SSL连接建立过程分析(5)

SSL连接建立过程分析(5)

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2.14 SSL_read

SSL结构(struct ssl_st)中的s2,s3指针分别指向SSL2和SSL3的状态结构，这些状态结构中都有用于读的rbuf，其中保存SSL会话接收到的原始数据，另外还有保存记录的rrec，用来保存解码后的数据。在SSL结构中有个指针packet是指向原始数据的。

SSL_read()实现从SSL通道中读取数据，送到应用程序的数据是已经经过SSL解封装的了。

/* ssl/ssl_lib.c */

int SSL_read(SSL *s,void *buf,int num)
 {
 if (s->handshake_func == 0)
  {
  SSLerr(SSL_F_SSL_READ, SSL_R_UNINITIALIZED);
  return -1;
  }
// 发现接收shutdown标志，接收结束
 if (s->shutdown & SSL_RECEIVED_SHUTDOWN)
  {
  s->rwstate=SSL_NOTHING;
  return(0);
  }
// 调用具体方法的接收函数, 如ssl3_read(), ssl2_read()等
 return(s->method->ssl_read(s,buf,num));
 }

下面以ssl3_read()函数进行详细说明，ssl3_read()函数本质是调用ssl3_read_internal()

/* ssl/s3_lib.c */
int ssl3_read(SSL *s, void *buf, int len)
 {
// 最后一个参数为0，表示是实实在在的读数据，不是偷看(peek)
// peek的意思是读数据,但不会把已读数据从缓冲区中去掉,因此下一次读还会读到
 return ssl3_read_internal(s, buf, len, 0);
 }

static int ssl3_read_internal(SSL *s, void *buf, int len, int peek)
 {
 int ret;
// 清除错误信息 
 clear_sys_error();
// 检查协商是否成功
 if (s->s3->renegotiate) ssl3_renegotiate_check(s);
// 标志现在在读应用层数据
 s->s3->in_read_app_data=1;
// ssl3读取数据, 类型是SSL3_RT_APPLICATION_DATA，应用数据
 ret=ssl3_read_bytes(s,SSL3_RT_APPLICATION_DATA,buf,len,peek);
 if ((ret == -1) && (s->s3->in_read_app_data == 2))
  {
// s->s3->in_read_app_data == 2表示要读协商数据
  /* ssl3_read_bytes decided to call s->handshake_func, which
   * called ssl3_read_bytes to read handshake data.
   * However, ssl3_read_bytes actually found application data
   * and thinks that application data makes sense here; so disable
   * handshake processing and try to read application data again. */
  s->in_handshake++;
// 重新读数据
  ret=ssl3_read_bytes(s,SSL3_RT_APPLICATION_DATA,buf,len,peek);
  s->in_handshake--;
  }
 else
  s->s3->in_read_app_data=0;

 return(ret);
 }

读取数据的主要函数实际为ssl3_read_bytes():

/* ssl/ssl3_pkt.c */
int ssl3_read_bytes(SSL *s, int type, unsigned char *buf, int len, int peek)
 {
 int al,i,j,ret;
 unsigned int n;
 SSL3_RECORD *rr;
 void (*cb)(const SSL *ssl,int type2,int val)=NULL;

// 检查是否分配了接收缓冲区
 if (s->s3->rbuf.buf == NULL) /* Not initialized yet */
  if (!ssl3_setup_buffers(s))
   return(-1);

// 只处理两种数据,或是应用层数据或者是SSL握手协商数据,
// peek只处理应用层数据，其他则出错
 if ((type && (type != SSL3_RT_APPLICATION_DATA) &&
     (type != SSL3_RT_HANDSHAKE) && type) ||
     (peek && (type != SSL3_RT_APPLICATION_DATA)))
  {
  SSLerr(SSL_F_SSL3_READ_BYTES, ERR_R_INTERNAL_ERROR);
  return -1;
  }

 if ((type == SSL3_RT_HANDSHAKE) && (s->s3->handshake_fragment_len > 0))
  /* (partially) satisfy request from storage */
  {
// 握手数据碎片, 处理完直到不再是碎片
  unsigned char *src = s->s3->handshake_fragment;
  unsigned char *dst = buf;
  unsigned int k;

