Linux內核--網絡內核實現分析(一)--與sk_buff有關的幾個重要的數據結構

本文分析基於Linux Kernel 3.2.1

幾個月之前做了關於Linux內核版本1.2.13網絡棧的結構框架分析並實現了基於Netfilter的包過濾防火牆，這裡以內核3.2.1內核為例來進一步分析，更全面的分析網絡棧的結構。

更多請查看 Linux內核--網絡內核實現分析

1、先說一下sk_buff結構體

這個結構體是套接字的緩沖區，詳細記錄了一個數據包的組成，時間、網絡設備、各層的首部及首部長度和數據的首尾指針。

下面是他的定義，挺長

1. struct sk_buff {
2. /* These two members must be first. */
3. struct sk_buff *next;
4. struct sk_buff *prev;
6. ktime_t tstamp;
8. struct sock *sk;
9. struct net_device *dev;
11. /*
12. * This is the control buffer. It is free to use for every
13. * layer. Please put your private variables there. If you
14. * want to keep them across layers you have to do a skb_clone()
15. * first. This is owned by whoever has the skb queued ATM.
16. */
17. char cb[48] __aligned(8);
19. unsigned long _skb_refdst;
20. #ifdef CONFIG_XFRM
21. struct sec_path *sp;
22. #endif
23. unsigned int len,
24. data_len;
25. __u16 mac_len,
26. hdr_len;
27. union {
28. __wsum csum;
29. struct {
30. __u16 csum_start;
31. __u16 csum_offset;
32. };
33. };
34. __u32 priority;
35. kmemcheck_bitfield_begin(flags1);
36. __u8 local_df:1,
37. cloned:1,
38. ip_summed:2,
39. nohdr:1,
40. nfctinfo:3;
41. __u8 pkt_type:3,
42. fclone:2,
43. ipvs_property:1,
44. peeked:1,
45. nf_trace:1;
46. kmemcheck_bitfield_end(flags1);
47. __be16 protocol;
49. void (*destructor)(struct sk_buff *skb);
50. #if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
51. struct nf_conntrack *nfct;
52. #endif
53. #ifdef NET_SKBUFF_NF_DEFRAG_NEEDED
54. struct sk_buff *nfct_reasm;
55. #endif
56. #ifdef CONFIG_BRIDGE_NETFILTER
57. struct nf_bridge_info *nf_bridge;
58. #endif
60. int skb_iif;
61. #ifdef CONFIG_NET_SCHED
62. __u16 tc_index; /* traffic control index */
63. #ifdef CONFIG_NET_CLS_ACT
64. __u16 tc_verd; /* traffic control verdict */
65. #endif
66. #endif
68. __u32 rxhash;
70. __u16 queue_mapping;
71. kmemcheck_bitfield_begin(flags2);
72. #ifdef CONFIG_IPV6_NDISC_NODETYPE
73. __u8 ndisc_nodetype:2;
74. #endif
75. __u8 ooo_okay:1;
76. __u8 l4_rxhash:1;
77. kmemcheck_bitfield_end(flags2);
79. /* 0/13 bit hole */
81. #ifdef CONFIG_NET_DMA
82. dma_cookie_t dma_cookie;
83. #endif
84. #ifdef CONFIG_NETWORK_SECMARK
85. __u32 secmark;
86. #endif
87. union {
88. __u32 mark;
89. __u32 dropcount;
90. };
92. __u16 vlan_tci;
94. sk_buff_data_t transport_header;
95. sk_buff_data_t network_header;
96. sk_buff_data_t mac_header;
97. /* These elements must be at the end, see alloc_skb() for details. */
98. sk_buff_data_t tail;
99. sk_buff_data_t end;
100. unsigned char *head,
101. *data;
102. unsigned int truesize;
103. atomic_t users;
104. };

可以看到新版本內核中發生了很多變化，其中數據包的首部在早期版本是以union的形式定義的，例如mac_header的定義方式如下：

1. union{
2. struct ethhdr *ethernet;
3. unsigned char *raw;
4. }mac;

這裡是以指針的形式給出的

1. #ifdef NET_SKBUFF_DATA_USES_OFFSET
2. typedef unsigned int sk_buff_data_t;
3. #else
4. typedef unsigned char *sk_buff_data_t;
5. #endif