netdev - 5.1 `ip route get addr/cidr` regression

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Date:   Fri, 17 May 2019 12:22:41 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     emersonbernier@...anota.com, Netdev <netdev@...r.kernel.org>,
        dsahern@...il.com, Stephen Hemminger <stephen@...workplumber.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        David Miller <davem@...emloft.net>
Cc:     piraty1@...ox.ru
Subject: 5.1 `ip route get addr/cidr` regression

Hi,

I'm back now and catching up with a lot of things. A few people have
mentioned to me that wg-quick(8), a bash script that makes a bunch of
iproute2 invocations, appears to be broken on 5.1. I've distilled the
behavior change down to the following.

Behavior on 5.0:

+ ip link add wg0 type dummy
+ ip address add 192.168.50.2/24 dev wg0
+ ip link set mtu 1420 up dev wg0
+ ip route get 192.168.50.0/24
broadcast 192.168.50.0 dev wg0 src 192.168.50.2 uid 0
   cache <local,brd>

Behavior on 5.1:

+ ip link add wg0 type dummy
+ ip address add 192.168.50.2/24 dev wg0
+ ip link set mtu 1420 up dev wg0
+ ip route get 192.168.50.0/24
RTNETLINK answers: Invalid argument

Upon investigating, I'm not sure that `ip route get` was ever suitable
for getting details on a particular route. So I'll adjust the
wg-quick(8) code accordingly. But FYI, this is unexpected userspace
breakage.

Jason

On Sat, Mar 23, 2019 at 2:00 PM <emersonbernier@...anota.com> wrote:
>
> After upgrading iproute2 from 4.20 to 5.0 the following error occurs:
>
> $ ip route show table default
> Error: ipv4: FIB table does not exist.
> Dump terminated
>
> The command works for all tables other than 'default' one. It seems related to this commit[1]
>
> I also saw "Error: ipv4: MR table does not exist." message in logs related to this commit[2] but don't know exact command to reproduce it. I've seen some fixups[3] for mentioned commits and wonder it they weren't complete.
>
> I reproduced it on Linux 4.20.17 and 5.0.3.
>
> [1] https://github.com/torvalds/linux/commit/18a8021a7be3207686851208f91a2f105b2d4703#diff-04a14e4f51765994f87e7e5e7681d0e1R861 <https://github.com/torvalds/linux/commit/18a8021a7be3207686851208f91a2f105b2d4703#diff-04a14e4f51765994f87e7e5e7681d0e1R861>
>
> [2] https://github.com/torvalds/linux/commit/cb167893f41e21e6bd283d78e53489289dc0592d#diff-9900db808ce5e5dd24a7341cd8ed1609R2545 <https://github.com/torvalds/linux/commit/cb167893f41e21e6bd283d78e53489289dc0592d#diff-9900db808ce5e5dd24a7341cd8ed1609R2545>
>
> [3] https://github.com/torvalds/linux/commit/73155879b3c1ac3ace35208a54a3a160ec520bef