  /* peek == 0 */
  n = 0;
  while ((len > 0) && (s->s3->handshake_fragment_len > 0))
   {
   *dst++ = *src++;
   len--; s->s3->handshake_fragment_len--;
   n++;
   }
  /* move any remaining fragment bytes: */
  for (k = 0; k < s->s3->handshake_fragment_len; k++)
   s->s3->handshake_fragment[k] = *src++;
  return n;
 }

 /* Now s->s3->handshake_fragment_len == 0 if type == SSL3_RT_HANDSHAKE. */

 if (!s->in_handshake && SSL_in_init(s))
  {
// 没进握手阶段但属于SSL初始化阶段, 调用SSL握手处理
  /* type == SSL3_RT_APPLICATION_DATA */
  i=s->handshake_func(s);
  if (i < 0) return(i);
  if (i == 0)
   {
   SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_SSL_HANDSHAKE_FAILURE);
   return(-1);
   }
  }
start:
// 正常的接收数据开始, 开始读写状态为NOTHING
 s->rwstate=SSL_NOTHING;

 /* s->s3->rrec.type     - is the type of record
  * s->s3->rrec.data,    - data
  * s->s3->rrec.off,     - offset into 'data' for next read
  * s->s3->rrec.length,  - number of bytes. */
// 记录类型指针
 rr = &(s->s3->rrec);

 /* get new packet if necessary */
 if ((rr->length == 0) || (s->rstate == SSL_ST_READ_BODY))
  {
// 当前没记录,获取新的记录信息, 该函数获取SSL记录数据
  ret=ssl3_get_record(s);
  if (ret <= 0) return(ret);
  }

 /* we now have a packet which can be read and processed */

 if (s->s3->change_cipher_spec /* set when we receive ChangeCipherSpec,
                                * reset by ssl3_get_finished */
  && (rr->type != SSL3_RT_HANDSHAKE))
  {
// 只允许在握手状态下修改当前的算法信息，否则出错
  al=SSL_AD_UNEXPECTED_MESSAGE;
  SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_DATA_BETWEEN_CCS_AND_FINISHED);
  goto err;
  }

 /* If the other end has shut down, throw anything we read away
  * (even in 'peek' mode) */
 if (s->shutdown & SSL_RECEIVED_SHUTDOWN)
  {
// 接收到对方的FIN信息, 接收结束
  rr->length=0;
  s->rwstate=SSL_NOTHING;
  return(0);
  }

 if (type == rr->type) /* SSL3_RT_APPLICATION_DATA or SSL3_RT_HANDSHAKE */
  {
// 读到是数据类型和期待读到的数据类型一致
// 这是正常大多数的情况
  /* make sure that we are not getting application data when we
   * are doing a handshake for the first time */
  if (SSL_in_init(s) && (type == SSL3_RT_APPLICATION_DATA) &&
   (s->enc_read_ctx == NULL))
   {
// 在初始阶段只应该读握手信息而不该读应用信息
   al=SSL_AD_UNEXPECTED_MESSAGE;
   SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_APP_DATA_IN_HANDSHAKE);
   goto f_err;
   }

  if (len <= 0) return(len);

// 在期待读取的数据长度len和已经读到的数据记录长度rr->length取小值作为返回的数据长度
  if ((unsigned int)len > rr->length)
   n = rr->length;
  else
   n = (unsigned int)len;

  memcpy(buf,&(rr->data[rr->off]),n);
  if (!peek)
   {
// 如果不是peek,移动记录缓冲的数据偏移指针，长度减
   rr->length-=n;
   rr->off+=n;
   if (rr->length == 0)
    {
    s->rstate=SSL_ST_READ_HEADER;
    rr->off=0;
    }
   }
// 正常返回
  return(n);
  }

