學習Linux-4.12核心網路協議棧（3.1）——網路層的收包處理
阿新 • • 發佈：2019-01-05
現在我們將進入傳輸層的分析：
在前面我們知道，資料包到達介面層的時候，它會根據ptype_base來查詢包的型別，並根據包的型別交給不同的網路層函式處理，比如ip_recv，icmp_recv等，現在我們就來看看網路層是怎麼處理不同型別的包的：
1. IP私有資訊控制塊
40 struct inet_skb_parm { 41 int iif; 42 struct ip_options opt; /* Compiled IP options */ IP頭部的選項都存放在這裡 43 u16 flags; 44 45 #define IPSKB_FORWARDED BIT(0) 46 #define IPSKB_XFRM_TUNNEL_SIZE BIT(1) 47 #define IPSKB_XFRM_TRANSFORMED BIT(2) 48 #define IPSKB_FRAG_COMPLETE BIT(3) 49 #define IPSKB_REROUTED BIT(4) 50 #define IPSKB_DOREDIRECT BIT(5) 51 #define IPSKB_FRAG_PMTU BIT(6) 52 #define IPSKB_L3SLAVE BIT(7) 53 54 u16 frag_max_size; 55 };
2. ip_rcv
404 /* 405 * Main IP Receive routine. 406 */ 407 int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt, struct net_device *orig_dev) 408 { 409 const struct iphdr *iph; //用於操作IP頭部 410 struct net *net; 411 u32 len; 412 413 /* When the interface is in promisc. mode, drop all the crap 414 * that it receives, do not try to analyse it. 415 */ 416 if (skb->pkt_type == PACKET_OTHERHOST) //丟掉不是去往本地的包，比如在抓包模式下有很多包不是到本地的 417 goto drop; 418 419 420 net = dev_net(dev);//獲取網路空間 421 __IP_UPD_PO_STATS(net, IPSTATS_MIB_IN, skb->len); 422 423 skb = skb_share_check(skb, GFP_ATOMIC); //如果資料包是一個共享資料包，則需要複製一個備份在作進一步處理，以免在處理過程中資料被修改 424 if (!skb) { 425 __IP_INC_STATS(net, IPSTATS_MIB_INDISCARDS); 426 goto out; 427 } 428 429 if (!pskb_may_pull(skb, sizeof(struct iphdr)))//通過資料包長度判斷資料包是否有效，不能小於IP首部長度 430 goto inhdr_error; 431 432 iph = ip_hdr(skb); //獲取IP首部 434 /* 435 * RFC1122: 3.2.1.2 MUST silently discard any IP frame that fails the checksum. 436 * 437 * Is the datagram acceptable? 438 * 439 * 1. Length at least the size of an ip header 440 * 2. Version of 4 441 * 3. Checksums correctly. [Speed optimisation for later, skip loopback checksums] 442 * 4. Doesn't have a bogus length 443 */ 444 445 if (iph->ihl < 5 || iph->version != 4) //IP首部至少20個bit，也就是說至少要2的5次方32才能放下，如果小於5說明頭部不對；協議版本號是IPV4 446 goto inhdr_error; 447 448 BUILD_BUG_ON(IPSTATS_MIB_ECT1PKTS != IPSTATS_MIB_NOECTPKTS + INET_ECN_ECT_1); 449 BUILD_BUG_ON(IPSTATS_MIB_ECT0PKTS != IPSTATS_MIB_NOECTPKTS + INET_ECN_ECT_0); 450 BUILD_BUG_ON(IPSTATS_MIB_CEPKTS != IPSTATS_MIB_NOECTPKTS + INET_ECN_CE); 451 __IP_ADD_STATS(net, 452 IPSTATS_MIB_NOECTPKTS + (iph->tos & INET_ECN_MASK), 453 max_t(unsigned short, 1, skb_shinfo(skb)->gso_segs)); 454 455 if (!pskb_may_pull(skb, iph->ihl*4)) //根據首部長度檢測資料包是否有效 456 goto inhdr_error; 457 458 iph = ip_hdr(skb); 459 460 if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))//檢測首部的校驗和是否有效 