lib/net/pbuf.c

/**
 * @file
 * Packet buffer management
 */


/*
 * Copyright (c) 2017, ETH Zurich.
 * All rights reserved.
 *
 * This file is distributed under the terms in the attached LICENSE file.
 * If you do not find this file, copies can be found by writing to:
 * ETH Zurich D-INFK, Universitaetsstrasse 6, CH-8092 Zurich. Attn: Systems Group.
 *
 *
 * This file contains modified pbuf functions:
 *
 *  pbuf_alloced_custom
 *  pbuf_alloc
 *  pbuf_realloc
 *  pbuf_free
 *
 */


/**
 * @defgroup pbuf Packet buffers (PBUF)
 * @ingroup infrastructure
 *
 * Packets are built from the pbuf data structure. It supports dynamic
 * memory allocation for packet contents or can reference externally
 * managed packet contents both in RAM and ROM. Quick allocation for
 * incoming packets is provided through pools with fixed sized pbufs.
 *
 * A packet may span over multiple pbufs, chained as a singly linked
 * list. This is called a "pbuf chain".
 *
 * Multiple packets may be queued, also using this singly linked list.
 * This is called a "packet queue".
 *
 * So, a packet queue consists of one or more pbuf chains, each of
 * which consist of one or more pbufs. CURRENTLY, PACKET QUEUES ARE
 * NOT SUPPORTED!!! Use helper structs to queue multiple packets.
 *
 * The differences between a pbuf chain and a packet queue are very
 * precise but subtle.
 *
 * The last pbuf of a packet has a ->tot_len field that equals the
 * ->len field. It can be found by traversing the list. If the last
 * pbuf of a packet has a ->next field other than NULL, more packets
 * are on the queue.
 *
 * Therefore, looping through a pbuf of a single packet, has an
 * loop end condition (tot_len == p->len), NOT (next == NULL).
 *
 * Example of custom pbuf usage for zero-copy RX:
  @code{.c}
typedef struct my_custom_pbuf
{
   struct pbuf_custom p;
   void* dma_descriptor;
} my_custom_pbuf_t;

LWIP_MEMPOOL_DECLARE(RX_POOL, 10, sizeof(my_custom_pbuf_t), "Zero-copy RX PBUF pool");

void my_pbuf_free_custom(void* p)
{
  my_custom_pbuf_t* my_puf = (my_custom_pbuf_t*)p;

  LOCK_INTERRUPTS();
  free_rx_dma_descriptor(my_pbuf->dma_descriptor);
  LWIP_MEMPOOL_FREE(RX_POOL, my_pbuf);
  UNLOCK_INTERRUPTS();
}

void eth_rx_irq()
{
  dma_descriptor*   dma_desc = get_RX_DMA_descriptor_from_ethernet();
  my_custom_pbuf_t* my_pbuf  = (my_custom_pbuf_t*)LWIP_MEMPOOL_ALLOC(RX_POOL);

  my_pbuf->p.custom_free_function = my_pbuf_free_custom;
  my_pbuf->dma_descriptor         = dma_desc;

  invalidate_cpu_cache(dma_desc->rx_data, dma_desc->rx_length);

  struct pbuf* p = pbuf_alloced_custom(PBUF_RAW,
     dma_desc->rx_length,
     PBUF_REF,
     &my_pbuf->p,
     dma_desc->rx_data,
     dma_desc->max_buffer_size);

  if(netif->input(p, netif) != ERR_OK) {
    pbuf_free(p);
  }
}
  @endcode
 */

/*
 * Copyright (c) 2001-2004 Swedish Institute of Computer Science.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
 * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
 * OF SUCH DAMAGE.
 *
 * This file is part of the lwIP TCP/IP stack.
 *
 * Author: Adam Dunkels <adam@sics.se>
 *
 *
 *
 *
 */

#include "lwip/opt.h"

#include "lwip/stats.h"
#include "lwip/def.h"
#include "lwip/mem.h"
#include "lwip/memp.h"
#include "lwip/pbuf.h"
#include "lwip/sys.h"
#if LWIP_TCP && TCP_QUEUE_OOSEQ
#include "lwip/priv/tcp_priv.h"
#endif
#if LWIP_CHECKSUM_ON_COPY
#include "lwip/inet_chksum.h"
#endif

#include "networking_internal.h"