 /* If we get here, then type != rr->type; if we have a handshake
  * message, then it was unexpected (Hello Request or Client Hello). */

 /* In case of record types for which we have 'fragment' storage,
  * fill that so that we can process the data at a fixed place.
  */
// 现在情况是读取的数据类型和期待读取的数据类型不一致
// 已经读到的数据不丢弃,而是作为碎片存储起来以后备用
  {
  unsigned int dest_maxlen = 0;
  unsigned char *dest = NULL;
  unsigned int *dest_len = NULL;

  if (rr->type == SSL3_RT_HANDSHAKE)
   {
// 握手类型数据存到握手碎片区
   dest_maxlen = sizeof s->s3->handshake_fragment;
   dest = s->s3->handshake_fragment;
   dest_len = &s->s3->handshake_fragment_len;
   }
  else if (rr->type == SSL3_RT_ALERT)
   {
// 告警信息存到告警碎片区
   dest_maxlen = sizeof s->s3->alert_fragment;
   dest = s->s3->alert_fragment;
   dest_len = &s->s3->alert_fragment_len;
   }

  if (dest_maxlen > 0)
   {
// 有缓冲区, 计算可用的最大缓冲区
   n = dest_maxlen - *dest_len; /* available space in 'dest' */
// 如果要写入的长度小于可用空间大小,可全部写入
   if (rr->length < n)
    n = rr->length; /* available bytes */
// 写入缓冲区
   /* now move 'n' bytes: */
   while (n-- > 0)
    {
    dest[(*dest_len)++] = rr->data[rr->off++];
    rr->length--;
    }

   if (*dest_len < dest_maxlen)
    goto start; /* fragment was too small */
   }
  }

 /* s->s3->handshake_fragment_len == 4  iff  rr->type == SSL3_RT_HANDSHAKE;
  * s->s3->alert_fragment_len == 2      iff  rr->type == SSL3_RT_ALERT.
  * (Possibly rr is 'empty' now, i.e. rr->length may be 0.) */

 /* If we are a client, check for an incoming 'Hello Request': */
 if ((!s->server) &&
  (s->s3->handshake_fragment_len >= 4) &&
  (s->s3->handshake_fragment[0] == SSL3_MT_HELLO_REQUEST) &&
  (s->session != NULL) && (s->session->cipher != NULL))
  {
// 本地是客户端
// 握手碎片长度至少是4字节, 类型是HELLO的握手信息时进行处理

  s->s3->handshake_fragment_len = 0;

  if ((s->s3->handshake_fragment[1] != 0) ||
   (s->s3->handshake_fragment[2] != 0) ||
   (s->s3->handshake_fragment[3] != 0))
   {
// SSL3_MT_HELLO_REQUEST类型后三个字节都要是0
   al=SSL_AD_DECODE_ERROR;
   SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_BAD_HELLO_REQUEST);
   goto err;
   }

  if (s->msg_callback)
   s->msg_callback(0, s->version, SSL3_RT_HANDSHAKE, s->s3->handshake_fragment, 4, s, s->msg_callback_arg);

  if (SSL_is_init_finished(s) &&
// 协商结束但没设置不需重协商算法标志
   !(s->s3->flags & SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS) &&
// 没设置该ssl3会话的重协商标志
   !s->s3->renegotiate)
   {
// 进行ssl3重协商,实际是将s->s3->renegotiate置1
   ssl3_renegotiate(s);
   if (ssl3_renegotiate_check(s))
    {
// 进行重协商
    i=s->handshake_func(s);
    if (i < 0) return(i);
    if (i == 0)
     {
     SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_SSL_HANDSHAKE_FAILURE);
     return(-1);
     }

