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

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Date:   Tue, 9 Feb 2021 11:43:40 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Mahesh Bandewar (महेश बंडेवार)" <maheshb@...gle.com>
Cc:     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, 9 Feb 2021 11:18:05 -0800 Mahesh Bandewar (महेश बंडेवार) wrote:
> On Tue, Feb 9, 2021 at 11:04 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > On Tue, 9 Feb 2021 10:49:23 -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:  
> > > > > 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?  
> >
> > The selftest is just a messenger. We all know Linus's stand on
> > regressions, IMO we can't make an honest argument that the change
> > does not break user space after it broke our own selftest. Maybe
> > others disagree..  
> 
> Actually that was the reason behind encompassing this behavior change
> with a sysctl.

Which as I explained to you is pointless for portable applications.