前分析过linux kernel 2.6.32的bridge转发逻辑，下面分析一下linux kernel 3.10的bridge转发逻辑。这样正是CentOS 5和CentOS 7对应的内核。3.10 kernel中bridge逻辑的最大改变就是增加了vlan处理逻辑以及brdige入口函数的设置。
1. netdev_rx_handler_register
在分析之前首先要介绍一个重要函数：netdev_rx_handler_register，这个函数是2.6内核所没有的。
netdev_rx_handler_register
/* * dev: 要注册接收函数的dev * rx_handler: 要注册的接收函数 * rx_handler_data: 指向rx_handler_data使用的数据 */ int netdev_rx_handler_register(struct net_device *dev, rx_handler_func_t *rx_handler, void *rx_handler_data) { ASSERT_RTNL(); if (dev->rx_handler) return -EBUSY; /* Note: rx_handler_data must be set before rx_handler */ rcu_assign_pointer(dev->rx_handler_data, rx_handler_data); rcu_assign_pointer(dev->rx_handler, rx_handler); return 0; } |
这个函数可以给设备（net_device）注册接收函数，然后在__netif_receive_skb函数中根据接收skb的设备接口，再调用这个被注册的接收函数。比如为网桥下的接口注册br_handle_frame函数，为bonding接口注册bond_handle_frame函数。这相对于老式的网桥处理更灵活，有了这个机制也可以在模块中自行注册处理函数。比如3.10中的openvswitch（OpenvSwitch在3.10已经合入了内核）创建netdev vport的函数netdev_create。
netdev_create
static struct vport *netdev_create(const struct vport_parms *parms)
{
struct vport *vport;
/....../
err = netdev_rx_handler_register(netdev_vport->dev, netdev_frame_hook,vport);
/....../
}
这个函数在创建netdev vport时将设备的接收函数设置为netdev_frame_hook函数，这也是整个openvswitch的入口函数，如果查看OpenvSwitch的源码可以看到当安装于2.6内核时这里是替换掉bridge的br_handle_frame_hook函数，从而由bridge逻辑进入OpenvSwitch逻辑。
2. Bridge转发逻辑分析
还是先从netif_receive_skb函数分析，这个函数算是进入协议栈的入口。
netif_receive_skb
int netif_receive_skb(struct sk_buff *skb) { int ret; if (skb_defer_rx_timestamp(skb)) return NET_RX_SUCCESS; rcu_read_lock(); /*RPS逻辑处理，现在内核中使用了RPS机制, 将报文分散到各个cpu的接收队列中进行负载均衡处理*/ #ifdef CONFIG_RPS if (static_key_false(&rps_needed)) { struct rps_dev_flow voidflow, *rflow = &voidflow; int cpu = get_rps_cpu(skb->dev, skb, &rflow); if (cpu >= 0) { ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail); rcu_read_unlock(); return ret; } } #endif ret = __netif_receive_skb(skb); rcu_read_unlock(); return ret; } |
netif_receive_skb只是对数据包进行了RPS的处理，然后调用__netif_receive_skb。
__netif_receive_skb并没有其他多余的处理逻辑，主要调用 __netif_receive_skb_core，这个函数才真正相当于2.6内核的netif_receive_skb。以下代码省略了和bridge无关的逻辑。
__netif_receive_skb_core
static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc) { struct packet_type *ptype, *pt_prev; rx_handler_func_t *rx_handler; struct net_device *orig_dev; struct net_device *null_or_dev; bool deliver_exact = false; int ret = NET_RX_DROP; __be16 type; /*......*/ orig_dev = skb->dev; skb_reset_network_header(skb); pt_prev = NULL; skb->skb_iif = skb->dev->ifindex; /*ptype_all协议处理，tcpdump抓包就在这里*/ list_for_each_entry_rcu(ptype, &ptype_all, list) { if (!ptype->dev || ptype->dev == skb->dev) { if (pt_prev) ret = deliver_skb(skb, pt_prev, orig_dev); pt_prev = ptype; } } /*调用接收设备的rx_handler*/ rx_handler = rcu_dereference(skb->dev->rx_handler); if (rx_handler) { if (pt_prev) { ret = deliver_skb(skb, pt_prev, orig_dev); pt_prev = NULL; } switch (rx_handler(&skb)) { case RX_HANDLER_CONSUMED: ret = NET_RX_SUCCESS; goto out; case RX_HANDLER_ANOTHER: goto another_round; case RX_HANDLER_EXACT: deliver_exact = true; case