was accidental.

We should align input/output behavior by reverting the omission, or
alternatively we may reconsider the entire IPTOS_RT_MASK thing.

Regards,
Shmulik
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