asn1_dec.c

LwIP是瑞士计算机科学院(Swedish Institute of Computer Science)的AdamDunkels等开发的一套用于嵌入式系统的开放源代码TCP/IP协议栈。LwIP的含义

/**
 * @file
 * Abstract Syntax Notation One (ISO 8824, 8825) decoding
 *
 * @todo not optimised (yet), favor correctness over speed, favor speed over size
 */

/*
 * Copyright (c) 2006 Axon Digital Design B.V., The Netherlands.
 * 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.
 *
 * Author: Christiaan Simons <[e-mail]>
 */

#include "lwip/opt.h"

#if LWIP_SNMP
#include "lwip/snmp_asn1.h"

/**
 * Retrieves type field from incoming pbuf chain.
 *
 * @param p points to a pbuf holding an ASN1 coded type field
 * @param ofs points to the offset within the pbuf chain of the ASN1 coded type field
 * @param type return ASN1 type
 * @return ERR_OK if successfull, ERR_ARG if we can't (or won't) decode
 */
err_t
snmp_asn1_dec_type(struct pbuf *p, u16_t ofs, u8_t *type)
{
  u16_t plen, base;
  u8_t *msg_ptr;

  plen = 0;
  while (p != NULL)
  {
    base = plen;
    plen += p->len;
    if (ofs < plen)
    {
      msg_ptr = p->payload;
      msg_ptr += ofs - base;
      *type = *msg_ptr;
      return ERR_OK;
    }
    p = p->next;
  }
  /* p == NULL, ofs >= plen */
  return ERR_ARG;
}

/**
 * Decodes length field from incoming pbuf chain into host length.
 *
 * @param p points to a pbuf holding an ASN1 coded length
 * @param ofs points to the offset within the pbuf chain of the ASN1 coded length
 * @param octets_used returns number of octets used by the length code
 * @param length return host order length, upto 64k
 * @return ERR_OK if successfull, ERR_ARG if we can't (or won't) decode
 */
err_t
snmp_asn1_dec_length(struct pbuf *p, u16_t ofs, u8_t *octets_used, u16_t *length)
{
  u16_t plen, base;
  u8_t *msg_ptr;

  plen = 0;
  while (p != NULL)
  {
    base = plen;
    plen += p->len;
    if (ofs < plen)
    {
      msg_ptr = p->payload;
      msg_ptr += ofs - base;

      if (*msg_ptr < 0x80)
      {
        /* primitive definite length format */
        *octets_used = 1;
        *length = *msg_ptr;
        return ERR_OK;
      }
      else if (*msg_ptr == 0x80)
      {
        /* constructed indefinite length format, termination with two zero octets */
        u8_t zeros;
        u8_t i;

        *length = 0;
        zeros = 0;
        while (zeros != 2)
        {
          i = 2;
          while (i > 0)
          {
            i--;
            (*length) += 1;
            ofs += 1;
            if (ofs >= plen)
            {
              /* next octet in next pbuf */
              p = p->next;
              if (p == NULL) { return ERR_ARG; }
              msg_ptr = p->payload;
              plen += p->len;
            }
            else
            {
              /* next octet in same pbuf */
              msg_ptr++;
            }
            if (*msg_ptr == 0)
            {
              zeros++;
              if (zeros == 2)
              {
                /* stop while (i > 0) */
                i = 0;
              }
            }
            else
            {
              zeros = 0;
            }
          }
        }
        *octets_used = 1;
        return ERR_OK;
      }
      else if (*msg_ptr == 0x81)
      {
        /* constructed definite length format, one octet */
        ofs += 1;
        if (ofs >= plen)
        {
          /* next octet in next pbuf */
          p = p->next;
          if (p == NULL) { return ERR_ARG; }
          msg_ptr = p->payload;
        }
        else
        {
          /* next octet in same pbuf */
          msg_ptr++;
        }
        *length = *msg_ptr;
        *octets_used = 2;
        return ERR_OK;
      }
      else if (*msg_ptr == 0x82)
      {
        u8_t i;

