netdev - Re: [PATCH net] ipv4: Update exception handling for multipath routes via same device

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Date:   Tue, 15 Sep 2020 15:46:18 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     dsahern@...nel.org
Cc:     netdev@...r.kernel.org, kuba@...nel.org, mastertheknife@...il.com
Subject: Re: [PATCH net] ipv4: Update exception handling for multipath
 routes via same device

From: David Ahern <dsahern@...nel.org>
Date: Mon, 14 Sep 2020 21:03:54 -0600

> Kfir reported that pmtu exceptions are not created properly for
> deployments where multipath routes use the same device.
> 
> After some digging I see 2 compounding problems:
> 1. ip_route_output_key_hash_rcu is updating the flowi4_oif *after*
>    the route lookup. This is the second use case where this has
>    been a problem (the first is related to use of vti devices with
>    VRF). I can not find any reason for the oif to be changed after the
>    lookup; the code goes back to the start of git. It does not seem
>    logical so remove it.
> 
> 2. fib_lookups for exceptions do not call fib_select_path to handle
>    multipath route selection based on the hash.
> 
> The end result is that the fib_lookup used to add the exception
> always creates it based using the first leg of the route.
 ...
> Before this patch the cache always shows exceptions against the first
> leg in the multipath route; 192.168.252.250 per this example. Since the
> hash has an initial random seed, you may need to vary the final octet
> more than what is listed. In my tests, using addresses between 11 and 19
> usually found 1 that used both legs.
> 
> With this patch, the cache will have exceptions for both legs.
> 
> Fixes: 4895c771c7f0 ("ipv4: Add FIB nexthop exceptions")
> Reported-by: Kfir Itzhak <mastertheknife@...il.com>
> Signed-off-by: David Ahern <dsahern@...nel.org>

Applied and queued up for -stable, thanks David.

The example topology and commands look like a good addition for
selftests perhaps?

Thanks again.