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

Jian Yang [off-list ref] writes:

From: Jian Yang <redacted>

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
    -- 
    2.26.2

This now fails because the ip commands are run within a "set -e" block,
and kernel rejects addition of a duplicate address.