1 /*
2 * Routines having to do with the 'struct sk_buff' memory handlers.
3 *
4 * Authors: Alan Cox <iiitac@pyr.swan.ac.uk>
5 * Florian La Roche <rzsfl@rz.uni-sb.de>
6 *
7 * Version: $Id: skbuff.c,v 1.90 2001/11/07 05:56:19 davem Exp $
8 *
9 * Fixes:
10 * Alan Cox : Fixed the worst of the load
11 * balancer bugs.
12 * Dave Platt : Interrupt stacking fix.
13 * Richard Kooijman : Timestamp fixes.
14 * Alan Cox : Changed buffer format.
15 * Alan Cox : destructor hook for AF_UNIX etc.
16 * Linus Torvalds : Better skb_clone.
17 * Alan Cox : Added skb_copy.
18 * Alan Cox : Added all the changed routines Linus
19 * only put in the headers
20 * Ray VanTassle : Fixed --skb->lock in free
21 * Alan Cox : skb_copy copy arp field
22 * Andi Kleen : slabified it.
23 * Robert Olsson : Removed skb_head_pool
24 *
25 * NOTE:
26 * The __skb_ routines should be called with interrupts
27 * disabled, or you better be *real* sure that the operation is atomic
28 * with respect to whatever list is being frobbed (e.g. via lock_sock()
29 * or via disabling bottom half handlers, etc).
30 *
31 * This program is free software; you can redistribute it and/or
32 * modify it under the terms of the GNU General Public License
33 * as published by the Free Software Foundation; either version
34 * 2 of the License, or (at your option) any later version.
35 */
36
37 /*
38 * The functions in this file will not compile correctly with gcc 2.4.x
39 */
40
41 #include <linux/config.h>
42 #include <linux/module.h>
43 #include <linux/types.h>
44 #include <linux/kernel.h>
45 #include <linux/sched.h>
46 #include <linux/mm.h>
47 #include <linux/interrupt.h>
48 #include <linux/in.h>
49 #include <linux/inet.h>
50 #include <linux/slab.h>
51 #include <linux/netdevice.h>
52 #ifdef CONFIG_NET_CLS_ACT
53 #include <net/pkt_sched.h>
54 #endif
55 #include <linux/string.h>
56 #include <linux/skbuff.h>
57 #include <linux/cache.h>
58 #include <linux/rtnetlink.h>
59 #include <linux/init.h>
60 #include <linux/highmem.h>
61
62 #include <net/protocol.h>
63 #include <net/dst.h>
64 #include <net/sock.h>
65 #include <net/checksum.h>
66 #include <net/xfrm.h>
67
68 #include <asm/uaccess.h>
69 #include <asm/system.h>
70
71 static kmem_cache_t *skbuff_head_cache;
72
73 /*
74 * Keep out-of-line to prevent kernel bloat.
75 * __builtin_return_address is not used because it is not always
76 * reliable.
77 */
78
79 /**
80 * skb_over_panic - private function
81 * @skb: buffer
82 * @sz: size
83 * @here: address
84 *
85 * Out of line support code for skb_put(). Not user callable.
86 */
87 void skb_over_panic(struct sk_buff *skb, int sz, void *here)
88 {
89 printk(KERN_EMERG "skb_over_panic: text:%p len:%d put:%d head:%p "
90 "data:%p tail:%p end:%p dev:%s/n",
91 here, skb->len, sz, skb->head, skb->data, skb->tail, skb->end,
92 skb->dev ? skb->dev->name : "<NULL>");
93 BUG();
94 }
95
96 /**
97 * skb_under_panic - private function
98 * @skb: buffer
99 * @sz: size
100 * @here: address
101 *
102 * Out of line support code for skb_push(). Not user callable.
103 */
104
105 void skb_under_panic(struct sk_buff *skb, int sz, void *here)
106 {
107 printk(KERN_EMERG "skb_under_panic: text:%p len:%d put:%d head:%p "
108 "data:%p tail:%p end:%p dev:%s/n",
109 here, skb->len, sz, skb->head, skb->data, skb->tail, skb->end,
110 skb->dev ? skb->dev->name : "<NULL>");
111 BUG();
112 }
113
114 /* Allocate a new skbuff. We do this ourselves so we can fill in a few
115 * 'private' fields and also do memory statistics to find all the
116 * [BEEP] leaks.
