sk

/* * sk_buff 完全注释 * 金伟注释 blog -- http://jinweidavid.cublog.cn 转载请注明 */

/* * 本文件取自linux 2.6.13内核的skbuff.c */

/* * Routines having to do with the 'struct sk_buff' memory handlers. * * Authors: Alan Cox <iiitac@pyr.swan.ac.uk> * Florian La Roche <rzsfl@rz.uni-sb.de> * * Version: $Id: skbuff.c,v 1.90 2001/11/07 05:56:19 davem Exp $ * * Fixes: * Alan Cox : Fixed the worst of the load * balancer bugs. * Dave Platt : Interrupt stacking fix. * Richard Kooijman : Timestamp fixes. * Alan Cox : Changed buffer format. * Alan Cox : destructor hook for AF_UNIX etc. * Linus Torvalds : Better skb_clone. * Alan Cox : Added skb_copy. * Alan Cox : Added all the changed routines Linus * only put in the headers * Ray VanTassle : Fixed --skb->lock in free * Alan Cox : skb_copy copy arp field * Andi Kleen : slabified it. * Robert Olsson : Removed skb_head_pool * * NOTE: * The __skb_ routines should be called with interrupts * disabled, or you better be *real* sure that the operation is atomic * with respect to whatever list is being frobbed (e.g. via lock_sock() * or via disabling bottom half handlers, etc). * * This program is free software; you can redistribute it and/or * modify it under the terms of the GNU General Public License * as published by the Free Software Foundation; either version * 2 of the License, or (at your option) any later version. */

/* * The functions in this file will not compile correctly with gcc 2.4.x */

#include <linux/config.h>#include <linux/module.h>#include <linux/types.h>#include <linux/kernel.h>#include <linux/sched.h>#include <linux/mm.h>#include <linux/interrupt.h>#include <linux/in.h>#include <linux/inet.h>#include <linux/slab.h>#include <linux/netdevice.h>#ifdef CONFIG_NET_CLS_ACT#include <net/pkt_sched.h>#endif#include <linux/string.h>#include <linux/skbuff.h>#include <linux/cache.h>#include <linux/rtnetlink.h>#include <linux/init.h>#include <linux/highmem.h>

#include <net/protocol.h>#include <net/dst.h>#include <net/sock.h>#include <net/checksum.h>#include <net/xfrm.h>

#include <asm/uaccess.h>#include <asm/system.h>

static kmem_cache_t *skbuff_head_cache;

/* * Keep out-of-line to prevent kernel bloat. * __builtin_return_address is not used because it is not always * reliable. */

/** * skb_over_panic - private function * @skb: buffer * @sz: size * @here: address * * Out of line support code for skb_put(). Not user callable. */void skb_over_panic(struct sk_buff *skb, int sz, void *here){    printk(KERN_EMERG "skb_over_panic: text:%p len:%d put:%d head:%p "     "data:%p tail:%p end:%p dev:%s/n",     here, skb->len, sz, skb->head, skb->data, skb->tail, skb->end,     skb->dev ? skb->dev->name : "<NULL>");    BUG();}

/** * skb_under_panic - private function * @skb: buffer * @sz: size * @here: address * * Out of line support code for skb_push(). Not user callable. */

void skb_under_panic(struct sk_buff *skb, int sz, void *here){    printk(KERN_EMERG "skb_under_panic: text:%p len:%d put:%d head:%p "     "data:%p tail:%p end:%p dev:%s/n",     here, skb->len, sz, skb->head, skb->data, skb->tail, skb->end,     skb->dev ? skb->dev->name : "<NULL>");    BUG();}

/* Allocate a new skbuff. We do this ourselves so we can fill in a few * 'private' fields and also do memory statistics to find all the * [BEEP] leaks. * */

/** * alloc_skb - allocate a network buffer * @size: size to allocate * @gfp_mask: allocation mask * * Allocate a new &sk_buff. The returned buffer has no headroom and a * tail room of size bytes. The object has a reference count of one. * The return is the buffer. On a failure the return is %NULL. * * Buffers may only be allocated from interrupts using a @gfp_mask of * %GFP_ATOMIC. */struct sk_buff *alloc_skb(unsigned int size, int gfp_mask){    struct sk_buff *skb;    u8 *data;

