IPV4_DEVCONF
#define IPV4_DEVCONF_RO(cnf, attr) READ_ONCE(IPV4_DEVCONF(cnf, attr))
IPV4_DEVCONF((*(net)->ipv4.devconf_all), attr)
if (IPV4_DEVCONF(in_dev->cnf, FORWARDING))
IPV4_DEVCONF((*net->ipv4.devconf_dflt), attr)
IPV4_DEVCONF(in_dev->cnf, RP_FILTER) = 0;
IPV4_DEVCONF(in_dev->cnf, MC_FORWARDING)--;
IPV4_DEVCONF(in_dev->cnf, MC_FORWARDING)++;