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/*
* 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
* Florian La Roche
*
* 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 : "");
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 : "");
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;
}
待续未完...
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