    /* Get the HEAD */    /* 从cache缓冲池中获取内存 */    skb = kmem_cache_alloc(skbuff_head_cache,             gfp_mask & ~__GFP_DMA);    if (!skb)        goto out;

    /* Get the DATA. Size must match skb_add_mtu(). */

    /* 对其size */    size = SKB_DATA_ALIGN(size);

    /* 分配的缓冲长度包含skb_shared_info的长度 */    data = kmalloc(size + sizeof(struct skb_shared_info), gfp_mask);    if (!data)        goto nodata;        /*      * offsetof是一个编译器宏或者是自定义的宏，用于计算member在struct中的偏移量。     * 把在truesize前面的field全部清零。     */    memset(skb, 0, offsetof(struct sk_buff, truesize));        /* truesize是广义SKB的大小，包含了4个部分的长度：skb自身，header，page frags，frag list */    skb->truesize = size + sizeof(struct sk_buff);        /* users初始化成1 */    atomic_set(&skb->users, 1);

    /* 初始化所有数据指针 */    skb->head = data;    skb->data = data;    skb->tail = data;    skb->end = data + size;        /*      * skb_shinfo是个宏，#define skb_shinfo(SKB) ((struct skb_shared_info *)((SKB)->end))     * 所以用这个宏的时候必须等skb->end已经初始化。     * skb_shinfo 接在skb->end指向的内存空间后面。      */

    /* 初始化skb_shared_info结构体 */    atomic_set(&(skb_shinfo(skb)->dataref), 1);    skb_shinfo(skb)->nr_frags = 0;    skb_shinfo(skb)->tso_size = 0;    skb_shinfo(skb)->tso_segs = 0;    skb_shinfo(skb)->frag_list = NULL;out:    return skb;nodata:    kmem_cache_free(skbuff_head_cache, skb);    skb = NULL;    goto out;}

/** * alloc_skb_from_cache - allocate a network buffer * @cp: kmem_cache from which to allocate the data area * (object size must be big enough for @size bytes + skb overheads) * @size: size to allocate * @gfp_mask: allocation mask * * Allocate a new &sk_buff. The returned buffer has no headroom and * tail room of size bytes. The object has a reference count of one. * The return is the buffer. On a failure the return is %NULL. * * Buffers may only be allocated from interrupts using a @gfp_mask of * %GFP_ATOMIC. */struct sk_buff *alloc_skb_from_cache(kmem_cache_t *cp,                 unsigned int size, int gfp_mask){    struct sk_buff *skb;    u8 *data;

    /* Get the HEAD */    skb = kmem_cache_alloc(skbuff_head_cache,             gfp_mask & ~__GFP_DMA);    if (!skb)        goto out;

    /* Get the DATA. */    size = SKB_DATA_ALIGN(size);        /* 这个函数和上面函数不同的地方就在下面这句，不用kmalloc，而用kmem_cache_alloc。 */    data = kmem_cache_alloc(cp, gfp_mask);    if (!data)        goto nodata;

    memset(skb, 0, offsetof(struct sk_buff, truesize));    skb->truesize = size + sizeof(struct sk_buff);    atomic_set(&skb->users, 1);    skb->head = data;    skb->data = data;    skb->tail = data;    skb->end = data + size;

    atomic_set(&(skb_shinfo(skb)->dataref), 1);    skb_shinfo(skb)->nr_frags = 0;    skb_shinfo(skb)->tso_size = 0;    skb_shinfo(skb)->tso_segs = 0;    skb_shinfo(skb)->frag_list = NULL;out:    return skb;nodata:    kmem_cache_free(skbuff_head_cache, skb);    skb = NULL;    goto out;}

/* 这个函数是用来释放当前skb的frag_list区的 */static void skb_drop_fraglist(struct sk_buff *skb){    struct sk_buff *list = skb_shinfo(skb)->frag_list;

    skb_shinfo(skb)->frag_list = NULL;        /* 循环前进，直到没有为止。 */    do {        struct sk_buff *this = list;        list = list->next;        kfree_skb(this);    } while (list);}

static void skb_clone_fraglist(struct sk_buff *skb){    struct sk_buff *list;    /* 对当前skb的frag_list区链上的每个skb增加引用计数。 */    for (list = skb_shinfo(skb)->frag_list; list; list = list->next)        skb_get(list);}

void skb_release_data(struct sk_buff *skb){    /* 查看skb是否被clone？skb_shinfo的dataref是否为0？     * 如果是，那么就释放skb非线性区域和线性区域。 */    if (!skb->cloned ||     !atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1,             &skb_shinfo(skb)->dataref)) {                /* 释放page frags区 */        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);        }