117 *
118 */
119
120 /**
121 * alloc_skb - allocate a network buffer
122 * @size: size to allocate
123 * @gfp_mask: allocation mask
124 *
125 * Allocate a new &sk_buff. The returned buffer has no headroom and a
126 * tail room of size bytes. The object has a reference count of one.
127 * The return is the buffer. On a failure the return is %NULL.
128 *
129 * Buffers may only be allocated from interrupts using a @gfp_mask of
130 * %GFP_ATOMIC.
131 */
132 struct sk_buff *alloc_skb(unsigned int size, int gfp_mask)
133 {
134 struct sk_buff *skb;
135 u8 *data;
136
137 /* Get the HEAD */
138 /* 从cache缓冲池中获取内存 */
139 skb = kmem_cache_alloc(skbuff_head_cache,
140 gfp_mask & ~__GFP_DMA);
141 if (!skb)
142 goto out;
143
144 /* Get the DATA. Size must match skb_add_mtu(). */
145
146 /* 对其size */
147 size = SKB_DATA_ALIGN(size);
148
149 /* 分配的缓冲长度包含skb_shared_info的长度 */
150 data = kmalloc(size + sizeof(struct skb_shared_info), gfp_mask);
151 if (!data)
152 goto nodata;
153
154 /*
155 * 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。
156 * 把在truesize前面的field全部清零。
157 */
158 memset(skb, 0, offsetof(struct sk_buff, truesize));
159
160 /* 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 */
161 skb->truesize = size + sizeof(struct sk_buff);
162
163 /* users初始化成1 */
164 atomic_set(&skb->users, 1);
165
166 /* 初始化所有数据指针 */
167 skb->head = data;
168 skb->data = data;
169 skb->tail = data;
170 skb->end = data + size;
171
172 /*
173 * skb_shinfo是个宏，#define skb_shinfo(SKB) ((struct skb_shared_info *)((SKB)->end))
174 * 所以用这个宏的时候必须等skb->end已经初始化。
175 * skb_shinfo 接在skb->end指向的内存空间后面。
176 */
177
178 /* 初始化skb_shared_info结构体 */
179 atomic_set(&(skb_shinfo(skb)->dataref), 1);
180 skb_shinfo(skb)->nr_frags = 0;
181 skb_shinfo(skb)->tso_size = 0;
182 skb_shinfo(skb)->tso_segs = 0;
183 skb_shinfo(skb)->frag_list = NULL;
184 out:
185 return skb;
186 nodata:
187 kmem_cache_free(skbuff_head_cache, skb);
188 skb = NULL;
189 goto out;
190 }
191
192 /**
193 * alloc_skb_from_cache - allocate a network buffer
194 * @cp: kmem_cache from which to allocate the data area
195 * (object size must be big enough for @size bytes + skb overheads)
196 * @size: size to allocate
197 * @gfp_mask: allocation mask
198 *
199 * Allocate a new &sk_buff. The returned buffer has no headroom and
200 * tail room of size bytes. The object has a reference count of one.
201 * The return is the buffer. On a failure the return is %NULL.
202 *
203 * Buffers may only be allocated from interrupts using a @gfp_mask of
204 * %GFP_ATOMIC.