#include <string.h>

#define SIZEOF_STRUCT_PBUF        LWIP_MEM_ALIGN_SIZE(sizeof(struct pbuf))
/* Since the pool is created in memp, PBUF_POOL_BUFSIZE will be automatically
   aligned there. Therefore, PBUF_POOL_BUFSIZE_ALIGNED can be used here. */
#define PBUF_POOL_BUFSIZE_ALIGNED LWIP_MEM_ALIGN_SIZE(PBUF_POOL_BUFSIZE)

#if !LWIP_TCP || !TCP_QUEUE_OOSEQ || !PBUF_POOL_FREE_OOSEQ
#define PBUF_POOL_IS_EMPTY()
#else /* !LWIP_TCP || !TCP_QUEUE_OOSEQ || !PBUF_POOL_FREE_OOSEQ */

#if !NO_SYS
#ifndef PBUF_POOL_FREE_OOSEQ_QUEUE_CALL
#include "lwip/tcpip.h"
#define PBUF_POOL_FREE_OOSEQ_QUEUE_CALL()  do { \
  if (tcpip_callback_with_block(pbuf_free_ooseq_callback, NULL, 0) != ERR_OK) { \
      SYS_ARCH_PROTECT(old_level); \
      pbuf_free_ooseq_pending = 0; \
      SYS_ARCH_UNPROTECT(old_level); \
  } } while(0)
#endif /* PBUF_POOL_FREE_OOSEQ_QUEUE_CALL */
#endif /* !NO_SYS */

volatile u8_t pbuf_free_ooseq_pending;
#define PBUF_POOL_IS_EMPTY() pbuf_pool_is_empty()

/**
 * Attempt to reclaim some memory from queued out-of-sequence TCP segments
 * if we run out of pool pbufs. It's better to give priority to new packets
 * if we're running out.
 *
 * This must be done in the correct thread context therefore this function
 * can only be used with NO_SYS=0 and through tcpip_callback.
 */
#if !NO_SYS
static
#endif /* !NO_SYS */
void
pbuf_free_ooseq(void)
{
  struct tcp_pcb* pcb;
  SYS_ARCH_SET(pbuf_free_ooseq_pending, 0);

  for (pcb = tcp_active_pcbs; NULL != pcb; pcb = pcb->next) {
    if (NULL != pcb->ooseq) {
      /** Free the ooseq pbufs of one PCB only */
      LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_free_ooseq: freeing out-of-sequence pbufs\n"));
      tcp_segs_free(pcb->ooseq);
      pcb->ooseq = NULL;
      return;
    }
  }
}

#if !NO_SYS
/**
 * Just a callback function for tcpip_callback() that calls pbuf_free_ooseq().
 */
static void
pbuf_free_ooseq_callback(void *arg)
{
  LWIP_UNUSED_ARG(arg);
  pbuf_free_ooseq();
}
#endif /* !NO_SYS */

/** Queue a call to pbuf_free_ooseq if not already queued. */
static void
pbuf_pool_is_empty(void)
{
#ifndef PBUF_POOL_FREE_OOSEQ_QUEUE_CALL
  SYS_ARCH_SET(pbuf_free_ooseq_pending, 1);
#else /* PBUF_POOL_FREE_OOSEQ_QUEUE_CALL */
  u8_t queued;
  SYS_ARCH_DECL_PROTECT(old_level);
  SYS_ARCH_PROTECT(old_level);
  queued = pbuf_free_ooseq_pending;
  pbuf_free_ooseq_pending = 1;
  SYS_ARCH_UNPROTECT(old_level);

  if (!queued) {
    /* queue a call to pbuf_free_ooseq if not already queued */
    PBUF_POOL_FREE_OOSEQ_QUEUE_CALL();
  }
#endif /* PBUF_POOL_FREE_OOSEQ_QUEUE_CALL */
}
#endif /* !LWIP_TCP || !TCP_QUEUE_OOSEQ || !PBUF_POOL_FREE_OOSEQ */