461 goto csum_error; 463 len = ntohs(iph->tot_len); 464 if (skb->len < len) { //根據IP頭部的總長度檢測資料包是否有效 465 __IP_INC_STATS(net, IPSTATS_MIB_INTRUNCATEDPKTS); 466 goto drop; 467 } else if (len < (iph->ihl*4)) 468 goto inhdr_error; 469 470 /* Our transport medium may have padded the buffer out. Now we know it 471 * is IP we can trim to the true length of the frame. 472 * Note this now means skb->len holds ntohs(iph->tot_len). 473 */ 474 if (pskb_trim_rcsum(skb, len)) { //根據IP資料首部的總長度重新設定skb的長度 475 __IP_INC_STATS(net, IPSTATS_MIB_INDISCARDS); 476 goto drop; 477 } 478 479 skb->transport_header = skb->network_header + iph->ihl*4; 480 481 /* Remove any debris in the socket control block */ 482 memset(IPCB(skb), 0, sizeof(struct inet_skb_parm)); 483 IPCB(skb)->iif = skb->skb_iif;//將skb中的IP控制塊清0，以便後續對IP選項的處理 484 485 /* Must drop socket now because of tproxy. */ 486 skb_orphan(skb); /*首先呼叫skb_orphan把skb孤立，使它跟傳送socket和協議棧不再有任何聯絡，也即對本機來說, **這個skb的資料內容已經發送出去了，而skb相當於已經被釋放掉了。對於環回裝置介面來說, **資料的傳送工作至此已經全部完成，接下來，只要把這個實際上還未被釋放的skb傳回給協議棧 **的接收函式即可。 487 488 return NF_HOOK(NFPROTO_IPV4, NF_INET_PRE_ROUTING, //通過netfilter處理函式呼叫ip_rcv_finish 489 net, NULL, skb, dev, NULL, 490 ip_rcv_finish); 491 492 csum_error: 493 __IP_INC_STATS(net, IPSTATS_MIB_CSUMERRORS); 494 inhdr_error: 495 __IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS); 496 drop: 497 kfree_skb(skb); 498 out: 499 return NET_RX_DROP; 500 }
3. ip_rcv_finish
311 static int ip_rcv_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
312 {
313 const struct iphdr *iph = ip_hdr(skb); //獲取IP頭部
314 struct rtable *rt; //管理路由表
315 struct net_device *dev = skb->dev; //獲取網路裝置資訊
316 void (*edemux)(struct sk_buff *skb);
317
318 /* if ingress device is enslaved to an L3 master device pass the
319 * skb to its handler for processing
320 */
321 skb = l3mdev_ip_rcv(skb); //如果這個包隸屬於第三層主裝置，則交給第三層，並且將SKB設成NULL
322 if (!skb) //如果交給了第三層，則return
323 return NET_RX_SUCCESS;
324
325 if (net->ipv4.sysctl_ip_early_demux &&
326 !skb_dst(skb) &&
327 !skb->sk &&
328 !ip_is_fragment(iph)) {
329 const struct net_protocol *ipprot; //獲取協議型別
330 int protocol = iph->protocol;
331
332 ipprot = rcu_dereference(inet_protos[protocol]);
333 if (ipprot && (edemux = READ_ONCE(ipprot->early_demux))) {
334 edemux(skb);
335 /* must reload iph, skb->head might have changed */
336 iph = ip_hdr(skb);
337 }
338 }
340 /*
341 * Initialise the virtual path cache for the packet. It describes
342 * how the packet travels inside Linux networking.