    if (!(s->mode & SSL_MODE_AUTO_RETRY))
// 该SSL会话不是自动重试模式
     {
     if (s->s3->rbuf.left == 0) /* no read-ahead left? */
      {
      BIO *bio;
  /* In the case where we try to read application data,
   * but we trigger an SSL handshake, we return -1 with
   * the retry option set.  Otherwise renegotiation may
   * cause nasty problems in the blocking world */
      s->rwstate=SSL_READING;
      bio=SSL_get_rbio(s);
// 清除读BIO重试标志
      BIO_clear_retry_flags(bio);
// 设置读BIO重试标志
      BIO_set_retry_read(bio);
      return(-1);
      }
     }
    }
   }
 /* we either finished a handshake or ignored the request,
  * now try again to obtain the (application) data we were asked for */
// 重新读数据
  goto start;
  }

 if (s->s3->alert_fragment_len >= 2)
  {
// 处理告警碎片信息, 长度大于等于2字节时就处理
  int alert_level = s->s3->alert_fragment[0];
  int alert_descr = s->s3->alert_fragment[1];

  s->s3->alert_fragment_len = 0;

// 分别处理msg和info的回调
  if (s->msg_callback)
   s->msg_callback(0, s->version, SSL3_RT_ALERT, s->s3->alert_fragment, 2, s, s->msg_callback_arg);

  if (s->info_callback != NULL)
   cb=s->info_callback;
  else if (s->ctx->info_callback != NULL)
   cb=s->ctx->info_callback;

  if (cb != NULL)
   {
   j = (alert_level << 8) | alert_descr;
   cb(s, SSL_CB_READ_ALERT, j);
   }

  if (alert_level == 1) /* warning */
   {
// 普通报警信息
   s->s3->warn_alert = alert_descr;
   if (alert_descr == SSL_AD_CLOSE_NOTIFY)
    {
    s->shutdown |= SSL_RECEIVED_SHUTDOWN;
    return(0);
    }
   }
  else if (alert_level == 2) /* fatal */
   {
// 严重错误信息, 断开SSL会话
   char tmp[16];

   s->rwstate=SSL_NOTHING;
   s->s3->fatal_alert = alert_descr;
   SSLerr(SSL_F_SSL3_READ_BYTES, SSL_AD_REASON_OFFSET + alert_descr);
   BIO_snprintf(tmp,sizeof tmp,"%d",alert_descr);
   ERR_add_error_data(2,"SSL alert number ",tmp);
   s->shutdown|=SSL_RECEIVED_SHUTDOWN;
   SSL_CTX_remove_session(s->ctx,s->session);
   return(0);
   }
  else
   {
   al=SSL_AD_ILLEGAL_PARAMETER;
   SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_UNKNOWN_ALERT_TYPE);
   goto f_err;
   }

  goto start;
  }

 if (s->shutdown & SSL_SENT_SHUTDOWN) /* but we have not received a shutdown */
  {
// SSL会话设置发送SHUTDOWN, 表示不再发送数据
  s->rwstate=SSL_NOTHING;
  rr->length=0;
  return(0);
  }

 if (rr->type == SSL3_RT_CHANGE_CIPHER_SPEC)
  {
// 数据记录类型是更改算法参数
  /* 'Change Cipher Spec' is just a single byte, so we know
   * exactly what the record payload has to look like */
  if ( (rr->length != 1) || (rr->off != 0) ||
   (rr->data[0] != SSL3_MT_CCS))
   {
// 错误检查
   i=SSL_AD_ILLEGAL_PARAMETER;
   SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_BAD_CHANGE_CIPHER_SPEC);
   goto err;
   }

  rr->length=0;

  if (s->msg_callback)
   s->msg_callback(0, s->version, SSL3_RT_CHANGE_CIPHER_SPEC, rr->data, 1, s, s->msg_callback_arg);

// 加密算法更改处理,成功时转到重新接收数据
  s->s3->change_cipher_spec=1;
  if (!do_change_cipher_spec(s))
   goto err;
  else
   goto start;
  }

