skb管理函数之skb_putskb_pushskb_pullskb_reserve

Posted 2020-10-07 Alex

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 }