343 */
344 if (!skb_valid_dst(skb)) { //如果還沒為該資料查詢路由快取，則呼叫下列函式為其查詢，如果查詢失敗則丟棄該報文
345 int err = ip_route_input_noref(skb, iph->daddr, iph->saddr,
346 iph->tos, dev);
347 if (unlikely(err)) {
348 if (err == -EXDEV)
349 __NET_INC_STATS(net, LINUX_MIB_IPRPFILTER);
350 goto drop;
351 }
352 }
353
354 #ifdef CONFIG_IP_ROUTE_CLASSID //與路由表classifier相關
355 if (unlikely(skb_dst(skb)->tclassid)) {
356 struct ip_rt_acct *st = this_cpu_ptr(ip_rt_acct);
357 u32 idx = skb_dst(skb)->tclassid;
358 st[idx&0xFF].o_packets++;
359 st[idx&0xFF].o_bytes += skb->len;
360 st[(idx>>16)&0xFF].i_packets++;
361 st[(idx>>16)&0xFF].i_bytes += skb->len;
362 }
363 #endif
364
365 if (iph->ihl > 5 && ip_rcv_options(skb))//判斷IP首部是否有option，如果有則呼叫option處理函式
366 goto drop;
367
368 rt = skb_rtable(skb);
369 if (rt->rt_type == RTN_MULTICAST) {
370 __IP_UPD_PO_STATS(net, IPSTATS_MIB_INMCAST, skb->len);
371 } else if (rt->rt_type == RTN_BROADCAST) {
372 __IP_UPD_PO_STATS(net, IPSTATS_MIB_INBCAST, skb->len);
373 } else if (skb->pkt_type == PACKET_BROADCAST ||
374 skb->pkt_type == PACKET_MULTICAST) {
375 struct in_device *in_dev = __in_dev_get_rcu(dev);
376
377 /* RFC 1122 3.3.6:
378 *
379 * When a host sends a datagram to a link-layer broadcast
380 * address, the IP destination address MUST be a legal IP
381 * broadcast or IP multicast address.
382 *
383 * A host SHOULD silently discard a datagram that is received
384 * via a link-layer broadcast (see Section 2.4) but does not
385 * specify an IP multicast or broadcast destination address.
386 *
387 * This doesn't explicitly say L2 *broadcast*, but broadcast is
388 * in a way a form of multicast and the most common use case for
389 * this is 802.11 protecting against cross-station spoofing (the
390 * so-called "hole-196" attack) so do it for both.
391 */
392 if (in_dev &&
393 IN_DEV_ORCONF(in_dev, DROP_UNICAST_IN_L2_MULTICAST))
394 goto drop;
395 }
396
397 return dst_input(skb); //根據路由快取將資料包交給本地處理還是轉發出去，本地處理將呼叫ip_local_deliver，如果要轉發出去則呼叫ip_forward
399 drop:
400 kfree_skb(skb);
401 return NET_RX_DROP;
402 }
4. ip_local_deliver
242 /*
243 * Deliver IP Packets to the higher protocol layers.
244 */
245 int ip_local_deliver(struct sk_buff *skb)
246 {
247 /*
248 * Reassemble IP fragments.
249 */
250 struct net *net = dev_net(skb->dev); //獲取網路空間
251
252 if (ip_is_fragment(ip_hdr(skb))) { //如果收到的是分片包，則呼叫ip_defrag進行重組，對應的標示是IP_DEFRAG_LOCAL_DELIVER
253 if (ip_defrag(net, skb, IP_DEFRAG_LOCAL_DELIVER))
254 return 0; //如果返回0表示分片尚未到齊，重組沒有完成
255 }
256
257 return NF_HOOK(NFPROTO_IPV4, NF_INET_LOCAL_IN,
258 net, NULL, skb, skb->dev, NULL,
259 ip_local_deliver_finish); //如果重組完成，則呼叫ip_local_deliver_finish將組裝完成的資料包交給傳輸層處理
260 }
5. ip_local_deliver_finish
呼叫這個函式後，資料包的處理結果有三種：交給raw套接字處理，交給傳輸層處理，因異常而被丟棄
192 static int ip_local_deliver_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
193 {
194 __skb_pull(skb, skb_network_header_len(skb)); //在交給傳輸層之前先去掉IP頭部
195
196 rcu_read_lock();
197 {
198 int protocol = ip_hdr(skb)->protocol; //獲取傳輸層協議號，用於計算雜湊值
199 const struct net_protocol *ipprot;
200 int raw;
201
202 resubmit:
203 raw = raw_local_deliver(skb, protocol); //根據protocol得到雜湊值，然後檢視raw_v4_htable散列表中以該值為關鍵字
的雜湊桶是否為空。如果不為空，則說明建立了RAW套介面，複製該資料包的副本輸入到註冊該桶中的所有套介面
