Re: [PATCH] net: ignore sysctl_devconf_inherit_init_net without SYSCTL

[Arnd Bergmann]

On Mon, Mar 4, 2019 at 10:00 PM Christian Brauner <christian@xxxxxxxxxx> 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@xxxxxxxx>
> > ---
> >  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