sk_buff结构用来描述已接收或者待发送的数据报文信息；skb在不同网络协议层之间传递，可被用于不同网络协议，如二层的mac或其他链路层协议，三层的ip，四层的tcp或者udp协议，其中某些成员变量会在该结构从一层向另一层传递时发生改变，从上层向下层传递需要添加首部，从下层向上层传递需要移除首部；
1 /* skb头结构 */
2 struct sk_buff_head {
3 /* These two members must be first. */
4 /* 通过下面两个指针成员将skb连接成双向链表 */
5 struct sk_buff *next; /* 指向后一个skb */
6 struct sk_buff *prev; /* 指向前一个skb */
7
8 __u32 qlen; /* 链表中元素个数 */
9 spinlock_t lock; /* 自旋锁 */
10 };
1 /**
2 * struct sk_buff - socket buffer
3 * @next: Next buffer in list
4 * @prev: Previous buffer in list
5 * @tstamp: Time we arrived/left
6 * @rbnode: RB tree node, alternative to next/prev for netem/tcp
7 * @sk: Socket we are owned by
8 * @dev: Device we arrived on/are leaving by
9 * @cb: Control buffer. Free for use by every layer. Put private vars here
10 * @_skb_refdst: destination entry (with norefcount bit)
11 * @sp: the security path, used for xfrm
12 * @len: Length of actual data
13 * @data_len: Data length
14 * @mac_len: Length of link layer header
15 * @hdr_len: writable header length of cloned skb
16 * @csum: Checksum (must include start/offset pair)
17 * @csum_start: Offset from skb->head where checksumming should start
18 * @csum_offset: Offset from csum_start where checksum should be stored
19 * @priority: Packet queueing priority
20 * @ignore_df: allow local fragmentation
21 * @cloned: Head may be cloned (check refcnt to be sure)
22 * @ip_summed: Driver fed us an IP checksum
23 * @nohdr: Payload reference only, must not modify header
24 * @pkt_type: Packet class
25 * @fclone: skbuff clone status
26 * @ipvs_property: skbuff is owned by ipvs
27 * @tc_skip_classify: do not classify packet. set by IFB device
28 * @tc_at_ingress: used within tc_classify to distinguish in/egress
29 * @tc_redirected: packet was redirected by a tc action
30 * @tc_from_ingress: if tc_redirected, tc_at_ingress at time of redirect
31 * @peeked: this packet has been seen already, so stats have been
32 * done for it, don‘t do them again
33 * @nf_trace: netfilter packet trace flag
34 * @protocol: Packet protocol from driver
35 * @destructor: Destruct function
36 * @_nfct: Associated connection, if any (with nfctinfo bits)
37 * @nf_bridge: Saved data about a bridged frame - see br_netfilter.c
38 * @skb_iif: ifindex of device we arrived on
39 * @tc_index: Traffic control index
40 * @hash: the packet hash
41 * @queue_mapping: Queue mapping for multiqueue devices
42 * @xmit_more: More SKBs are pending for this queue
43 * @ndisc_nodetype: router type (from link layer)
44 * @ooo_okay: allow the mapping of a socket to a queue to be changed
45 * @l4_hash: indicate hash is a canonical 4-tuple hash over transport
46 * ports.
47 * @sw_hash: indicates hash was computed in software stack
48 * @wifi_acked_valid: wifi_acked was set
49 * @wifi_acked: whether frame was acked on wifi or not
50 * @no_fcs: Request NIC to treat last 4 bytes as Ethernet FCS
51 * @dst_pending_confirm: need to confirm neighbour
52 * @napi_id: id of the NAPI struct this skb came from
53 * @secmark: security marking
54 * @mark: Generic packet mark
55 * @vlan_proto: vlan encapsulation protocol
56 * @vlan_tci: vlan tag control information
57 * @inner_protocol: Protocol (encapsulation)
58 * @inner_transport_header: Inner transport layer header (encapsulation)
59 * @inner_network_header: Network layer header (encapsulation)
60 * @inner_mac_header: Link layer header (encapsulation)
61 * @transport_header: Transport layer header
62 * @network_header: Network layer header
63 * @mac_header: Link layer header
64 * @tail: Tail pointer
65 * @end: End pointer
66 * @head: Head of buffer
67 * @data: Data head pointer
68 * @truesize: Buffer size
69 * @users: User count - see {datagram,tcp}.c
70 */
71 /* skb结构 */
72 struct sk_buff {
73 union {
74 struct {
75 /* These two members must be first. */
76 struct sk_buff *next;
77 struct sk_buff *prev;
78
79 /* 报文到达或者离开的时间戳 */
80 union {
81 ktime_t tstamp;
82 struct skb_mstamp skb_mstamp;
83 };
84 };
85 struct rb_node rbnode; /* used in netem & tcp stack */
86 };
87
88 /*
89 指向缓冲区的套接字sock数据结构。当数据在本地产生或者正由本地进程接收时，
90 该数据以及套接字相关信息会被L4(tcp或者udp)以及用户应用程序使用
91 当缓冲区只是被转发时(本地机器不是来源也不是目的地)，该指针为NULL
92 */
93 struct sock *sk;
94
95 union {
96 /* 报文到达或者离开时的网络设备 */
97 struct net_device *dev;
98 /* Some protocols might use this space to store information,
99 * while device pointer would be NULL.
