skb管理函数之skb_put、skb_push、skb_pull、skb_reserve

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；

 /**
  *    skb_put - add data to a buffer
  *    @skb: buffer to use
  *    @len: amount of data to add
  *
  *    This function extends the used data area of the buffer. If this would
  *    exceed the total buffer size the kernel will panic. A pointer to the
  *    first byte of the extra data is returned.
  */
 /*
     向skb尾部添加数据
 */
 unsigned char *skb_put(struct sk_buff *skb, unsigned int len)
 {
     /* 获取当前skb->tail */
     unsigned char *tmp = skb_tail_pointer(skb);

     /* 要求skb数据区必须为线性 */
     SKB_LINEAR_ASSERT(skb);

     /* skb尾部增加len字节 */
     skb->tail += len;
     /* skb数据总长度增加len字节 */
     skb->len  += len;

     /* 如果增加之后的tail > end ，则panic */
     if (unlikely(skb->tail > skb->end))
         skb_over_panic(skb, len, __builtin_return_address());

     //返回添加数据的第一个字节位置
     return tmp;
 }

 /**
  *    skb_push - add data to the start of a buffer
  *    @skb: buffer to use
  *    @len: amount of data to add
  *
  *    This function extends the used data area of the buffer at the buffer
  *    start. If this would exceed the total buffer headroom the kernel will
  *    panic. A pointer to the first byte of the extra data is returned.
  */
 /*
     向skb数据区头部添加数据
 */
 unsigned char *skb_push(struct sk_buff *skb, unsigned int len)
 {
     /* 数据区data指针前移len字节 */
     skb->data -= len;
     /* 数据总长度增加len字节 */
     skb->len  += len;

     /* 添加数据长度溢出过header ，panic*/
     if (unlikely(skb->data<skb->head))
         skb_under_panic(skb, len, __builtin_return_address());

     /* 返回新的data指针 */
     return skb->data;
 }

 /**
  *    skb_pull - remove data from the start of a buffer
  *    @skb: buffer to use
  *    @len: amount of data to remove
  *
  *    This function removes data from the start of a buffer, returning
  *    the memory to the headroom. A pointer to the next data in the buffer
  *    is returned. Once the data has been pulled future pushes will overwrite
  *    the old data.
  */
 /*
     从数据区头部移除数据
 */
 unsigned char *skb_pull(struct sk_buff *skb, unsigned int len)
 {
     return skb_pull_inline(skb, len);
 }

 /* 根据移除数据长度判断函数调用 */
 static inline unsigned char *skb_pull_inline(struct sk_buff *skb, unsigned int len)
 {
     /*
         移除长度> skb数据总长度，返回NULL
         否则，继续调用_skb_pull函数
     */
     return unlikely(len > skb->len) ? NULL : __skb_pull(skb, len);
 }

 /* 从skb数据区头部移除数据 */
 static inline unsigned char *__skb_pull(struct sk_buff *skb, unsigned int len)
 {
     /* 数据总长度减去len字节 */
     skb->len -= len;
     /* 数据总长度是否有异常 */
     BUG_ON(skb->len < skb->data_len);

     /*
         data指针移动len字节
         返回移除之后新的data指针
     */
     return skb->data += len;
 }

 /*
     保留头部空间，只能对空的skb使用
 */
 static inline void skb_reserve(struct sk_buff *skb, int len)
 {
     /* 数据区data指针增加len字节*/
     skb->data += len;
     /* 数据区tail指针增加len字节 */
     skb->tail += len;
 }