skb操作中的预留和对齐操作主要由skb_put、skb_push、skb_pull、skb_reserve完成；这几个函数的区别通过下面图(图片来自：深入理解linux网络技术内幕)可以清晰的区分；另外，需要注意的是skb_reserve只能操作空skb，即在分配了空间，尚未填充数据时调用；
以下为四个函数的源码分析；
1 /** 2 * skb_put - add data to a buffer 3 * @skb: buffer to use 4 * @len: amount of data to add 5 * 6 * This function extends the used data area of the buffer. If this would 7 * exceed the total buffer size the kernel will panic. A pointer to the 8 * first byte of the extra data is returned. 9 */ 10 /* 11 向skb尾部添加数据 12 */ 13 unsigned char *skb_put(struct sk_buff *skb, unsigned int len) 14 { 15 /* 获取当前skb->tail */ 16 unsigned char *tmp = skb_tail_pointer(skb); 17 18 /* 要求skb数据区必须为线性 */ 19 SKB_LINEAR_ASSERT(skb); 20 21 /* skb尾部增加len字节 */ 22 skb->tail += len; 23 /* skb数据总长度增加len字节 */ 24 skb->len += len; 25 26 /* 如果增加之后的tail > end ，则panic */ 27 if (unlikely(skb->tail > skb->end)) 28 skb_over_panic(skb, len, __builtin_return_address(0)); 29 30 //返回添加数据的第一个字节位置 31 return tmp; 32 }
1 /** 2 * skb_push - add data to the start of a buffer 3 * @skb: buffer to use 4 * @len: amount of data to add 5 * 6 * This function extends the used data area of the buffer at the buffer 7 * start. If this would exceed the total buffer headroom the kernel will 8 * panic. A pointer to the first byte of the extra data is returned. 9 */ 10 /* 11 向skb数据区头部添加数据 12 */ 13 unsigned char *skb_push(struct sk_buff *skb, unsigned int len) 14 { 15 /* 数据区data指针前移len字节 */ 16 skb->data -= len; 17 /* 数据总长度增加len字节 */ 18 skb->len += len; 19 20 /* 添加数据长度溢出过header ，panic*/ 21 if (unlikely(skb->data<skb->head)) 22 skb_under_panic(skb, len, __builtin_return_address(0)); 23 24 /* 返回新的data指针 */ 25 return skb->data; 26 }
1 /** 2 * skb_pull - remove data from the start of a buffer 3 * @skb: buffer to use 4 * @len: amount of data to remove 5 * 6 * This function removes data from the start of a buffer, returning 7 * the memory to the headroom. A pointer to the next data in the buffer 8 * is returned. Once the data has been pulled future pushes will overwrite 9 * the old data. 10 */ 11 /* 12 从数据区头部移除数据 13 */ 14 unsigned char *skb_pull(struct sk_buff *skb, unsigned int len) 15 { 16 return skb_pull_inline(skb, len); 17 }
1 /* 根据移除数据长度判断函数调用 */ 2 static inline unsigned char *skb_pull_inline(struct sk_buff *skb, unsigned int len) 3 { 4 /* 5 移除长度> skb数据总长度，返回NULL 6 否则，继续调用_skb_pull函数 7 */ 8 return unlikely(len > skb->len) ? NULL : __skb_pull(skb, len); 9 }
1 /* 从skb数据区头部移除数据 */ 2 static inline unsigned char *__skb_pull(struct sk_buff *skb, unsigned int len) 3 { 4 /* 数据总长度减去len字节 */ 5 skb->len -= len; 6 /* 数据总长度是否有异常 */ 7 BUG_ON(skb->len < skb->data_len); 8 9 /* 10 data指针移动len字节 11 返回移除之后新的data指针 12 */ 13 return skb->data += len; 14 }
1 /* 2 保留头部空间，只能对空的skb使用 3 */ 4 static inline void skb_reserve(struct sk_buff *skb, int len) 5 { 6 /* 数据区data指针增加len字节*/ 7 skb->data += len; 8 /* 数据区tail指针增加len字节 */ 9 skb->tail += len; 10 }