205 ipprot = rcu_dereference(inet_protos[protocol]);//查詢inet_protos陣列中是否註冊了與IP首部中傳輸層協議號一致的
傳輸層協議。如果查詢命中則呼叫對應的傳輸層接收函式
206 if (ipprot) {
207 int ret;
208
209 if (!ipprot->no_policy) {
210 if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb)) { //資料包安全策略檢測，與ipsec相關
211 kfree_skb(skb);
212 goto out;
213 }
214 nf_reset(skb);
215 }
216 ret = ipprot->handler(skb); //這個是傳輸層的接收函式，包括tcp_protocol, udp_protocol, icmp_protocol
217 if (ret < 0) {
218 protocol = -ret;
219 goto resubmit;
220 }
221 __IP_INC_STATS(net, IPSTATS_MIB_INDELIVERS);
222 } else {
223 if (!raw) {
224 if (xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb)) {
225 __IP_INC_STATS(net, IPSTATS_MIB_INUNKNOWNPROTOS);
226 icmp_send(skb, ICMP_DEST_UNREACH, //如果raw套接字沒有接收或者接收異常，則傳送一個協議不可達的報文給對方
227 ICMP_PROT_UNREACH, 0);
228 }
229 kfree_skb(skb); //釋放該報文。
230 } else {
231 __IP_INC_STATS(net, IPSTATS_MIB_INDELIVERS);
232 consume_skb(skb); //如果沒有相應的傳輸層協議接收該報文，則釋放該報文。
233 }
234 }
235 }
236 out:
237 rcu_read_unlock();
238
239 return 0;
240 }
6. ip_forward
現在來了解一下資料包被轉發出去的情況，執行的是ip_forward：
77 int ip_forward(struct sk_buff *skb)
78 {
79 u32 mtu;
80 struct iphdr *iph; /* Our header */
81 struct rtable *rt; /* Route we use */
82 struct ip_options *opt = &(IPCB(skb)->opt); //獲取資料包中IP的option
83 struct net *net;
84
85 /* that should never happen */
86 if (skb->pkt_type != PACKET_HOST) //不是發往本機的包丟棄
87 goto drop;
88
89 if (unlikely(skb->sk))
90 goto drop;
91
92 if (skb_warn_if_lro(skb)) //與GSO相關
93 goto drop;
94
95 if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb)) //查詢IPsec資料庫，如果查詢失敗則丟包
96 goto drop;
97
98 if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb)) //如果IP option中存在警告選項，則呼叫ip_call_ra_chain將資料包輸入給對路由警告選項感興趣的用
戶程序。如果成功則不再轉發該資料包
99 return NET_RX_SUCCESS;
100
101 skb_forward_csum(skb); //在轉發過程中可能會修改IP首部，則呼叫該函式skb->ip_summed = CHECKSUM_NONE，後面輸出時要計算新的校驗和
102 net = dev_net(skb->dev);
103
104 /*
105 * According to the RFC, we must first decrease the TTL field. If
106 * that reaches zero, we must reply an ICMP control message telling
107 * that the packet's lifetime expired.
108 */
109 if (ip_hdr(skb)->ttl <= 1) //如果跳數小於或等於1，則不轉發，並跳轉到傳送ICMP報文
110 goto too_many_hops;
111
112 if (!xfrm4_route_forward(skb)) //進行IPsec的路由選擇和轉發，如果失敗則丟包
113 goto drop;
114
115 rt = skb_rtable(skb); //獲取路由
116
117 if (opt->is_strictroute && rt->rt_uses_gateway) //如果資料包啟動嚴格路由選項，且路由的下一跳不是閘道器，則傳送超時ICMP報文到傳送方
118 goto sr_failed;
119
120 IPCB(skb)->flags |= IPSKB_FORWARDED; //設定IP管理塊中的flag為FORWORD
121 mtu = ip_dst_mtu_maybe_forward(&rt->dst, true); //獲取當前系統的最小MTU
122 if (ip_exceeds_mtu(skb, mtu)) { //如果該包的大小超過了最新MTU,則傳送ICMP給傳送端，並且停止轉發
123 IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);
124 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
125 htonl(mtu));
126 goto drop;
127 }
128
129 /* We are about to mangle packet. Copy it! */
130 if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len)) //確保SKB有指定長度的headroom，當他的headroom小於指定長度或者克隆SKB時，
會新建SKB緩衝，並且釋放對原包的引用
131 goto drop;
132 iph = ip_hdr(skb); //獲取IP頭部
134 /* Decrease ttl after skb cow done */
135 ip_decrease_ttl(iph); //將TTL減1
136
137 /*
138 * We now generate an ICMP HOST REDIRECT giving the route
139 * we calculated.
