netdev - Re: [PATCH net] ipv4: Fix tos mask in inet_rtm

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Date:   Sat, 28 Nov 2020 13:17:16 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Guillaume Nault <gnault@...hat.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Russell Strong <russell@...ong.id.au>
Subject: Re: [PATCH net] ipv4: Fix tos mask in inet_rtm_getroute()

On Sat, 28 Nov 2020 10:03:42 -0700 David Ahern wrote:
> On 11/26/20 11:09 AM, Guillaume Nault wrote:
> > When inet_rtm_getroute() was converted to use the RCU variants of
> > ip_route_input() and ip_route_output_key(), the TOS parameters
> > stopped being masked with IPTOS_RT_MASK before doing the route lookup.
> > 
> > As a result, "ip route get" can return a different route than what
> > would be used when sending real packets.
> > 
> > For example:
> > 
> >     $ ip route add 192.0.2.11/32 dev eth0
> >     $ ip route add unreachable 192.0.2.11/32 tos 2
> >     $ ip route get 192.0.2.11 tos 2
> >     RTNETLINK answers: No route to host
> > 
> > But, packets with TOS 2 (ECT(0) if interpreted as an ECN bit) would
> > actually be routed using the first route:
> > 
> >     $ ping -c 1 -Q 2 192.0.2.11
> >     PING 192.0.2.11 (192.0.2.11) 56(84) bytes of data.
> >     64 bytes from 192.0.2.11: icmp_seq=1 ttl=64 time=0.173 ms
> > 
> >     --- 192.0.2.11 ping statistics ---
> >     1 packets transmitted, 1 received, 0% packet loss, time 0ms
> >     rtt min/avg/max/mdev = 0.173/0.173/0.173/0.000 ms
> > 
> > This patch re-applies IPTOS_RT_MASK in inet_rtm_getroute(), to
> > return results consistent with real route lookups.
> > 
> > Fixes: 3765d35ed8b9 ("net: ipv4: Convert inet_rtm_getroute to rcu versions of route lookup")
> > Signed-off-by: Guillaume Nault <gnault@...hat.com>
> 
> Reviewed-by: David Ahern <dsahern@...nel.org>

Applied, thanks!

Should the discrepancy between the behavior of ip_route_input_rcu() and
ip_route_input() be addressed, possibly?