linux-kernel - Re: [PATCH] net: ignore sysctl_devconf_inherit_init

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Date:   Mon, 4 Mar 2019 22:07:17 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Christian Brauner <christian@...uner.io>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Zhu Yanjun <Yanjun.Zhu@...driver.com>,
        Tonghao Zhang <xiangxia.m.yue@...il.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        David Ahern <dsahern@...il.com>,
        Xin Long <lucien.xin@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: ignore sysctl_devconf_inherit_init_net without SYSCTL

On Mon, Mar 4, 2019 at 10:00 PM Christian Brauner <christian@...uner.io> wrote:
>
> On Mon, Mar 04, 2019 at 09:38:03PM +0100, Arnd Bergmann wrote:
> > When CONFIG_SYSCTL is turned off, we get a link failure for
> > the newly introduced tuning knob.
> >
> > net/ipv6/addrconf.o: In function `addrconf_init_net':
> > addrconf.c:(.text+0x31dc): undefined reference to `sysctl_devconf_inherit_init_net'
> >
> > Add an IS_ENABLED() check to fall back to the default behavior
> > (sysctl_devconf_inherit_init_net=0) here.
> >
> > Fixes: 856c395cfa63 ("net: introduce a knob to control whether to inherit devconf config")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  net/ipv4/devinet.c  | 4 +++-
> >  net/ipv6/addrconf.c | 3 ++-
> >  2 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
> > index cd9033245b98..eb514f312e6f 100644
> > --- a/net/ipv4/devinet.c
> > +++ b/net/ipv4/devinet.c
> > @@ -2614,7 +2614,9 @@ static __net_init int devinet_init_net(struct net *net)
> >       tbl[0].extra2 = net;
> >  #endif
> >
> > -     if (sysctl_devconf_inherit_init_net != 2 && !net_eq(net, &init_net)) {
> > +     if ((!IS_ENABLED(CONFIG_SYSCTL) ||
> > +          sysctl_devconf_inherit_init_net != 2) &&
>
> I'm sure I'm just being stupid here but wouldn't this still trigger the
> warning or is sysctl_devconf_inherit_init_net unconditionally defined?
> The error you're reporting makes it look like it isn't.

The declaration is unconditional in include/linux/netdevice.h, so the
compiler does not see a problem, it simply doesn't create a reference
when it sees that the code path is dead.

The error message I quoted is from the linker, so this is in turn
not a problem after there is no reference to the variable.

      Arnd