140 */
141 if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr && //如果該資料包的輸出路由存在重定向標誌，且該資料報中不存在源路由選項，則向傳送端傳送
142 !skb_sec_path(skb)) //重定向ICMP報文
143 ip_rt_send_redirect(skb);
144
145 skb->priority = rt_tos2priority(iph->tos);
146
147 return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD,
148 net, NULL, skb, skb->dev, rt->dst.dev,
149 ip_forward_finish); //經過netfilter處理後，呼叫ip_forward_finish進一步處理
150
151 sr_failed:
152 /*
153 * Strict routing permits no gatewaying
154 */
155 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
156 goto drop;
157
158 too_many_hops:
159 /* Tell the sender its packet died... */
160 __IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
161 icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
162 drop:
163 kfree_skb(skb);
164 return NET_RX_DROP;
165 }
7. ip_forward_finish
64 static int ip_forward_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
65 {
66 struct ip_options *opt = &(IPCB(skb)->opt);
67
68 __IP_INC_STATS(net, IPSTATS_MIB_OUTFORWDATAGRAMS);
69 __IP_ADD_STATS(net, IPSTATS_MIB_OUTOCTETS, skb->len);
70
71 if (unlikely(opt->optlen)) //如果option的長度不為0
72 ip_forward_options(skb); //處理轉發IP資料報中的IP選項，包括記錄路由選項和時間戳選項
73
74 return dst_output(net, sk, skb); //通過路由快取將資料包輸出，最終會呼叫單播的輸出函式ip_output或組播的輸出函式ip_mc_output
75 }
8. ip_output
395 int ip_output(struct net *net, struct sock *sk, struct sk_buff *skb)
396 {
397 struct net_device *dev = skb_dst(skb)->dev;
398
399 IP_UPD_PO_STATS(net, IPSTATS_MIB_OUT, skb->len);
400
401 skb->dev = dev; //設定資料包的輸出網路裝置
402 skb->protocol = htons(ETH_P_IP); //設定資料包網路層協議型別
403
404 return NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING,
405 net, sk, skb, NULL, dev,
406 ip_finish_output,
407 !(IPCB(skb)->flags & IPSKB_REROUTED)); //呼叫ip_finish_output繼續IP包的輸出
408 }
9. ip_finish_output
291 static int ip_finish_output(struct net *net, struct sock *sk, struct sk_buff *skb)
292 {
293 unsigned int mtu;
294 int ret;
295
296 ret = BPF_CGROUP_RUN_PROG_INET_EGRESS(sk, skb);
297 if (ret) {
298 kfree_skb(skb);
299 return ret;
300 }
301
302 #if defined(CONFIG_NETFILTER) && defined(CONFIG_XFRM)
303 /* Policy lookup after SNAT yielded a new policy */
304 if (skb_dst(skb)->xfrm) {
305 IPCB(skb)->flags |= IPSKB_REROUTED;
306 return dst_output(net, sk, skb); //根據安全策略需要重傳
307 }
308 #endif
309 mtu = ip_skb_dst_mtu(sk, skb); //獲取最小MTU的大小
310 if (skb_is_gso(skb))
311 return ip_finish_output_gso(net, sk, skb, mtu); //如果需要支援GSO特性，則進行GSO的組裝，關於GSO的特性我們後面分開講
312
313 if (skb->len > mtu || (IPCB(skb)->flags & IPSKB_FRAG_PMTU))
314 return ip_fragment(net, sk, skb, mtu, ip_finish_output2); //如果長度不滿足要求則進行IP包的分段
315
316 return ip_finish_output2(net, sk, skb); //呼叫該函式繼續處理
317 }
10. ip_finish_output2