/**
 * @ingroup pbuf
 * Allocates a pbuf of the given type (possibly a chain for PBUF_POOL type).
 *
 * The actual memory allocated for the pbuf is determined by the
 * layer at which the pbuf is allocated and the requested size
 * (from the size parameter).
 *
 * @param layer flag to define header size
 * @param length size of the pbuf's payload
 * @param type this parameter decides how and where the pbuf
 * should be allocated as follows:
 *
 * - PBUF_RAM: buffer memory for pbuf is allocated as one large
 *             chunk. This includes protocol headers as well.
 * - PBUF_ROM: no buffer memory is allocated for the pbuf, even for
 *             protocol headers. Additional headers must be prepended
 *             by allocating another pbuf and chain in to the front of
 *             the ROM pbuf. It is assumed that the memory used is really
 *             similar to ROM in that it is immutable and will not be
 *             changed. Memory which is dynamic should generally not
 *             be attached to PBUF_ROM pbufs. Use PBUF_REF instead.
 * - PBUF_REF: no buffer memory is allocated for the pbuf, even for
 *             protocol headers. It is assumed that the pbuf is only
 *             being used in a single thread. If the pbuf gets queued,
 *             then pbuf_take should be called to copy the buffer.
 * - PBUF_POOL: the pbuf is allocated as a pbuf chain, with pbufs from
 *              the pbuf pool that is allocated during pbuf_init().
 *
 * @return the allocated pbuf. If multiple pbufs where allocated, this
 * is the first pbuf of a pbuf chain.
 */
struct pbuf *
pbuf_alloc(pbuf_layer layer, u16_t length, pbuf_type type)
{
  struct pbuf *p, *q, *r;
  u16_t offset;
  s32_t rem_len; /* remaining length */
  LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc(length=%"U16_F")\n", length));

  /* determine header offset */
  switch (layer) {
  case PBUF_TRANSPORT:
    /* add room for transport (often TCP) layer header */
    offset = PBUF_LINK_ENCAPSULATION_HLEN + PBUF_LINK_HLEN + PBUF_IP_HLEN + PBUF_TRANSPORT_HLEN;
    break;
  case PBUF_IP:
    /* add room for IP layer header */
    offset = PBUF_LINK_ENCAPSULATION_HLEN + PBUF_LINK_HLEN + PBUF_IP_HLEN;
    break;
  case PBUF_LINK:
    /* add room for link layer header */
    offset = PBUF_LINK_ENCAPSULATION_HLEN + PBUF_LINK_HLEN;
    break;
  case PBUF_RAW_TX:
    /* add room for encapsulating link layer headers (e.g. 802.11) */
    offset = PBUF_LINK_ENCAPSULATION_HLEN;
    break;
  case PBUF_RAW:
    /* no offset (e.g. RX buffers or chain successors) */
    offset = 0;
    break;
  default:
    LWIP_ASSERT("pbuf_alloc: bad pbuf layer", 0);
    return NULL;
  }

  switch (type) {
  case PBUF_POOL:
    /* allocate head of pbuf chain into p */

    p  = net_buf_alloc(state.pool);
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc: allocated pbuf %p\n", (void *)p));
    if (p == NULL) {
      PBUF_POOL_IS_EMPTY();
      return NULL;
    }
    p->type = type;
    p->next = NULL;

    /* make the payload pointer point 'offset' bytes into pbuf data memory */
    p->payload = LWIP_MEM_ALIGN((void *)((u8_t *)p->payload + offset));
    LWIP_ASSERT("pbuf_alloc: pbuf p->payload properly aligned",
            ((mem_ptr_t)p->payload % MEM_ALIGNMENT) == 0);
    /* the total length of the pbuf chain is the requested size */
    p->tot_len = length;
    /* set the length of the first pbuf in the chain */
    p->len = LWIP_MIN(length, PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset));
    LWIP_ASSERT("check p->payload + p->len does not overflow pbuf",
                ((u8_t*)p->payload + p->len <=
                 (u8_t*)p + SIZEOF_STRUCT_PBUF + PBUF_POOL_BUFSIZE_ALIGNED));
    LWIP_ASSERT("PBUF_POOL_BUFSIZE must be bigger than MEM_ALIGNMENT",
      (PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset)) > 0 );
    /* set reference count (needed here in case we fail) */
    p->ref = 1;