        /* constructed definite length format, two octets */
        i = 2;
        while (i > 0)
        {
          i--;
          ofs += 1;
          if (ofs >= plen)
          {
            /* next octet in next pbuf */
            p = p->next;
            if (p == NULL) { return ERR_ARG; }
            msg_ptr = p->payload;
            plen += p->len;
          }
          else
          {
            /* next octet in same pbuf */
            msg_ptr++;
          }
          if (i == 0)
          {
            /* least significant length octet */
            *length |= *msg_ptr;
          }
          else
          {
            /* most significant length octet */
            *length = (*msg_ptr) << 8;
          }
        }
        *octets_used = 3;
        return ERR_OK;
      }
      else
      {
        /* constructed definite length format 3..127 octets, this is too big (>64k) */
        /**  @todo: do we need to accept inefficient codings with many leading zero's? */
        *octets_used = 1 + ((*msg_ptr) & 0x7f);
        return ERR_ARG;
      }
    }
    p = p->next;
  }

  /* p == NULL, ofs >= plen */
  return ERR_ARG;
}

/**
 * Decodes positive integer (counter, gauge, timeticks) into u32_t.
 *
 * @param p points to a pbuf holding an ASN1 coded integer
 * @param ofs points to the offset within the pbuf chain of the ASN1 coded integer
 * @param len length of the coded integer field
 * @param value return host order integer
 * @return ERR_OK if successfull, ERR_ARG if we can't (or won't) decode
 *
 * @note ASN coded integers are _always_ signed. E.g. +0xFFFF is coded
 * as 0x00,0xFF,0xFF. Note the leading sign octet. A positive value
 * of 0xFFFFFFFF is preceded with 0x00 and the length is 5 octets!!
 */
err_t
snmp_asn1_dec_u32t(struct pbuf *p, u16_t ofs, u16_t len, u32_t *value)
{
  u16_t plen, base;
  u8_t *msg_ptr;

  plen = 0;
  while (p != NULL)
  {
    base = plen;
    plen += p->len;
    if (ofs < plen)
    {
      msg_ptr = p->payload;
      msg_ptr += ofs - base;
      if ((len > 0) && (len < 6))
      {
        /* start from zero */
        *value = 0;
        if (*msg_ptr & 0x80)
        {
          /* negative, expecting zero sign bit! */
          return ERR_ARG;
        }
        else
        {
          /* positive */
          if ((len > 1) && (*msg_ptr == 0))
          {
            /* skip leading "sign byte" octet 0x00 */
            len--;
            ofs += 1;
            if (ofs >= plen)
            {
              /* next octet in next pbuf */
              p = p->next;
              if (p == NULL) { return ERR_ARG; }
              msg_ptr = p->payload;
              plen += p->len;
            }
            else
            {
              /* next octet in same pbuf */
              msg_ptr++;
            }
          }
        }
        /* OR octets with value */
        while (len > 1)
        {
          len--;
          *value |= *msg_ptr;
          *value <<= 8;
          ofs += 1;
          if (ofs >= plen)
          {
            /* next octet in next pbuf */
            p = p->next;
            if (p == NULL) { return ERR_ARG; }
            msg_ptr = p->payload;
            plen += p->len;
          }
          else
          {
            /* next octet in same pbuf */
            msg_ptr++;
          }
        }
        *value |= *msg_ptr;
        return ERR_OK;
      }
      else
      {
        return ERR_ARG;
      }
    }
    p = p->next;
  }
  /* p == NULL, ofs >= plen */
  return ERR_ARG;
}

/**
 * Decodes integer into s32_t.
 *
 * @param p points to a pbuf holding an ASN1 coded integer
 * @param ofs points to the offset within the pbuf chain of the ASN1 coded integer
 * @param len length of the coded integer field
 * @param value return host order integer
 * @return ERR_OK if successfull, ERR_ARG if we can't (or won't) decode