100 * UDP receive path is one user.
101 */
102 unsigned long dev_scratch;
103 };
104 /*
105 * This is the control buffer. It is free to use for every
106 * layer. Please put your private variables there. If you
107 * want to keep them across layers you have to do a skb_clone()
108 * first. This is owned by whoever has the skb queued ATM.
109 */
110 /*
111 控制缓冲区，用于存储私有信息，每层协议自己维护并使用，
112 并且只在本层有有效
113 */
114 char cb[48] __aligned(8);
115
116 /* 路由缓存，输入或者输出报文都要查询到目的路由缓存项，才能确定流向 */
117 unsigned long _skb_refdst;
118
119 /*
120 当缓冲区被删除时，可以完成某些清理工作
121 当缓冲区不属于一个套接字时，该函数通常不被初始化
122 属于一个套接字时，通常设置为sock_rfree或sock_wfree
123 sock_xxx函数用于更新套接字队列中所持有的内存
124 */
125 void (*destructor)(struct sk_buff *skb);
126 #ifdef CONFIG_XFRM
127 struct sec_path *sp;
128 #endif
129 #if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
130 unsigned long _nfct;
131 #endif
132 #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
133 struct nf_bridge_info *nf_bridge;
134 #endif
135 /*
136 缓冲区的数据区块大小，该长度包括主缓冲区(head指针指向)的数据
137 以及一些片段(fragment)的数据，当缓冲区从一个网络分层移动到下一个
138 网络分层时，该值会发生变化，因为在协议栈中向上层移动时报头会被丢弃
139 向下层移动时报头会添加，len也会把协议报头算在内，与"数据预留和对齐"操作
140 */
141 unsigned int len,
142 /* 片段(fragment)中的数据大小 */
143 data_len;
144 /* mac报头大小 */
145 __u16 mac_len,
146 /* 克隆skb时可写报文头部长度 */
147 hdr_len;
148
149 /* Following fields are _not_ copied in __copy_skb_header()
150 * Note that queue_mapping is here mostly to fill a hole.
151 */
152 kmemcheck_bitfield_begin(flags1);
153 __u16 queue_mapping;
154
155 /* if you move cloned around you also must adapt those constants */
156 #ifdef __BIG_ENDIAN_BITFIELD
157 #define CLONED_MASK (1 << 7)
158 #else
159 #define CLONED_MASK 1
160 #endif
161 #define CLONED_OFFSET() offsetof(struct sk_buff, __cloned_offset)
162
163 __u8 __cloned_offset[0];
164 /* 表示该skb是另外一个skb的克隆 */
165 __u8 cloned:1,
166 /*
167 payload是否被单独引用，不存在协议首部，如果被引用，则不能修改协议首部，也不能通过skb->data来访问协议首部
168 */
169 nohdr:1,
170 /*
171 当前克隆状态
172 SKB_FCLONE_UNAVAILABLE-skb未被克隆
173 SKB_FCLONE_ORIG-在skbuff_fclone_cache分配的父skb，可以被克隆
174 SKB_FCLONE_CLONE-在skbuff_fclone_cache分配的子skb，从父skb克隆得到
175 */
176 fclone:2,
177 peeked:1,
178 head_frag:1,
179 xmit_more:1,
180 __unused:1; /* one bit hole */
181 kmemcheck_bitfield_end(flags1);
182
183 /* fields enclosed in headers_start/headers_end are copied
184 * using a single memcpy() in __copy_skb_header()
185 */
186 /* private: */
187 __u32 headers_start[0];
188 /* public: */
189
190 /* if you move pkt_type around you also must adapt those constants */
191 #ifdef __BIG_ENDIAN_BITFIELD
192 #define PKT_TYPE_MAX (7 << 5)
193 #else
194 #define PKT_TYPE_MAX 7
195 #endif
196 #define PKT_TYPE_OFFSET() offsetof(struct sk_buff, __pkt_type_offset)
197
198 __u8 __pkt_type_offset[0];
199 /*
200 此字段根据l2的目的地址进行划分
201 PACKET_HOST-mac地址与接收设备mac地址相等，说明是发给该主机的
202 PACKET_BROADCAST-mac地址是接收设备的广播地址
203 PACKET_MULTICAST-mac地址接收改设备注册的多播地址之一
204 PACKET_OTHERHOST-mac地址不属于接收设备的地址，启用转发则转发，否则丢弃
205 PACKET_OUTGOING-数据包将被发出，用到这个标记的功能包括decnet，
206 或者为每个网络tab都复制一份发出包的函数
207 PACKET_LOOPBACK-数据包发往回环设备，有此标识，处理回环设备时，
208 可以跳过一些真实设备所需的操作
209 PACKET_USER-发送到用户空间，netlink使用