    /* now allocate the tail of the pbuf chain */

    /* remember first pbuf for linkage in next iteration */
    r = p;
    /* remaining length to be allocated */
    rem_len = length - p->len;
    /* any remaining pbufs to be allocated? */
    while (rem_len > 0) {
      q  = net_buf_alloc(state.pool);
      if (q == NULL) {
        PBUF_POOL_IS_EMPTY();
        /* free chain so far allocated */
        pbuf_free(p);
        /* bail out unsuccessfully */
        return NULL;
      }
      q->type = type;
      q->flags = 0;
      q->next = NULL;
      /* make previous pbuf point to this pbuf */
      r->next = q;
      /* set total length of this pbuf and next in chain */
      LWIP_ASSERT("rem_len < max_u16_t", rem_len < 0xffff);
      q->tot_len = (u16_t)rem_len;
      /* this pbuf length is pool size, unless smaller sized tail */
      q->len = LWIP_MIN((u16_t)rem_len, PBUF_POOL_BUFSIZE_ALIGNED);
      LWIP_ASSERT("pbuf_alloc: pbuf q->payload properly aligned",
              ((mem_ptr_t)q->payload % MEM_ALIGNMENT) == 0);
      LWIP_ASSERT("check p->payload + p->len does not overflow pbuf",
                  ((u8_t*)p->payload + p->len <=
                   (u8_t*)p + SIZEOF_STRUCT_PBUF + PBUF_POOL_BUFSIZE_ALIGNED));
      q->ref = 1;
      /* calculate remaining length to be allocated */
      rem_len -= q->len;
      /* remember this pbuf for linkage in next iteration */
      r = q;
    }
    /* end of chain */
    /*r->next = NULL;*/

    break;
  case PBUF_RAM:
    {
      mem_size_t alloc_len = LWIP_MEM_ALIGN_SIZE(SIZEOF_STRUCT_PBUF + offset) + LWIP_MEM_ALIGN_SIZE(length);

      /* bug #50040: Check for integer overflow when calculating alloc_len */
      if (alloc_len < LWIP_MEM_ALIGN_SIZE(length)) {
        return NULL;
      }

      /* If pbuf is to be allocated in RAM, allocate memory for it. */
      p  = net_buf_alloc(state.pool);
    }

    if (p == NULL) {
      return NULL;
    }
    /* Set up internal structure of the pbuf. */
    p->payload = LWIP_MEM_ALIGN((void *)((u8_t *)p->payload + offset));
    p->len = p->tot_len = length;
    p->next = NULL;
    p->type = type;

    LWIP_ASSERT("pbuf_alloc: pbuf->payload properly aligned",
           ((mem_ptr_t)p->payload % MEM_ALIGNMENT) == 0);
    break;
  /* pbuf references existing (non-volatile static constant) ROM payload? */
  case PBUF_ROM:
  /* pbuf references existing (externally allocated) RAM payload? */
  case PBUF_REF:
    /* only allocate memory for the pbuf structure */
    p = (struct pbuf *)memp_malloc(MEMP_PBUF);
    if (p == NULL) {
      LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
                  ("pbuf_alloc: Could not allocate MEMP_PBUF for PBUF_%s.\n",
                  (type == PBUF_ROM) ? "ROM" : "REF"));
      return NULL;
    }
    /* caller must set this field properly, afterwards */
    p->payload = NULL;
    p->len = p->tot_len = length;
    p->next = NULL;
    p->type = type;
    break;
  default:
    LWIP_ASSERT("pbuf_alloc: erroneous type", 0);
    return NULL;
  }
  /* set reference count */
  p->ref = 1;
  p->type = PBUF_REF;
  /* set flags */
  LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc(length=%"U16_F") == %p\n", length, (void *)p));

  assert(p->next == NULL);
  return p;
}

#if LWIP_SUPPORT_CUSTOM_PBUF
/**
 * @ingroup pbuf
 * Initialize a custom pbuf (already allocated).
 *
 * @param l flag to define header size
 * @param length size of the pbuf's payload
 * @param type type of the pbuf (only used to treat the pbuf accordingly, as
 *        this function allocates no memory)
 * @param p pointer to the custom pbuf to initialize (already allocated)
 * @param payload_mem pointer to the buffer that is used for payload and headers,
 *        must be at least big enough to hold 'length' plus the header size,
 *        may be NULL if set later.
 *        ATTENTION: The caller is responsible for correct alignment of this buffer!!
 * @param payload_mem_len the size of the 'payload_mem' buffer, must be at least
 *        big enough to hold 'length' plus the header size
 */
struct pbuf*
pbuf_alloced_custom(pbuf_layer l, u16_t length, pbuf_type type, struct pbuf_custom *p,
                    void *payload_mem, u16_t payload_mem_len)
{
  u16_t offset;
  LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloced_custom(length=%"U16_F")\n", length));