 /* Unexpected handshake message (Client Hello, or protocol violation) */
 if ((s->s3->handshake_fragment_len >= 4) && !s->in_handshake)
  {
// 异常的握手信息
// SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS标志表示不需要重新协商加密算法
  if (((s->state&SSL_ST_MASK) == SSL_ST_OK) &&
   !(s->s3->flags & SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS))
   {
#if 0 /* worked only because C operator preferences are not as expected (and
       * because this is not really needed for clients except for detecting
       * protocol violations): */
   s->state=SSL_ST_BEFORE|(s->server)
    ?SSL_ST_ACCEPT
    :SSL_ST_CONNECT;
#else
// 如果是服务器端,SSL状态转为接受;如果是客户端, 转为连接
   s->state = s->server ? SSL_ST_ACCEPT : SSL_ST_CONNECT;
#endif
// 重新进行新会话
   s->new_session=1;
   }
// 重新握手协商
  i=s->handshake_func(s);
  if (i < 0) return(i);
  if (i == 0)
   {
   SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_SSL_HANDSHAKE_FAILURE);
   return(-1);
   }

  if (!(s->mode & SSL_MODE_AUTO_RETRY))
   {
   if (s->s3->rbuf.left == 0) /* no read-ahead left? */
    {
    BIO *bio;
    /* In the case where we try to read application data,
     * but we trigger an SSL handshake, we return -1 with
     * the retry option set.  Otherwise renegotiation may
     * cause nasty problems in the blocking world */
    s->rwstate=SSL_READING;
    bio=SSL_get_rbio(s);
    BIO_clear_retry_flags(bio);
    BIO_set_retry_read(bio);
    return(-1);
    }
   }
  goto start;
  }

// 最后再根据记录类型进行处理
 switch (rr->type)
  {
 default:
#ifndef OPENSSL_NO_TLS
  /* TLS just ignores unknown message types */
// 如果会话是TLS1版本转为重新接收数据
// 其他版本则认为出错
  if (s->version == TLS1_VERSION)
   {
   rr->length = 0;
   goto start;
   }
#endif
  al=SSL_AD_UNEXPECTED_MESSAGE;
  SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_UNEXPECTED_RECORD);
  goto f_err;
 case SSL3_RT_CHANGE_CIPHER_SPEC:
 case SSL3_RT_ALERT:
 case SSL3_RT_HANDSHAKE:
// 这几种类型前面已经处理过了, 到这还有的话是哪出错了
  /* we already handled all of these, with the possible exception
   * of SSL3_RT_HANDSHAKE when s->in_handshake is set, but that
   * should not happen when type != rr->type */
  al=SSL_AD_UNEXPECTED_MESSAGE;
  SSLerr(SSL_F_SSL3_READ_BYTES,ERR_R_INTERNAL_ERROR);
  goto f_err;
  case SSL3_RT_APPLICATION_DATA:
// 这时情况是收到的是应用数据,而期望接收的是握手数据
  /* At this point, we were expecting handshake data,
   * but have application data.  If the library was
   * running inside ssl3_read() (i.e. in_read_app_data
   * is set) and it makes sense to read application data
   * at this point (session renegotiation not yet started),
   * we will indulge it.
   */
// 要接收应用数据
  if (s->s3->in_read_app_data &&
// 需要重协商
   (s->s3->total_renegotiations != 0) &&
   ((
    (s->state & SSL_ST_CONNECT) &&
    (s->state >= SSL3_ST_CW_CLNT_HELLO_A) &&
    (s->state <= SSL3_ST_CR_SRVR_HELLO_A)
    ) || (
     (s->state & SSL_ST_ACCEPT) &&
     (s->state <= SSL3_ST_SW_HELLO_REQ_A) &&
     (s->state >= SSL3_ST_SR_CLNT_HELLO_A)
     )
    ))
   {
// in_read_app_data设置为2，会重新执行ssl3_read_bytes()
   s->s3->in_read_app_data=2;
   return(-1);
   }
  else
   {
   al=SSL_AD_UNEXPECTED_MESSAGE;
   SSLerr(SSL_F_SSL3_READ_BYTES,SSL_R_UNEXPECTED_RECORD);
   goto f_err;
   }
  }
 /* not reached */

f_err:
// 错误时发送告警信息
 ssl3_send_alert(s,SSL3_AL_FATAL,al);
err:
 return(-1);
 }

可以看出接收数据的重点函数是ssl3_get_record():