        /* 释放frag_list区 */        if (skb_shinfo(skb)->frag_list)            skb_drop_fraglist(skb);

        /* 释放线性区域 */        kfree(skb->head);    }}

/* * Free an skbuff by memory without cleaning the state. */

/* 把skb自身和线性，非线性区域全部释放 */void kfree_skbmem(struct sk_buff *skb){    skb_release_data(skb);    kmem_cache_free(skbuff_head_cache, skb);}

/** * __kfree_skb - private function * @skb: buffer * * Free an sk_buff. Release anything attached to the buffer. * Clean the state. This is an internal helper function. Users should * always call kfree_skb *//* 这个函数应该也能算是一个wrapper函数 */

void __kfree_skb(struct sk_buff *skb){    BUG_ON(skb->list != NULL);

    dst_release(skb->dst);#ifdef CONFIG_XFRM    secpath_put(skb->sp);#endif    if (skb->destructor) {        WARN_ON(in_irq());        skb->destructor(skb);    }#ifdef CONFIG_NETFILTER    nf_conntrack_put(skb->nfct);#ifdef CONFIG_BRIDGE_NETFILTER    nf_bridge_put(skb->nf_bridge);#endif#endif/* XXX: IS this still necessary? - JHS */#ifdef CONFIG_NET_SCHED    skb->tc_index = 0;#ifdef CONFIG_NET_CLS_ACT    skb->tc_verd = 0;    skb->tc_classid = 0;#endif#endif

    kfree_skbmem(skb);}

/** * skb_clone - duplicate an sk_buff * @skb: buffer to clone * @gfp_mask: allocation priority * * Duplicate an &sk_buff. The new one is not owned by a socket. Both * copies share the same packet data but not structure. The new * buffer has a reference count of 1. If the allocation fails the * function returns %NULL otherwise the new buffer is returned. * * If this function is called from an interrupt gfp_mask() must be * %GFP_ATOMIC. */

struct sk_buff *skb_clone(struct sk_buff *skb, int gfp_mask){    /* 从cache池中分配一个skb */    struct sk_buff *n = kmem_cache_alloc(skbuff_head_cache, gfp_mask);

    if (!n)         return NULL;        /* 这个C(x) 就是clone的意思 */#define C(x) n->x = skb->x

    n->next = n->prev = NULL;    n->list = NULL;    n->sk = NULL;    /* 把skb中各个成员都clone过去 */    C(stamp);    C(dev);    C(real_dev);    C(h);    C(nh);    C(mac);    C(dst);    dst_clone(skb->dst);    C(sp);#ifdef CONFIG_INET    secpath_get(skb->sp);#endif    memcpy(n->cb, skb->cb, sizeof(skb->cb));    C(len);    C(data_len);    C(csum);    C(local_df);    /* 新分配的skb是clone的 */    n->cloned = 1;    n->nohdr = 0;    C(pkt_type);    C(ip_summed);    C(priority);    C(protocol);    C(security);    n->destructor = NULL;#ifdef CONFIG_NETFILTER    C(nfmark);    C(nfcache);    C(nfct);    nf_conntrack_get(skb->nfct);    C(nfctinfo);#ifdef CONFIG_NETFILTER_DEBUG    C(nf_debug);#endif#ifdef CONFIG_BRIDGE_NETFILTER    C(nf_bridge);    nf_bridge_get(skb->nf_bridge);#endif#endif /*CONFIG_NETFILTER*/#if defined(CONFIG_HIPPI)    C(private);#endif#ifdef CONFIG_NET_SCHED    C(tc_index);#ifdef CONFIG_NET_CLS_ACT    n->tc_verd = SET_TC_VERD(skb->tc_verd,0);    n->tc_verd = CLR_TC_OK2MUNGE(skb->tc_verd);    n->tc_verd = CLR_TC_MUNGED(skb->tc_verd);    C(input_dev);    C(tc_classid);#endif