205 */
206 struct sk_buff *alloc_skb_from_cache(kmem_cache_t *cp,
207 unsigned int size, int gfp_mask)
208 {
209 struct sk_buff *skb;
210 u8 *data;
211
212 /* Get the HEAD */
213 skb = kmem_cache_alloc(skbuff_head_cache,
214 gfp_mask & ~__GFP_DMA);
215 if (!skb)
216 goto out;
217
218 /* Get the DATA. */
219 size = SKB_DATA_ALIGN(size);
220
221 /* 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。 */
222 data = kmem_cache_alloc(cp, gfp_mask);
223 if (!data)
224 goto nodata;
225
226 memset(skb, 0, offsetof(struct sk_buff, truesize));
227 skb->truesize = size + sizeof(struct sk_buff);
228 atomic_set(&skb->users, 1);
229 skb->head = data;
230 skb->data = data;
231 skb->tail = data;
232 skb->end = data + size;
233
234 atomic_set(&(skb_shinfo(skb)->dataref), 1);
235 skb_shinfo(skb)->nr_frags = 0;
236 skb_shinfo(skb)->tso_size = 0;
237 skb_shinfo(skb)->tso_segs = 0;
238 skb_shinfo(skb)->frag_list = NULL;
239 out:
240 return skb;
241 nodata:
242 kmem_cache_free(skbuff_head_cache, skb);
243 skb = NULL;
244 goto out;
245 }
246
247 /* 这个函数是用来释放当前skb的frag_list区的 */
248 static void skb_drop_fraglist(struct sk_buff *skb)
249 {
250 struct sk_buff *list = skb_shinfo(skb)->frag_list;
251
252 skb_shinfo(skb)->frag_list = NULL;
253
254 /* 循环前进，直到没有为止。 */
255 do {
256 struct sk_buff *this = list;
257 list = list->next;
258 kfree_skb(this);
259 } while (list);
260 }
261
262 static void skb_clone_fraglist(struct sk_buff *skb)
263 {
264 struct sk_buff *list;
265 /* 对当前skb的frag_list区链上的每个skb增加引用计数。 */
266 for (list = skb_shinfo(skb)->frag_list; list; list = list->next)
267 skb_get(list);
268 }
269
270 void skb_release_data(struct sk_buff *skb)
271 {
272 /* 查看skb是否被clone？skb_shinfo的dataref是否为0？
273 * 如果是，那么就释放skb非线性区域和线性区域。 */
274 if (!skb->cloned ||
275 !atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1,
276 &skb_shinfo(skb)->dataref)) {
277
278 /* 释放page frags区 */
279 if (skb_shinfo(skb)->nr_frags) {
280 int i;
281 for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
282 put_page(skb_shinfo(skb)->frags[i].page);
283 }
284
285 /* 释放frag_list区 */
286 if (skb_shinfo(skb)->frag_list)
287 skb_drop_fraglist(skb);
288
289 /* 释放线性区域 */
290 kfree(skb->head);
291 }
292 }
293
294 /*
295 * Free an skbuff by memory without cleaning the state.
296 */
297
298 /* 把skb自身和线性，非线性区域全部释放 */
299 void kfree_skbmem(struct sk_buff *skb)
300 {
301 skb_release_data(skb);
302 kmem_cache_free(skbuff_head_cache, skb);
303 }
304
305 /**
306 * __kfree_skb - private function
307 * @skb: buffer
308 *
309 * Free an sk_buff. Release anything attached to the buffer.
310 * Clean the state. This is an internal helper function. Users should
311 * always call kfree_skb
312 */
313 /* 这个函数应该也能算是一个wrapper函数 */
314
315 void __kfree_skb(struct sk_buff *skb)
316 {
317 BUG_ON(skb->list != NULL);
318
319 dst_release(skb->dst);
320 #ifdef CONFIG_XFRM
321 secpath_put(skb->sp);
322 #endif
323 if (skb->destructor) {
324 WARN_ON(in_irq());
325 skb->destructor(skb);
326 }
327 #ifdef CONFIG_NETFILTER
328 nf_conntrack_put(skb->nfct);
329 #ifdef CONFIG_BRIDGE_NETFILTER
330 nf_bridge_put(skb->nf_bridge);
331 #endif
332 #endif
333 /* XXX: IS this still necessary? - JHS */
334 #ifdef CONFIG_NET_SCHED
335 skb->tc_index = 0;
336 #ifdef CONFIG_NET_CLS_ACT
337 skb->tc_verd = 0;
338 skb->tc_classid = 0;
339 #endif
340 #endif
341
342 kfree_skbmem(skb);
343 }
344
345 /**
346 * skb_clone - duplicate an sk_buff
347 * @skb: buffer to clone
348 * @gfp_mask: allocation priority
349 *
350 * Duplicate an &sk_buff. The new one is not owned by a socket. Both
351 * copies share the same packet data but not structure. The new
352 * buffer has a reference count of 1. If the allocation fails the
353 * function returns %NULL otherwise the new buffer is returned.
