netdev - Re: [PATCH net-next v3] net-loopback: set lo dev initial state to UP

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Date:   Tue, 9 Feb 2021 21:06:27 +0200
From:   Ido Schimmel <idosch@...sch.org>
To:     Mahesh Bandewar
         (महेश बंडेवार) <maheshb@...gle.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, Petr Machata <petrm@...dia.com>,
        Jian Yang <jianyang.kernel@...il.com>,
        David Miller <davem@...emloft.net>,
        linux-netdev <netdev@...r.kernel.org>,
        Jian Yang <jianyang@...gle.com>
Subject: Re: [PATCH net-next v3] net-loopback: set lo dev initial state to UP

On Tue, Feb 09, 2021 at 10:49:23AM -0800, Mahesh Bandewar (महेश बंडेवार) wrote:
> On Tue, Feb 9, 2021 at 8:23 AM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Tue, 9 Feb 2021 12:54:59 +0100 Petr Machata wrote:
> > > Jian Yang <jianyang.kernel@...il.com> writes:
> > >
> > > > From: Jian Yang <jianyang@...gle.com>
> > > >
> > > > Traditionally loopback devices come up with initial state as DOWN for
> > > > any new network-namespace. This would mean that anyone needing this
> > > > device would have to bring this UP by issuing something like 'ip link
> > > > set lo up'. This can be avoided if the initial state is set as UP.
> > >
> > > This will break user scripts, and it fact breaks kernel's very own
> > > selftest. We currently have this internally:
> > >
> > >     diff --git a/tools/testing/selftests/net/fib_nexthops.sh b/tools/testing/selftests/net/fib_nexthops.sh
> > >     index 4c7d33618437..bf8ed24ab3ba 100755
> > >     --- a/tools/testing/selftests/net/fib_nexthops.sh
> > >     +++ b/tools/testing/selftests/net/fib_nexthops.sh
> > >     @@ -121,8 +121,6 @@ create_ns()
> > >       set -e
> > >       ip netns add ${n}
> > >       ip netns set ${n} $((nsid++))
> > >     - ip -netns ${n} addr add 127.0.0.1/8 dev lo
> > >     - ip -netns ${n} link set lo up
> > >
> > >       ip netns exec ${n} sysctl -qw net.ipv4.ip_forward=1
> > >       ip netns exec ${n} sysctl -qw net.ipv4.fib_multipath_use_neigh=1
> > >
> > > This now fails because the ip commands are run within a "set -e" block,
> > > and kernel rejects addition of a duplicate address.
> >
> > Thanks for the report, could you send a revert with this explanation?
> Rather than revert, shouldn't we just fix the self-test in that regard?

I reviewed such a patch internally and asked Petr to report it as a
regression instead. At the time the new behavior was added under a
sysctl, but nobody had examples for behavior that will break, so the
sysctl was removed. Now we have such an example, so the revert / sysctl
are needed.