#endif    C(truesize);    /* 新skb的users初始化为1 */    atomic_set(&n->users, 1);    C(head);    C(data);    C(tail);    C(end);        /* 增加被clone的skb的数据引用 */    atomic_inc(&(skb_shinfo(skb)->dataref));    /* 设置原skb也是被clone了 */    skb->cloned = 1;

    return n;}

static void copy_skb_header(struct sk_buff *new, const struct sk_buff *old){    /*     * Shift between the two data areas in bytes     */    /* 为了等一下要给网络各层的指针赋值，现在要先算出两个data的偏移量 */    unsigned long offset = new->data - old->data;

    new->list = NULL;    new->sk = NULL;    new->dev = old->dev;    new->real_dev = old->real_dev;    new->priority = old->priority;    new->protocol = old->protocol;    new->dst = dst_clone(old->dst);#ifdef CONFIG_INET    new->sp = secpath_get(old->sp);#endif    /* 用上面算出来的offset来算 */    new->h.raw = old->h.raw + offset;    new->nh.raw = old->nh.raw + offset;    new->mac.raw = old->mac.raw + offset;

    /* 拷贝control block */    memcpy(new->cb, old->cb, sizeof(old->cb));

    new->local_df = old->local_df;    new->pkt_type = old->pkt_type;    new->stamp = old->stamp;    new->destructor = NULL;    new->security = old->security;#ifdef CONFIG_NETFILTER    new->nfmark = old->nfmark;    new->nfcache = old->nfcache;    new->nfct = old->nfct;    nf_conntrack_get(old->nfct);    new->nfctinfo = old->nfctinfo;#ifdef CONFIG_NETFILTER_DEBUG    new->nf_debug = old->nf_debug;#endif#ifdef CONFIG_BRIDGE_NETFILTER    new->nf_bridge = old->nf_bridge;    nf_bridge_get(old->nf_bridge);#endif#endif#ifdef CONFIG_NET_SCHED#ifdef CONFIG_NET_CLS_ACT    new->tc_verd = old->tc_verd;#endif    new->tc_index = old->tc_index;#endif    /* 设置新的skb的users为1 */    atomic_set(&new->users, 1);

    /* 把skb_shinfo的东西也一起copy过去 */    skb_shinfo(new)->tso_size = skb_shinfo(old)->tso_size;    skb_shinfo(new)->tso_segs = skb_shinfo(old)->tso_segs;}

/** * skb_copy - create private copy of an sk_buff * @skb: buffer to copy * @gfp_mask: allocation priority * * Make a copy of both an &sk_buff and its data. This is used when the * caller wishes to modify the data and needs a private copy of the * data to alter. Returns %NULL on failure or the pointer to the buffer * on success. The returned buffer has a reference count of 1. * * As by-product this function converts non-linear &sk_buff to linear * one, so that &sk_buff becomes completely private and caller is allowed * to modify all the data of returned buffer. This means that this * function is not recommended for use in circumstances when only * header is going to be modified. Use pskb_copy() instead. */

struct sk_buff *skb_copy(const struct sk_buff *skb, int gfp_mask){    int headerlen = skb->data - skb->head;    /*     * Allocate the copy buffer     */

    /*      * 分配内存包含线性数据区的长度和非线性数据区的长度      * data_len是指非线性数据区的长度。     */    struct sk_buff *n = alloc_skb(skb->end - skb->head + skb->data_len,                 gfp_mask);    if (!n)        return NULL;

    /* Set the data pointer */    /* 预留头的长度 */    skb_reserve(n, headerlen);    /* Set the tail pointer and length */    /* len是指线性和非线性数据的总长，把tail往后推 */    skb_put(n, skb->len);    n->csum = skb->csum;    n->ip_summed = skb->ip_summed;    /* 因为 skb_copy_bits 函数中 offset是对有效负载的，即skb->data。     * 因此这里的offset为-headerlen。目的是从skb->data向前推headerlen。     * 从skb的head处拷贝到n的head处。这个函数把skb的线性和非线性部分全部拷贝到     * n的线性部分去了。     */    if (skb_copy_bits(skb, -headerlen, n->head, headerlen + skb->len))        BUG();        /* 把skb的本身复制到n的本身 */    copy_skb_header(n, skb);    return n;}