354 *
355 * If this function is called from an interrupt gfp_mask() must be
356 * %GFP_ATOMIC.
357 */
358
359 struct sk_buff *skb_clone(struct sk_buff *skb, int gfp_mask)
360 {
361 /* 从cache池中分配一个skb */
362 struct sk_buff *n = kmem_cache_alloc(skbuff_head_cache, gfp_mask);
363
364 if (!n)
365 return NULL;
366
367 /* 这个C(x) 就是clone的意思 */
368 #define C(x) n->x = skb->x
369
370 n->next = n->prev = NULL;
371 n->list = NULL;
372 n->sk = NULL;
373 /* 把skb中各个成员都clone过去 */
374 C(stamp);
375 C(dev);
376 C(real_dev);
377 C(h);
378 C(nh);
379 C(mac);
380 C(dst);
381 dst_clone(skb->dst);
382 C(sp);
383 #ifdef CONFIG_INET
384 secpath_get(skb->sp);
385 #endif
386 memcpy(n->cb, skb->cb, sizeof(skb->cb));
387 C(len);
388 C(data_len);
389 C(csum);
390 C(local_df);
391 /* 新分配的skb是clone的 */
392 n->cloned = 1;
393 n->nohdr = 0;
394 C(pkt_type);
395 C(ip_summed);
396 C(priority);
397 C(protocol);
398 C(security);
399 n->destructor = NULL;
400 #ifdef CONFIG_NETFILTER
401 C(nfmark);
402 C(nfcache);
403 C(nfct);
404 nf_conntrack_get(skb->nfct);
405 C(nfctinfo);
406 #ifdef CONFIG_NETFILTER_DEBUG
407 C(nf_debug);
408 #endif
409 #ifdef CONFIG_BRIDGE_NETFILTER
410 C(nf_bridge);
411 nf_bridge_get(skb->nf_bridge);
412 #endif
413 #endif /*CONFIG_NETFILTER*/
414 #if defined(CONFIG_HIPPI)
415 C(private);
416 #endif
417 #ifdef CONFIG_NET_SCHED
418 C(tc_index);
419 #ifdef CONFIG_NET_CLS_ACT
420 n->tc_verd = SET_TC_VERD(skb->tc_verd,0);
421 n->tc_verd = CLR_TC_OK2MUNGE(skb->tc_verd);
422 n->tc_verd = CLR_TC_MUNGED(skb->tc_verd);
423 C(input_dev);
424 C(tc_classid);
425 #endif
426
427 #endif
428 C(truesize);
429 /* 新skb的users初始化为1 */
430 atomic_set(&n->users, 1);
431 C(head);
432 C(data);
433 C(tail);
434 C(end);
435
436 /* 增加被clone的skb的数据引用 */
437 atomic_inc(&(skb_shinfo(skb)->dataref));
438 /* 设置原skb也是被clone了 */
439 skb->cloned = 1;
440
441 return n;
442 }
443
444
445 static void copy_skb_header(struct sk_buff *new, const struct sk_buff *old)
446 {
447 /*
448 * Shift between the two data areas in bytes
449 */
450 /* 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 */
451 unsigned long offset = new->data - old->data;
452
453 new->list = NULL;
454 new->sk = NULL;
455 new->dev = old->dev;
456 new->real_dev = old->real_dev;
457 new->priority = old->priority;
458 new->protocol = old->protocol;
459 new->dst = dst_clone(old->dst);
460 #ifdef CONFIG_INET
461 new->sp = secpath_get(old->sp);
462 #endif
463 /* 用上面算出来的offset来算 */
464 new->h.raw = old->h.raw + offset;
465 new->nh.raw = old->nh.raw + offset;
466 new->mac.raw = old->mac.raw + offset;
467
468 /* 拷贝control block */
469 memcpy(new->cb, old->cb, sizeof(old->cb));
470
471 new->local_df = old->local_df;
472 new->pkt_type = old->pkt_type;
473 new->stamp = old->stamp;
474 new->destructor = NULL;
475 new->security = old->security;
476 #ifdef CONFIG_NETFILTER
477 new->nfmark = old->nfmark;
478 new->nfcache = old->nfcache;
479 new->nfct = old->nfct;
