netdev - Re: [PATCH net-next] net-loopback: allow lo dev initial state to be controlled

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Date:   Mon, 16 Nov 2020 12:50:22 -0800
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     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] net-loopback: allow lo dev initial state to be controlled

On Mon, Nov 16, 2020 at 12:17 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 16 Nov 2020 12:02:48 -0800 Mahesh Bandewar (महेश बंडेवार) wrote:
> > On Sat, Nov 14, 2020 at 10:17 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > > On Wed, 11 Nov 2020 12:43:08 -0800 Jian Yang wrote:
> > > > From: Mahesh Bandewar <maheshb@...gle.com>
> > > >
> > > > Traditionally loopback devices comes up with initial state as DOWN for
> > > > any new network-namespace. This would mean that anyone needing this
> > > > device (which is mostly true except sandboxes where networking in not
> > > > needed at all), would have to bring this UP by issuing something like
> > > > 'ip link set lo up' which can be avoided if the initial state can be set
> > > > as UP. Also ICMP error propagation needs loopback to be UP.
> > > >
> > > > The default value for this sysctl is set to ZERO which will preserve the
> > > > backward compatible behavior for the root-netns while changing the
> > > > sysctl will only alter the behavior of the newer network namespaces.
> >
> > > Any reason why the new sysctl itself is not per netns?
> > >
> > Making it per netns would not be very useful since its effect is only
> > during netns creation.
>
> I must be confused. Are all namespaces spawned off init_net, not the
> current netns the process is in?
The namespace hierarchy is maintained in user-ns while we have per-ns
sysctls hanging off of a netns object and we don't have parent (netns)
reference when initializing newly created netns values. One can copy
the current value of the settings from root-ns but I don't think
that's a good practice since there is no clear way to affect those
values when the root-ns changes them. Also from the isolation
perspective (I think) it's better to have this behavior (sysctl
values) stand on it's own i.e. have default values and then alter
values on it's own without linking to any other netns values.