  /* determine header offset */
  switch (l) {
  case PBUF_TRANSPORT:
    /* add room for transport (often TCP) layer header */
    offset = PBUF_LINK_ENCAPSULATION_HLEN + PBUF_LINK_HLEN + PBUF_IP_HLEN + PBUF_TRANSPORT_HLEN;
    break;
  case PBUF_IP:
    /* add room for IP layer header */
    offset = PBUF_LINK_ENCAPSULATION_HLEN + PBUF_LINK_HLEN + PBUF_IP_HLEN;
    break;
  case PBUF_LINK:
    /* add room for link layer header */
    offset = PBUF_LINK_ENCAPSULATION_HLEN + PBUF_LINK_HLEN;
    break;
  case PBUF_RAW_TX:
    /* add room for encapsulating link layer headers (e.g. 802.11) */
    offset = PBUF_LINK_ENCAPSULATION_HLEN;
    break;
  case PBUF_RAW:
    offset = 0;
    break;
  default:
    LWIP_ASSERT("pbuf_alloced_custom: bad pbuf layer", 0);
    return NULL;
  }

  if (LWIP_MEM_ALIGN_SIZE(offset) + length > payload_mem_len) {
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_LEVEL_WARNING, ("pbuf_alloced_custom(length=%"U16_F") buffer too short\n", length));
    return NULL;
  }

  p->pbuf.next = NULL;
  if (payload_mem != NULL) {
    p->pbuf.payload = (u8_t *)payload_mem + LWIP_MEM_ALIGN_SIZE(offset);
  } else {
    p->pbuf.payload = NULL;
  }
  p->pbuf.flags = PBUF_FLAG_IS_CUSTOM;
  p->pbuf.len = p->pbuf.tot_len = length;
  p->pbuf.type = PBUF_REF;
  p->pbuf.ref = 1;
  return &p->pbuf;
}
#endif /* LWIP_SUPPORT_CUSTOM_PBUF */

/**
 * @ingroup pbuf
 * Shrink a pbuf chain to a desired length.
 *
 * @param p pbuf to shrink.
 * @param new_len desired new length of pbuf chain
 *
 * Depending on the desired length, the first few pbufs in a chain might
 * be skipped and left unchanged. The new last pbuf in the chain will be
 * resized, and any remaining pbufs will be freed.
 *
 * @note If the pbuf is ROM/REF, only the ->tot_len and ->len fields are adjusted.
 * @note May not be called on a packet queue.
 *
 * @note Despite its name, pbuf_realloc cannot grow the size of a pbuf (chain).
 */
void
pbuf_realloc(struct pbuf *p, u16_t new_len)
{
  struct pbuf *q;
  u16_t rem_len; /* remaining length */
  s32_t grow;

  LWIP_ASSERT("pbuf_realloc: p != NULL", p != NULL);
  LWIP_ASSERT("pbuf_realloc: sane p->type", p->type == PBUF_POOL ||
              p->type == PBUF_ROM ||
              p->type == PBUF_RAM ||
              p->type == PBUF_REF);

  /* desired length larger than current length? */
  if (new_len >= p->tot_len) {
    /* enlarging not yet supported */
    return;
  }

  /* the pbuf chain grows by (new_len - p->tot_len) bytes
   * (which may be negative in case of shrinking) */
  grow = new_len - p->tot_len;

  /* first, step over any pbufs that should remain in the chain */
  rem_len = new_len;
  q = p;
  /* should this pbuf be kept? */
  while (rem_len > q->len) {
    /* decrease remaining length by pbuf length */
    rem_len -= q->len;
    /* decrease total length indicator */
    LWIP_ASSERT("grow < max_u16_t", grow < 0xffff);
    q->tot_len += (u16_t)grow;
    /* proceed to next pbuf in chain */
    q = q->next;
    LWIP_ASSERT("pbuf_realloc: q != NULL", q != NULL);
  }
  /* we have now reached the new last pbuf (in q) */
  /* rem_len == desired length for pbuf q */

  /* shrink allocated memory for PBUF_RAM */
  /* (other types merely adjust their length fields */
  if ((q->type == PBUF_RAM) && (rem_len != q->len)
#if LWIP_SUPPORT_CUSTOM_PBUF
      && ((q->flags & PBUF_FLAG_IS_CUSTOM) == 0)
#endif /* LWIP_SUPPORT_CUSTOM_PBUF */
     ) {
    USER_PANIC("SHOULD Not trim memory\n");
    /* reallocate and adjust the length of the pbuf that will be split */
    q = (struct pbuf *)mem_trim(q, (u16_t)((u8_t *)q->payload - (u8_t *)q) + rem_len);
    LWIP_ASSERT("mem_trim returned q == NULL", q != NULL);
  }
  /* adjust length fields for new last pbuf */
  q->len = rem_len;
  q->tot_len = q->len;

