From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stefano Brivio <sbrivio@redhat.com>
To: passt-dev@passt.top
Subject: Re: [PATCH 17/18] tests: Correct determination of host interface name
 in tests
Date: Tue, 19 Jul 2022 21:05:52 +0200
Message-ID: <20220719210552.45329b70@elisabeth>
In-Reply-To: <YtZNPZcskrY+P+Cv@yekko>
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On Tue, 19 Jul 2022 16:20:45 +1000
David Gibson <david(a)gibson.dropbear.id.au> wrote:

> On Fri, Jul 15, 2022 at 03:21:40PM +1000, David Gibson wrote:
> > By default, passt itself attaches to the first host interface with a
> > default route.  However, when determining the host interface name the tes=
ts
> > implicitly select the *last* host interface: they use a jq expression whi=
ch
> > will list all interfaces with default routes, but the way output detection
> > works in the scripts, it will only pick up the last line.
> >=20
> > If there are multiple interfaces with default routes on the host, and they
> > each have a different address, this can cause spurious test
> > failures. =20
>=20
> It seems this change is not enough to always fix the tests when there
> are multiple default routes.  I'm still sometimes getting failures,
> now because passt itself doesn't seem to be picking the interface
> with the first default route.
>=20
> I'm wondering if this is because ip(8) is sorting the output, not just
> presenting it in the same order that the underlying netlink interface
> does.

I don't see that happening at least in my environment (and I also can't
see any code that would sort it, that pretty much comes from
rtnl_dump_filter_l() in lib/libnetlink.c).

The netlink "filter", though, is slightly different. For IPv4:

$ strace -e sendto ip route show >/dev/null
sendto(3, [{{len=3D36, type=3DRTM_GETROUTE, flags=3DNLM_F_REQUEST|NLM_F_DUMP,=
 seq=3D1658257027, pid=3D0}, {rtm_family=3DAF_INET, rtm_dst_len=3D0, rtm_src_=
len=3D0, rtm_tos=3D0, rtm_table=3DRT_TABLE_UNSPEC, rtm_protocol=3DRTPROT_UNSP=
EC, rtm_scope=3DRT_SCOPE_UNIVERSE, rtm_type=3DRTN_UNSPEC, rtm_flags=3D0}, {{n=
la_len=3D8, nla_type=3DRTA_TABLE}, RT_TABLE_MAIN}}, {len=3D0, type=3D0 /* NLM=
SG_??? */, flags=3D0, seq=3D0, pid=3D0}], 156, 0, NULL, 0) =3D 156

$ strace -e sendto ./passt -f
sendto(5, {{len=3D28, type=3DRTM_GETROUTE, flags=3DNLM_F_REQUEST|NLM_F_DUMP, =
seq=3D0, pid=3D0}, {rtm_family=3DAF_INET, rtm_dst_len=3D0, rtm_src_len=3D0, r=
tm_tos=3D0, rtm_table=3DRT_TABLE_MAIN, rtm_protocol=3DRTPROT_UNSPEC, rtm_scop=
e=3DRT_SCOPE_UNIVERSE, rtm_type=3DRTN_UNICAST, rtm_flags=3D0}}, 28, 0, NULL, =
0) =3D 28
[...]

and, while I don't think the FIB trie is actually descended in a
different way, we might still have a slightly different result from the
kernel (if I recall correctly, I didn't check right now).

But letting that aside for a moment: if you have two default routes, I
suppose they have different metrics. If not, what's the intended usage?

If yes, we should probably implement a sorting logic in passt, so that
the route with the lowest metric is picked, and then adjust the jq
expression to also pick that one.

--=20
Stefano


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