183 static int ip_finish_output2(struct net *net, struct sock *sk, struct sk_buff *skb)
184 {
185 struct dst_entry *dst = skb_dst(skb);
186 struct rtable *rt = (struct rtable *)dst;
187 struct net_device *dev = dst->dev;
188 unsigned int hh_len = LL_RESERVED_SPACE(dev);
189 struct neighbour *neigh;
190 u32 nexthop;
191
192 if (rt->rt_type == RTN_MULTICAST) {
193 IP_UPD_PO_STATS(net, IPSTATS_MIB_OUTMCAST, skb->len);
194 } else if (rt->rt_type == RTN_BROADCAST)
195 IP_UPD_PO_STATS(net, IPSTATS_MIB_OUTBCAST, skb->len);
196
197 /* Be paranoid, rather than too clever. */
198 if (unlikely(skb_headroom(skb) < hh_len && dev->header_ops)) { //檢測skb頭部空間是否還有空間存放鏈路層首部，如果不夠，則重新分配更大的儲存空間，
199 struct sk_buff *skb2; //並釋放原來的SKB
200
201 skb2 = skb_realloc_headroom(skb, LL_RESERVED_SPACE(dev));
202 if (!skb2) {
203 kfree_skb(skb);
204 return -ENOMEM;
205 }
206 if (skb->sk)
207 skb_set_owner_w(skb2, skb->sk);
208 consume_skb(skb);
209 skb = skb2;
210 }
211
212 if (lwtunnel_xmit_redirect(dst->lwtstate)) {
213 int res = lwtunnel_xmit(skb);
214
215 if (res < 0 || res == LWTUNNEL_XMIT_DONE)
216 return res;
217 }
219 rcu_read_lock_bh();
220 nexthop = (__force u32) rt_nexthop(rt, ip_hdr(skb)->daddr); //獲取下一跳路由
221 neigh = __ipv4_neigh_lookup_noref(dev, nexthop); //查詢鄰居子系統
222 if (unlikely(!neigh))
223 neigh = __neigh_create(&arp_tbl, &nexthop, dev, false); //如果沒有查到，則根據下一跳路由建立一跳新的記錄
224 if (!IS_ERR(neigh)) { //如果建立成功
225 int res;
226
227 sock_confirm_neigh(skb, neigh); //確認路由，如果不能確認則設定對應的標識
228 res = neigh_output(neigh, skb); //呼叫該函式交給鄰居子系統
229
230 rcu_read_unlock_bh();
231 return res;
232 }
233 rcu_read_unlock_bh();
234
235 net_dbg_ratelimited("%s: No header cache and no neighbour!\n",
236 __func__);
237 kfree_skb(skb);
238 return -EINVAL;
239 }
472 static inline int neigh_output(struct neighbour *n, struct sk_buff *skb)
473 {
474 const struct hh_cache *hh = &n->hh;
475
476 if ((n->nud_state & NUD_CONNECTED) && hh->hh_len) //如果快取了鏈路層的首部，則呼叫neigh_hh_output,否則呼叫鄰居項的輸出方法輸出
477 return neigh_hh_output(hh, skb); //呼叫該函式輸出資料包
478 else
479 return n->output(n, skb); //使用鄰居項的輸出方法
480 }
450 static inline int neigh_hh_output(const struct hh_cache *hh, struct sk_buff *skb)
451 {
452 unsigned int seq;
453 unsigned int hh_len;
454
455 do {
456 seq = read_seqbegin(&hh->hh_lock);
457 hh_len = hh->hh_len;
458 if (likely(hh_len <= HH_DATA_MOD)) {
459 /* this is inlined by gcc */
460 memcpy(skb->data - HH_DATA_MOD, hh->hh_data, HH_DATA_MOD);
461 } else {
462 unsigned int hh_alen = HH_DATA_ALIGN(hh_len);
463
464 memcpy(skb->data - hh_alen, hh->hh_data, hh_alen);
465 }
466 } while (read_seqretry(&hh->hh_lock, seq));
467
468 skb_push(skb, hh_len);
469 return dev_queue_xmit(skb); //到這裡將包從網路層傳送給介面層
470 }