socket buffer结构解析

struct sk_buff_head {
        /* These two members must be first. */
        struct sk_buff  *next;
        struct sk_buff  *prev;

        __u32           qlen;
        spinlock_t      lock;
};

#define TCP_SKB_CB(__skb)       ((struct tcp_skb_cb *)&((__skb)->cb[0]))

// net/ipv4/tcp_output.c
TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

// include/linux/if_packet.h
/* Packet types */
#define PACKET_HOST             0               /* To us                */
#define PACKET_BROADCAST        1               /* To all               */
#define PACKET_MULTICAST        2               /* To group             */
#define PACKET_OTHERHOST        3               /* To someone else      */
#define PACKET_OUTGOING         4               /* Outgoing of any type */

/**
 *      __alloc_skb     -       allocate a network buffer
 *      @size: size to allocate
 *      @gfp_mask: allocation mask
 *      @fclone: allocate from fclone cache instead of head cache
 *              and allocate a cloned (child) skb
 *      @node: numa node to allocate memory on
 */
struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
                            int fclone, int node)
{
        struct kmem_cache *cache;
        struct skb_shared_info *shinfo;
        struct sk_buff *skb;
        u8 *data;

        cache = fclone ? skbuff_fclone_cache : skbuff_head_cache;

        /* Get the HEAD */
        skb = kmem_cache_alloc_node(cache, gfp_mask & ~__GFP_DMA, node);
        if (!skb)
                goto out;

        size = SKB_DATA_ALIGN(size);
        data = kmalloc_node_track_caller(size + sizeof(struct skb_shared_info),
                        gfp_mask, node);
        if (!data)
                goto nodata;

        /*
         * See comment in sk_buff definition, just before the 'tail' member
         */
        memset(skb, 0, offsetof(struct sk_buff, tail));
        skb->truesize = size + sizeof(struct sk_buff);
        atomic_set(&skb->users, 1);
        skb->head = data;
        skb->data = data;
        skb_reset_tail_pointer(skb);
        skb->end = skb->tail + size;
        /* make sure we initialize shinfo sequentially */
        shinfo = skb_shinfo(skb);
        atomic_set(&shinfo->dataref, 1);
        shinfo->nr_frags  = 0;
        shinfo->gso_size = 0;
        shinfo->gso_segs = 0;
        shinfo->gso_type = 0;
        shinfo->ip6_frag_id = 0;
        shinfo->frag_list = NULL;

        if (fclone) {
                struct sk_buff *child = skb + 1;
                atomic_t *fclone_ref = (atomic_t *) (child + 1);

                skb->fclone = SKB_FCLONE_ORIG;
                atomic_set(fclone_ref, 1);

                child->fclone = SKB_FCLONE_UNAVAILABLE;
        }
out:
        return skb;
nodata:
        kmem_cache_free(cache, skb);
        skb = NULL;
        goto out;
}

  void __kfree_skb(struct sk_buff *skb)
  {
    skb_release_all(skb);
    kfree_skbmem(skb);
  }
static void skb_release_all(struct sk_buff *skb)
{
        dst_release(skb->dst);
#ifdef CONFIG_XFRM
        secpath_put(skb->sp);
#endif
        if (skb->destructor) {
                WARN_ON(in_irq());
                skb->destructor(skb);
        }
#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
        nf_conntrack_put(skb->nfct);
        nf_conntrack_put_reasm(skb->nfct_reasm);
#endif
#ifdef CONFIG_BRIDGE_NETFILTER
        nf_bridge_put(skb->nf_bridge);
#endif
/* XXX: IS this still necessary? - JHS */
#ifdef CONFIG_NET_SCHED
        skb->tc_index = 0;
#ifdef CONFIG_NET_CLS_ACT
        skb->tc_verd = 0;
#endif
#endif
        skb_release_data(skb);
}
static void skb_release_data(struct sk_buff *skb)
{
        if (!skb->cloned ||
            !atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1,
                               &skb_shinfo(skb)->dataref)) {
                if (skb_shinfo(skb)->nr_frags) {
                        int i;
                        for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
                                put_page(skb_shinfo(skb)->frags[i].page);
                }

                if (skb_shinfo(skb)->frag_list)
                        skb_drop_fraglist(skb);

                kfree(skb->head);
        }
}

struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
{
        struct sk_buff *skb2;
        int delta = headroom - skb_headroom(skb);

        if (delta <= 0)
                skb2 = pskb_copy(skb, GFP_ATOMIC);
        else {
                skb2 = skb_clone(skb, GFP_ATOMIC);
                if (skb2 && pskb_expand_head(skb2, SKB_DATA_ALIGN(delta), 0,
                                             GFP_ATOMIC)) {
                        kfree_skb(skb2);
                        skb2 = NULL;
                }
        }
        return skb2;
}

/* not NET_SKBUFF_DATA_USES_OFFSET */
static inline unsigned char *skb_transport_header(const struct sk_buff *skb)
{
        return skb->transport_header;
}

static inline void skb_reset_transport_header(struct sk_buff *skb)
{
        skb->transport_header = skb->data;
}

static inline void skb_set_transport_header(struct sk_buff *skb,
                                            const int offset)
{
        skb->transport_header = skb->data + offset;
}

static inline unsigned char *skb_network_header(const struct sk_buff *skb)
{
        return skb->network_header;
}

static inline void skb_reset_network_header(struct sk_buff *skb)
{
        skb->network_header = skb->data;
}

static inline void skb_set_network_header(struct sk_buff *skb, const int offset)
{
        skb->network_header = skb->data + offset;
}

static inline unsigned char *skb_mac_header(const struct sk_buff *skb)
{
        return skb->mac_header;
}

static inline int skb_mac_header_was_set(const struct sk_buff *skb)
{
        return skb->mac_header != NULL;
}

static inline void skb_reset_mac_header(struct sk_buff *skb)
{
        skb->mac_header = skb->data;
}

static inline void skb_set_mac_header(struct sk_buff *skb, const int offset)
{
        skb->mac_header = skb->data + offset;
}