480 nf_conntrack_get(old->nfct);
481 new->nfctinfo = old->nfctinfo;
482 #ifdef CONFIG_NETFILTER_DEBUG
483 new->nf_debug = old->nf_debug;
484 #endif
485 #ifdef CONFIG_BRIDGE_NETFILTER
486 new->nf_bridge = old->nf_bridge;
487 nf_bridge_get(old->nf_bridge);
488 #endif
489 #endif
490 #ifdef CONFIG_NET_SCHED
491 #ifdef CONFIG_NET_CLS_ACT
492 new->tc_verd = old->tc_verd;
493 #endif
494 new->tc_index = old->tc_index;
495 #endif
496 /* 设置新的skb的users为1 */
497 atomic_set(&new->users, 1);
498
499 /* 把skb_shinfo的东西也一起copy过去 */
500 skb_shinfo(new)->tso_size = skb_shinfo(old)->tso_size;
501 skb_shinfo(new)->tso_segs = skb_shinfo(old)->tso_segs;
502 }
503
504 /**
505 * skb_copy - create private copy of an sk_buff
506 * @skb: buffer to copy
507 * @gfp_mask: allocation priority
508 *
509 * Make a copy of both an &sk_buff and its data. This is used when the
510 * caller wishes to modify the data and needs a private copy of the
511 * data to alter. Returns %NULL on failure or the pointer to the buffer
512 * on success. The returned buffer has a reference count of 1.
513 *
514 * As by-product this function converts non-linear &sk_buff to linear
515 * one, so that &sk_buff becomes completely private and caller is allowed
516 * to modify all the data of returned buffer. This means that this
517 * function is not recommended for use in circumstances when only
518 * header is going to be modified. Use pskb_copy() instead.
519 */
520
521 struct sk_buff *skb_copy(const struct sk_buff *skb, int gfp_mask)
522 {
523 int headerlen = skb->data - skb->head;
524 /*
525 * Allocate the copy buffer
526 */
527
528 /*
529 * 分配内存包含线性数据区的长度和非线性数据区的长度
530 * data_len是指非线性数据区的长度。
531 */
532 struct sk_buff *n = alloc_skb(skb->end - skb->head + skb->data_len,
533 gfp_mask);
534 if (!n)
535 return NULL;
536
537 /* Set the data pointer */
538 /* 预留头的长度 */
539 skb_reserve(n, headerlen);
540 /* Set the tail pointer and length */
541 /* len是指线性和非线性数据的总长，把tail往后推 */
542 skb_put(n, skb->len);
543 n->csum = skb->csum;
544 n->ip_summed = skb->ip_summed;
545 /* 因为 skb_copy_bits 函数中 offset是对有效负载的，即skb->data。
546 * 因此这里的offset为-headerlen。目的是从skb->data向前推headerlen。
547 * 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
548 * n的线性部分去了。
549 */
550 if (skb_copy_bits(skb, -headerlen, n->head, headerlen + skb->len))
551 BUG();
552
553 /* 把skb的本身复制到n的本身 */
554 copy_skb_header(n, skb);
555 return n;
556 }
557
558
559 /**
560 * pskb_copy - create copy of an sk_buff with private head.
561 * @skb: buffer to copy
562 * @gfp_mask: allocation priority
563 *
564 * Make a copy of both an &sk_buff and part of its data, located
565 * in header. Fragmented data remain shared. This is used when
566 * the caller wishes to modify only header of &sk_buff and needs
567 * private copy of the header to alter. Returns %NULL on failure
568 * or the pointer to the buffer on success.
569 * The returned buffer has a reference count of 1.