/** * pskb_copy - create copy of an sk_buff with private head. * @skb: buffer to copy * @gfp_mask: allocation priority * * Make a copy of both an &sk_buff and part of its data, located * in header. Fragmented data remain shared. This is used when * the caller wishes to modify only header of &sk_buff and needs * private copy of the header to alter. Returns %NULL on failure * or the pointer to the buffer on success. * The returned buffer has a reference count of 1. */

struct sk_buff *pskb_copy(struct sk_buff *skb, int gfp_mask){    /*     * Allocate the copy buffer     */    /* 分配一个新的skb_buff n,它的线性区长度是和原skb长度一样 */    struct sk_buff *n = alloc_skb(skb->end - skb->head, gfp_mask);

    if (!n)        goto out;

    /* Set the data pointer */    /* 预留head到data之间的空隙 */    skb_reserve(n, skb->data - skb->head);

    /* Set the tail pointer and length */    /* 准备向n放数据，试放数据长度是skb的header section的长度 */    skb_put(n, skb_headlen(skb));

    /* Copy the bytes */    /* 拷贝有效负载，长度是n->len。上面skb_put中已经把n->len赋值成skb_headlen(skb)     * 所以这里拷贝线性区域的长度。     */    memcpy(n->data, skb->data, n->len);

    /* 复制skb本身信息到n */    n->csum = skb->csum;    n->ip_summed = skb->ip_summed;

    n->data_len = skb->data_len;    n->len = skb->len;

    /* 把skb中page frags的指针复制到n的page frags。 */    if (skb_shinfo(skb)->nr_frags) {        int i;

        for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {            skb_shinfo(n)->frags[i] = skb_shinfo(skb)->frags[i];            get_page(skb_shinfo(n)->frags[i].page);        }        skb_shinfo(n)->nr_frags = i;    }

    /* 把skb中frag_list地址复制到n的frag_list */    if (skb_shinfo(skb)->frag_list) {        skb_shinfo(n)->frag_list = skb_shinfo(skb)->frag_list;        skb_clone_fraglist(n);    }

    /* 把skb的本身复制到n的本身 */    copy_skb_header(n, skb);out:    return n;}

/** * pskb_expand_head - reallocate header of &sk_buff * @skb: buffer to reallocate * @nhead: room to add at head * @ntail: room to add at tail * @gfp_mask: allocation priority * * Expands (or creates identical copy, if &nhead and &ntail are zero) * header of skb. &sk_buff itself is not changed. &sk_buff MUST have * reference count of 1. Returns zero in the case of success or error, * if expansion failed. In the last case, &sk_buff is not changed. * * All the pointers pointing into skb header may change and must be * reloaded after call to this function. *//* 这个函数要注意的是原来的skb结构体并没有释放 * 释放的是header section数据区。 */int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail, int gfp_mask){    int i;    u8 *data;    /* 算出原来线性区的长度，再加上现在要求的增加的headroom和tailroom。 */    int size = nhead + (skb->end - skb->head) + ntail;    long off;

    if (skb_shared(skb))        BUG();        /* 对齐size的大小 */    size = SKB_DATA_ALIGN(size);

    /* 按照要求分配新的header section */    data = kmalloc(size + sizeof(struct skb_shared_info), gfp_mask);    if (!data)        goto nodata;

    /* Copy only real data... and, alas, header. This should be     * optimized for the cases when header is void. */    /* 拷贝payload到正确的位置上 */    memcpy(data + nhead, skb->head, skb->tail - skb->head);    memcpy(data + size, skb->end, sizeof(struct skb_shared_info));

    /* 下面复制page frags区域和fraglist区域的指针 */    for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)        get_page(skb_shinfo(skb)->frags[i].page);