  /* any remaining pbufs in chain? */
  if (q->next != NULL) {
    /* free remaining pbufs in chain */
    pbuf_free(q->next);
  }
  /* q is last packet in chain */
  q->next = NULL;
}


/**
 * @ingroup pbuf
 * Dereference a pbuf chain or queue and deallocate any no-longer-used
 * pbufs at the head of this chain or queue.
 *
 * Decrements the pbuf reference count. If it reaches zero, the pbuf is
 * deallocated.
 *
 * For a pbuf chain, this is repeated for each pbuf in the chain,
 * up to the first pbuf which has a non-zero reference count after
 * decrementing. So, when all reference counts are one, the whole
 * chain is free'd.
 *
 * @param p The pbuf (chain) to be dereferenced.
 *
 * @return the number of pbufs that were de-allocated
 * from the head of the chain.
 *
 * @note MUST NOT be called on a packet queue (Not verified to work yet).
 * @note the reference counter of a pbuf equals the number of pointers
 * that refer to the pbuf (or into the pbuf).
 *
 * @internal examples:
 *
 * Assuming existing chains a->b->c with the following reference
 * counts, calling pbuf_free(a) results in:
 *
 * 1->2->3 becomes ...1->3
 * 3->3->3 becomes 2->3->3
 * 1->1->2 becomes ......1
 * 2->1->1 becomes 1->1->1
 * 1->1->1 becomes .......
 *
 */
u8_t
pbuf_free(struct pbuf *p)
{
  u16_t type;
  struct pbuf *q;
  u8_t count;

  if (p == NULL) {
    LWIP_ASSERT("p != NULL", p != NULL);
    /* if assertions are disabled, proceed with debug output */
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
      ("pbuf_free(p == NULL) was called.\n"));
    return 0;
  }
  LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_free(%p)\n", (void *)p));

  PERF_START;

  LWIP_ASSERT("pbuf_free: sane type",
    p->type == PBUF_RAM || p->type == PBUF_ROM ||
    p->type == PBUF_REF || p->type == PBUF_POOL);

  count = 0;
  /* de-allocate all consecutive pbufs from the head of the chain that
   * obtain a zero reference count after decrementing*/
  while (p != NULL) {
    u16_t ref;
    SYS_ARCH_DECL_PROTECT(old_level);
    /* Since decrementing ref cannot be guaranteed to be a single machine operation
     * we must protect it. We put the new ref into a local variable to prevent
     * further protection. */
    SYS_ARCH_PROTECT(old_level);
    /* all pbufs in a chain are referenced at least once */
    LWIP_ASSERT("pbuf_free: p->ref > 0", p->ref > 0);
    /* decrease reference count (number of pointers to pbuf) */
    ref = --(p->ref);
    SYS_ARCH_UNPROTECT(old_level);
    /* this pbuf is no longer referenced to? */
    if (ref == 0) {
      /* remember next pbuf in chain for next iteration */
      q = p->next;
      LWIP_DEBUGF( PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_free: deallocating %p\n", (void *)p));
      type = p->type;
#if LWIP_SUPPORT_CUSTOM_PBUF
      /* is this a custom pbuf? */
      if ((p->flags & PBUF_FLAG_IS_CUSTOM) != 0) {
        struct pbuf_custom *pc = (struct pbuf_custom*)p;
        LWIP_ASSERT("pc->custom_free_function != NULL", pc->custom_free_function != NULL);
        pc->custom_free_function(p);
      } else
#endif /* LWIP_SUPPORT_CUSTOM_PBUF */
      {
          USER_PANIC("SHOULD NOT REACH HERE...\n");
        /* is this a pbuf from the pool? */
        if (type == PBUF_POOL) {
          memp_free(MEMP_PBUF_POOL, p);
        /* is this a ROM or RAM referencing pbuf? */
        } else if (type == PBUF_ROM || type == PBUF_REF) {
          memp_free(MEMP_PBUF, p);
        /* type == PBUF_RAM */
        } else {
          mem_free(p);
        }
      }
      count++;
      /* proceed to next pbuf */
      p = q;
    /* p->ref > 0, this pbuf is still referenced to */
    /* (and so the remaining pbufs in chain as well) */
    } else {
      LWIP_DEBUGF( PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_free: %p has ref %"U16_F", ending here.\n", (void *)p, ref));
      /* stop walking through the chain */
      p = NULL;
    }
  }
  PERF_STOP("pbuf_free");
  /* return number of de-allocated pbufs */
  return count;
}