570 */
571
572 struct sk_buff *pskb_copy(struct sk_buff *skb, int gfp_mask)
573 {
574 /*
575 * Allocate the copy buffer
576 */
577 /* 分配一个新的skb_buff n,它的线性区长度是和原skb长度一样 */
578 struct sk_buff *n = alloc_skb(skb->end - skb->head, gfp_mask);
579
580 if (!n)
581 goto out;
582
583 /* Set the data pointer */
584 /* 预留head到data之间的空隙 */
585 skb_reserve(n, skb->data - skb->head);
586
587 /* Set the tail pointer and length */
588 /* 准备向n放数据，试放数据长度是skb的header section的长度 */
589 skb_put(n, skb_headlen(skb));
590
591 /* Copy the bytes */
592 /* 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)
593 * 所以这里拷贝线性区域的长度。
594 */
595 memcpy(n->data, skb->data, n->len);
596
597 /* 复制skb本身信息到n */
598 n->csum = skb->csum;
599 n->ip_summed = skb->ip_summed;
600
601 n->data_len = skb->data_len;
602 n->len = skb->len;
603
604 /* 把skb中page frags的指针复制到n的page frags。 */
605 if (skb_shinfo(skb)->nr_frags) {
606 int i;
607
608 for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
609 skb_shinfo(n)->frags[i] = skb_shinfo(skb)->frags[i];
610 get_page(skb_shinfo(n)->frags[i].page);
611 }
612 skb_shinfo(n)->nr_frags = i;
613 }
614
615 /* 把skb中frag_list地址复制到n的frag_list */
616 if (skb_shinfo(skb)->frag_list) {
617 skb_shinfo(n)->frag_list = skb_shinfo(skb)->frag_list;
618 skb_clone_fraglist(n);
619 }
620
621 /* 把skb的本身复制到n的本身 */
622 copy_skb_header(n, skb);
623 out:
624 return n;
625 }
626
627 /**
628 * pskb_expand_head - reallocate header of &sk_buff
629 * @skb: buffer to reallocate
630 * @nhead: room to add at head
631 * @ntail: room to add at tail
632 * @gfp_mask: allocation priority
633 *
634 * Expands (or creates identical copy, if &nhead and &ntail are zero)
635 * header of skb. &sk_buff itself is not changed. &sk_buff MUST have
636 * reference count of 1. Returns zero in the case of success or error,
637 * if expansion failed. In the last case, &sk_buff is not changed.
638 *
639 * All the pointers pointing into skb header may change and must be
640 * reloaded after call to this function.
641 */
642 /* 这个函数要注意的是原来的skb结构体并没有释放
643 * 释放的是header section数据区。
644 */
645 int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail, int gfp_mask)
646 {
647 int i;
648 u8 *data;
649 /* 算出原来线性区的长度，再加上现在要求的增加的headroom和tailroom。 */
650 int size = nhead + (skb->end - skb->head) + ntail;
651 long off;
652
653 if (skb_shared(skb))
654 BUG();
655
656 /* 对齐size的大小 */
657 size = SKB_DATA_ALIGN(size);
658
659 /* 按照要求分配新的header section */
660 data = kmalloc(size + sizeof(struct skb_shared_info), gfp_mask);
661 if (!data)
662 goto nodata;
663
664 /* Copy only real data... and, alas, header. This should be
665 * optimized for the cases when header is void. */
666 /* 拷贝payload到正确的位置上 */
667 memcpy(data + nhead, skb->head, skb->tail - skb->head);
668 memcpy(data + size, skb->end, sizeof(struct skb_shared_info));
669
670 /* 下面复制page frags区域和fraglist区域的指针 */
671 for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
672 get_page(skb_shinfo(skb)->frags[i].page);
673
674 if (skb_shinfo(skb)->frag_list)
675 skb_clone_fraglist(skb);
676
677 /* 释放原来的数据区 */
678 skb_release_data(skb);
679
680 /* 计算偏移量 */
681 off = (data + nhead) - skb->head;
682
683 skb->head = data;
684 skb->end = data + size;
685 skb->data += off;
686 skb->tail += off;
687 skb->mac.raw += off;
688 skb->h.raw += off;
689 skb->nh.raw += off;
690 skb->cloned = 0;
691 skb->nohdr = 0;
692 atomic_set(&skb_shinfo(skb)->dataref, 1);
693 return 0;
694
695 nodata:
696 return -ENOMEM;
697 }
698
699 /* Make private copy of skb with writable head and some headroom */
700
701 struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
702 {
703 struct sk_buff *skb2;
704 /* 计算现在要求的headroom 和原来headroom之间的差值 */
705 int delta = headroom - skb_headroom(skb);
706
707 /* 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，
708 * 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。
709 */
710 if (delta <= 0)
711 skb2 = pskb_copy(skb, GFP_ATOMIC);
712 else {
713 /* 如果要求的headroom比原来的headroom大的话，clone一个skb */
714 skb2 = skb_clone(skb, GFP_ATOMIC);
715 /* 把新clone的skb用pskb_expand_head扩大headroom */
716 if (skb2 && pskb_expand_head(skb2, SKB_DATA_ALIGN(delta), 0,
717 GFP_ATOMIC)) {
718 kfree_skb(skb2);
719 skb2 = NULL;
720 }
721 }
722 return skb2;
723 }
724
725
726 /**
727 * skb_copy_expand - copy and expand sk_buff
728 * @skb: buffer to copy
729 * @newheadroom: new free bytes at head
730 * @newtailroom: new free bytes at tail
731 * @gfp_mask: allocation priority
732 *
733 * Make a copy of both an &sk_buff and its data and while doing so
734 * allocate additional space.