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

    /* 释放原来的数据区 */    skb_release_data(skb);        /* 计算偏移量 */    off = (data + nhead) - skb->head;

    skb->head = data;    skb->end = data + size;    skb->data += off;    skb->tail += off;    skb->mac.raw += off;    skb->h.raw += off;    skb->nh.raw += off;    skb->cloned = 0;    skb->nohdr = 0;    atomic_set(&skb_shinfo(skb)->dataref, 1);    return 0;

nodata:    return -ENOMEM;}

/* Make private copy of skb with writable head and some headroom */

struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom){    struct sk_buff *skb2;    /* 计算现在要求的headroom 和原来headroom之间的差值 */    int delta = headroom - skb_headroom(skb);        /* 如果现在要求的headroom没有原来的headroom大，那说明原来的header section可以用，     * 所以只要用pskb_copy复制一份skb结构体和它的线性区域就可以了。     */    if (delta <= 0)        skb2 = pskb_copy(skb, GFP_ATOMIC);    else {        /* 如果要求的headroom比原来的headroom大的话，clone一个skb */        skb2 = skb_clone(skb, GFP_ATOMIC);        /* 把新clone的skb用pskb_expand_head扩大headroom */        if (skb2 && pskb_expand_head(skb2, SKB_DATA_ALIGN(delta), 0,                     GFP_ATOMIC)) {            kfree_skb(skb2);            skb2 = NULL;        }    }    return skb2;}

/** * skb_copy_expand - copy and expand sk_buff * @skb: buffer to copy * @newheadroom: new free bytes at head * @newtailroom: new free bytes at tail * @gfp_mask: allocation priority * * Make a copy of both an &sk_buff and its data and while doing so * allocate additional space. * * This is used when the caller wishes to modify the data and needs a * private copy of the data to alter as well as more space for new fields. * Returns %NULL on failure or the pointer to the buffer * on success. The returned buffer has a reference count of 1. * * You must pass %GFP_ATOMIC as the allocation priority if this function * is called from an interrupt. * * BUG ALERT: ip_summed is not copied. Why does this work? Is it used * only by netfilter in the cases when checksum is recalculated? --ANK */struct sk_buff *skb_copy_expand(const struct sk_buff *skb,                int newheadroom, int newtailroom, int gfp_mask){    /*     * Allocate the copy buffer     */    /* 分配一个新的skb结构体，header section长度是原来的skb所有数据长度加上新的skb要求的headroom     * 和要求的tailroom。目的是把原来的SKB线性化。     */    struct sk_buff *n = alloc_skb(newheadroom + skb->len + newtailroom,                 gfp_mask);    int head_copy_len, head_copy_off;

    if (!n)        return NULL;

    /* 新的sk_buff n的headroom长度为newheadroom */    skb_reserve(n, newheadroom);

    /* Set the tail pointer and length */    /* 设置tail指针和n->len */    skb_put(n, skb->len);

    /* 设置head_copy_len 为老的skb的headroom */    head_copy_len = skb_headroom(skb);    head_copy_off = 0;    /* 如果新的headroom比老的headroom小，     *　拷贝长度就为新的headroom的长度。     */    if (newheadroom <= head_copy_len)        head_copy_len = newheadroom;    else        head_copy_off = newheadroom - head_copy_len;

    /* Copy the linear header and data. */    /* offset为原来skb->data-head_copy_len */    if (skb_copy_bits(skb, -head_copy_len, n->head + head_copy_off,             skb->len + head_copy_len))        BUG();

    /* 拷贝skb结构体到n结构体 */    copy_skb_header(n, skb);

    return n;}

/** * skb_pad - zero pad the tail of an skb * @skb: buffer to pad * @pad: space to pad * * Ensure that a buffer is followed by a padding area that is zero * filled. Used by network drivers which may DMA or transfer data * beyond the buffer end onto the wire. * * May return NULL in out of memory cases. */ struct sk_buff *skb_pad(struct sk_buff *skb, int pad){    struct sk_buff *nskb;        /* If the skbuff is non linear tailroom is always zero.. */    /* 如果需要pad的长度比skb_tailroom小的话，     * 就直接从skb->data+skb->len,开始清零.     */    if (skb_tailroom(skb) >= pad) {        memset(skb->data+skb->len, 0, pad);        return skb;    }        /* 如果需要pad的长度比tailroom长的话，就skb_copy_expand */    nskb = skb_copy_expand(skb, skb_headroom(skb), skb_tailroom(skb) + pad, GFP_ATOMIC);    /* 释放原来的SKB */    kfree_skb(skb);    /* 清零 */    if (nskb)        memset(nskb->data+nskb->len, 0, pad);    return nskb;}

 

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