RX_HANDLER_PASS: break; default: BUG(); } } /*根据 skb->protocol传递给上层协议*/ type = skb->protocol; list_for_each_entry_rcu(ptype,&ptype_base[ntohs(type) & PTYPE_HASH_MASK], list) { if (ptype->type == type && (ptype->dev == null_or_dev || ptype->dev == skb->dev ||ptype->dev == orig_dev)) { if (pt_prev) ret = deliver_skb(skb, pt_prev, orig_dev); pt_prev = ptype; } } if (pt_prev) { if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC))) goto drop; else ret = pt_prev->func(skb, skb->dev, pt_prev, orig_dev); } else { drop: atomic_long_inc(&skb->dev->rx_dropped); kfree_skb(skb); ret = NET_RX_DROP; } out: return ret; } |
如果一个dev被添加到一个bridge（做为bridge的一个接口），的这个接口设备的rx_handler被设置为br_handle_frame函数，这是在br_add_if函数中设置的，而br_add_if (net/bridge/br_if.c)是在向网桥设备上添加接口时设置的。进入br_handle_frame也就进入了bridge的逻辑代码。
br_add_if
int br_add_if(struct net_bridge *br, struct net_device *dev) { /*......*/ err = netdev_rx_handler_register(dev, br_handle_frame, p); /*......*/ } |
br_handle_frame
rx_handler_result_t br_handle_frame(struct sk_buff **pskb) { struct net_bridge_port *p; struct sk_buff *skb = *pskb; const unsigned char *dest = eth_hdr(skb)->h_dest; br_should_route_hook_t *rhook; if (unlikely(skb->pkt_type == PACKET_LOOPBACK)) return RX_HANDLER_PASS; if (!is_valid_ether_addr(eth_hdr(skb)->h_source)) goto drop; skb = skb_share_check(skb, GFP_ATOMIC); if (!skb) return RX_HANDLER_CONSUMED; /*获取dev对应的bridge port*/ p = br_port_get_rcu(skb->dev); /*特殊目的mac地址的处理*/ if (unlikely(is_link_local_ether_addr(dest))) { /* * See IEEE 802.1D Table 7-10 Reserved addresses * * Assignment Value * Bridge Group Address 01-80-C2-00-00-00 * (MAC Control) 802.3 01-80-C2-00-00-01 * (Link Aggregation) 802.3 01-80-C2-00-00-02 * 802.1X PAE address 01-80-C2-00-00-03 * * 802.1AB LLDP 01-80-C2-00-00-0E * * Others reserved for future standardization */ switch (dest[5]) { case 0x00: /* Bridge Group Address */ /* If STP is turned off,then must forward to keep loop detection */ if (p->br->stp_enabled == BR_NO_STP) goto forward; break; case 0x01: /* IEEE MAC (Pause) */ goto drop; default: /* Allow selective forwarding for most other protocols */ if (p->br->group_fwd_mask & (1u << dest[5])) goto forward; } /* LOCAL_IN hook点，注意经过这个hook点并不代表发送到主机协议栈（只有特殊目的mac 01-80-C2才会走到这里）*/ if (NF_HOOK(NFPROTO_BRIDGE, NF_BR_LOCAL_IN, skb, skb->dev, NULL, br_handle_local_finish)) { return RX_HANDLER_CONSUMED; /* consumed by filter */ } else { *pskb = skb; return RX_HANDLER_PASS; /* continue processing */ } } /*转发逻辑*/ forward: switch (p->state) { case BR_STATE_FORWARDING: rhook = rcu_dereference(br_should_route_hook); if (rhook) { if ((*rhook)(skb)) { *pskb = skb; return RX_HANDLER_PASS; } dest = eth_hdr(skb)->h_dest; } /* fall through */ case BR_STATE_LEARNING: /*skb的目的mac和bridge的mac一样，则将skb发往本机协议栈*/ if (ether_addr_equal(p->br->dev->dev_addr, dest)) skb->pkt_type = PACKET_HOST; /*NF_BR_PRE_ROUTING hook点*/ NF_HOOK(NFPROTO_BRIDGE, NF_BR_PRE_ROUTING, skb, skb->dev, NULL,br_handle_frame_finish); break; default: drop: kfree_skb(skb); } return RX_HANDLER_CONSUMED; } |