210 PACKET_KERNEL-发送到内核空间，netlink使用
211 PACKET_FASTROUTE-未使用
212 */
213 __u8 pkt_type:3;
214 __u8 pfmemalloc:1;
215 __u8 ignore_df:1;
216
217 __u8 nf_trace:1;
218 /*
219 CHECKSUM_NONE-硬件不支持，完全由软件执行校验和
220 CHECKSUM_PARTIAL-由硬件来执行校验和
221 CHECKSUM_UNNECESSARY-没必要执行校验和
222 CHECKSUM_COMPLETE-已完成执行校验和
223 */
224 __u8 ip_summed:2;
225 __u8 ooo_okay:1;
226 __u8 l4_hash:1;
227 __u8 sw_hash:1;
228 __u8 wifi_acked_valid:1;
229 __u8 wifi_acked:1;
230
231 __u8 no_fcs:1;
232 /* Indicates the inner headers are valid in the skbuff. */
233 __u8 encapsulation:1;
234 __u8 encap_hdr_csum:1;
235 __u8 csum_valid:1;
236 __u8 csum_complete_sw:1;
237 __u8 csum_level:2;
238 __u8 csum_bad:1;
239
240 __u8 dst_pending_confirm:1;
241 #ifdef CONFIG_IPV6_NDISC_NODETYPE
242 __u8 ndisc_nodetype:2;
243 #endif
244 __u8 ipvs_property:1;
245 __u8 inner_protocol_type:1;
246 __u8 remcsum_offload:1;
247 #ifdef CONFIG_NET_SWITCHDEV
248 __u8 offload_fwd_mark:1;
249 #endif
250 #ifdef CONFIG_NET_CLS_ACT
251 __u8 tc_skip_classify:1;
252 __u8 tc_at_ingress:1;
253 __u8 tc_redirected:1;
254 __u8 tc_from_ingress:1;
255 #endif
256
257 #ifdef CONFIG_NET_SCHED
258 __u16 tc_index; /* traffic control index */
259 #endif
260
261 union {
262 /* 校验和，必须包含csum_start和csum_offset */
263 __wsum csum;
264 struct {
265 /* 校验开始位置，相对于header */
266 __u16 csum_start;
267 /* 校验和存储位置，相对于csum_start */
268 __u16 csum_offset;
269 };
270 };
271 /*
272 正在被传输的数据包QoS等级
273 数据包由本地产生，套接字会定义优先级的值
274 数据包在被转发，则在调用ip_forward函数时，会根据
275 ip头本身的ToS字段定义该值
276 */
277 __u32 priority;
278 int skb_iif;
279 __u32 hash;
280 __be16 vlan_proto;
281 __u16 vlan_tci;
282 #if defined(CONFIG_NET_RX_BUSY_POLL) || defined(CONFIG_XPS)
283 union {
284 unsigned int napi_id;
285 unsigned int sender_cpu;
286 };
287 #endif
288 #ifdef CONFIG_NETWORK_SECMARK
289 __u32 secmark;
290 #endif
291
292 union {
293 __u32 mark;
294 __u32 reserved_tailroom;
295 };
296
297 /* 封装的协议 */
298 union {
299 __be16 inner_protocol;
300 __u8 inner_ipproto;
301 };
302 /* 封装的传输层头部相对于head的偏移 */
303 __u16 inner_transport_header;
304 /* 封装的网络层头部相对于head的偏移 */
305 __u16 inner_network_header;
306 /* 封装的链路层头部相对于head的偏移 */
307 __u16 inner_mac_header;
308
309 /*
310 l3层协议值
311 如ETH_P_IP-ipv4报文
312 ETH_P_ARP-arp报文等
313 */
314 __be16 protocol;
315 /* 传输层头部相对于head的偏移 */
316 __u16 transport_header;
317 /* 网络层头部相对于head的偏移 */
318 __u16 network_header;
319 /* 链路层头部相对于head的偏移 */
320 __u16 mac_header;
321
322 /* private: */
323 __u32 headers_end[0];
324 /* public: */
325
326 /* These elements must be at the end, see alloc_skb() for details. */
327 /* 实际数据的尾部 */
328 sk_buff_data_t tail;
329 /* 缓冲区的尾部 */
330 sk_buff_data_t end;
331 /* 缓冲区的头部 */
332 unsigned char *head,
333 /* 实际数据的头部 */
334 *data;
335 /*
336 缓冲区的总大小，包括skb本身和实际数据len大小，alloc_skb函数将
337 该字段设置为len+sizeof(sk_buff)
338 每当len值更新，该值也要对应更新
339 */
340 unsigned int truesize;
341
342 /*
343 引用计数，在使用该skb缓冲区的实例个数，当引用计数为0时，skb才能被释放
344 skb_get()获取操作中会增加引用计数，kfree_skb释放过程中检查引用计数，
345 引用计数为0时，才真正释放skb
346 该计数器只计算sk_buff结构引用计数，缓冲区包含的实际数据由
347 skb_shared_info->dataref字段记录
348 */
349 atomic_t users;
350 };
时间： 2024-08-09 10:44:47