Debugging mac address of bridge

# strace brctl addbr br0
socket(PF_FILE, SOCK_STREAM, 0)         = 3
ioctl(3, SIOCBRADDBR, 0x7fff0b1240c6)   = 0
exit_group(0)                           = ?

# ifconfig br0 (got a random address)
br0       Link encap:Ethernet  HWaddr 5a:34:f1:ba:8f:40 

# strace brctl addif br0 eth0 (no ioctl releated with changing mac)
ioctl(4, SIOCGIFINDEX, {ifr_name="eth0", ifr_index=2}) = 0
close(4)                                = 0
ioctl(3, SIOCBRADDIF, 0x7fff57c335c0)   = 0
exit_group(0)                           = ?

# ifconfig br0 (got same addr as eth0)
br0       Link encap:Ethernet  HWaddr 00:22:68:16:c9:e8  

# grep SIOCBRADDIF -nr net/bridge/
net/bridge/br_ioctl.c:409:      case SIOCBRADDIF:
net/bridge/br_ioctl.c:411:              return add_del_if(br, rq->ifr_ifindex, cmd == SIOCBRADDIF);

int br_dev_ioctl(struct net_device *dev, struct ifreq *rq, int cmd)
        case SIOCBRDELIF:
                return add_del_if(br, rq->ifr_ifindex, cmd == SIOCBRADDIF);
net/bridge/br_ioctl.c      v
static int add_del_if(struct net_bridge *br, int ifindex, int isadd)
        if (isadd)
                ret = br_add_if(br, dev);
net/bridge/br_if.c:        v
int br_add_if(struct net_bridge *br, struct net_device *dev)
        changed_addr = br_stp_recalculate_bridge_id(br);
net/bridge/br_stp_if.c:    v
bool br_stp_recalculate_bridge_id(struct net_bridge *br)
        const unsigned char *br_mac_zero =
                        (const unsigned char *)br_mac_zero_aligned;
        const unsigned char *addr = br_mac_zero;
        struct net_bridge_port *p;
        /* user has chosen a value so keep it */ flags & BR_SET_MAC_ADDR)
                return false;
        list_for_each_entry(p, &br->port_list, list) {
                if (addr == br_mac_zero ||
                    memcmp(p->dev->dev_addr, addr, ETH_ALEN) dev->dev_addr; <---- a 'min' mac address in port_list will be set to bridge

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