735 *
736 * This is used when the caller wishes to modify the data and needs a
737 * private copy of the data to alter as well as more space for new fields.
738 * Returns %NULL on failure or the pointer to the buffer
739 * on success. The returned buffer has a reference count of 1.
740 *
741 * You must pass %GFP_ATOMIC as the allocation priority if this function
742 * is called from an interrupt.
743 *
744 * BUG ALERT: ip_summed is not copied. Why does this work? Is it used
745 * only by netfilter in the cases when checksum is recalculated? --ANK
746 */
747 struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
748 int newheadroom, int newtailroom, int gfp_mask)
749 {
750 /*
751 * Allocate the copy buffer
752 */
753 /* 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
754 * 和要求的tailroom。目的是把原来的SKB线性化。
755 */
756 struct sk_buff *n = alloc_skb(newheadroom + skb->len + newtailroom,
757 gfp_mask);
758 int head_copy_len, head_copy_off;
759
760 if (!n)
761 return NULL;
762
763 /* 新的sk_buff n的headroom长度为newheadroom */
764 skb_reserve(n, newheadroom);
765
766 /* Set the tail pointer and length */
767 /* 设置tail指针和n->len */
768 skb_put(n, skb->len);
769
770 /* 设置head_copy_len 为老的skb的headroom */
771 head_copy_len = skb_headroom(skb);
772 head_copy_off = 0;
773 /* 如果新的headroom比老的headroom小，
774 * 拷贝长度就为新的headroom的长度。
775 */
776 if (newheadroom <= head_copy_len)
777 head_copy_len = newheadroom;
778 else
779 head_copy_off = newheadroom - head_copy_len;
780
781 /* Copy the linear header and data. */
782 /* offset为原来skb->data-head_copy_len */
783 if (skb_copy_bits(skb, -head_copy_len, n->head + head_copy_off,
784 skb->len + head_copy_len))
785 BUG();
786
787 /* 拷贝skb结构体到n结构体 */
788 copy_skb_header(n, skb);
789
790 return n;
791 }
792
793 /**
794 * skb_pad - zero pad the tail of an skb
795 * @skb: buffer to pad
796 * @pad: space to pad
797 *
798 * Ensure that a buffer is followed by a padding area that is zero
799 * filled. Used by network drivers which may DMA or transfer data
800 * beyond the buffer end onto the wire.
801 *
802 * May return NULL in out of memory cases.
803 */
804
805 struct sk_buff *skb_pad(struct sk_buff *skb, int pad)
806 {
807 struct sk_buff *nskb;
808
809 /* If the skbuff is non linear tailroom is always zero.. */
810 /* 如果需要pad的长度比skb_tailroom小的话，
811 * 就直接从skb->data+skb->len,开始清零.
812 */
813 if (skb_tailroom(skb) >= pad) {
814 memset(skb->data+skb->len, 0, pad);
815 return skb;
816 }
817
818 /* 如果需要pad的长度比tailroom长的话，就skb_copy_expand */
819 nskb = skb_copy_expand(skb, skb_headroom(skb), skb_tailroom(skb) + pad,GFP_ATOMIC);
820 /* 释放原来的SKB */
821 kfree_skb(skb);
822 /* 清零 */
823 if (nskb)
824 memset(nskb->data+nskb->len, 0, pad);
825 return nskb;
826 }