/* ssl/s3_pkt.c */
static int ssl3_get_record(SSL *s)
 {
 int ssl_major,ssl_minor,al;
 int enc_err,n,i,ret= -1;
 SSL3_RECORD *rr;
 SSL_SESSION *sess;
 unsigned char *p;
 unsigned char md[EVP_MAX_MD_SIZE];
 short version;
 unsigned int mac_size;
 int clear=0;
 size_t extra;
 int decryption_failed_or_bad_record_mac = 0;
 unsigned char *mac = NULL;

// 所读数据的记录指针
 rr= &(s->s3->rrec);
 sess=s->session;

 if (s->options & SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER)
// MS的SSL实现可附带较大数据
  extra=SSL3_RT_MAX_EXTRA;
 else
  extra=0;
 if (extra != s->s3->rbuf.len - SSL3_RT_MAX_PACKET_SIZE)
// 普通情况下读缓冲大小就是SSL3_RT_MAX_PACKET_SIZE
  {
/* actually likely an application error: SLS_OP_MICROSOFT_BIG_SSLV3_BUFFER
 * set after ssl3_setup_buffers() was done */
  SSLerr(SSL_F_SSL3_GET_RECORD, ERR_R_INTERNAL_ERROR);
  return -1;
  }

again:
 /* check if we have the header */
 if ( (s->rstate != SSL_ST_READ_BODY) ||
// 接收的数据长度还不够记录长度
  (s->packet_length < SSL3_RT_HEADER_LENGTH))
  {
// 读至少SSL3_RT_HEADER_LENGTH长度,最大s->s3->rbuf.len
  n=ssl3_read_n(s, SSL3_RT_HEADER_LENGTH, s->s3->rbuf.len, 0);
  if (n <= 0) return(n); /* error or non-blocking */
  s->rstate=SSL_ST_READ_BODY;

// p现在是SSL收到的原始数据头
  p=s->packet;

  /* Pull apart the header into the SSL3_RECORD */
// SSL记录类型
  rr->type= *(p++);
// 版本信息
  ssl_major= *(p++);
  ssl_minor= *(p++);
  version=(ssl_major<<8)|ssl_minor;
// 数据长度
  n2s(p,rr->length);

  /* Lets check version */
  if (s->first_packet)
   {
   s->first_packet=0;
   }
  else
   {
// 已经不是第一个包了
// 检查版本是否等于SSL会话版本,这时应该协商好应该是相同的了
// 如果不同就是错的
   if (version != s->version)
    {
    SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_WRONG_VERSION_NUMBER);
    /* Send back error using their
     * version number :-) */
    s->version=version;
    al=SSL_AD_PROTOCOL_VERSION;
    goto f_err;
    }
   }
// 检查版本是否是SSL3
  if ((version>>8) != SSL3_VERSION_MAJOR)
   {
   SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_WRONG_VERSION_NUMBER);
   goto err;
   }
// 接收数据过长出错
  if (rr->length > SSL3_RT_MAX_ENCRYPTED_LENGTH+extra)
   {
   al=SSL_AD_RECORD_OVERFLOW;
   SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_PACKET_LENGTH_TOO_LONG);
   goto f_err;
   }

  /* now s->rstate == SSL_ST_READ_BODY */
  }

 /* s->rstate == SSL_ST_READ_BODY, get and decode the data */
// SSL3_RT_HEADER_LENGTH = 5
 if (rr->length > s->packet_length-SSL3_RT_HEADER_LENGTH)
  {
// 已收到的数据还不够记录长度, 继续读够记录长度
  /* now s->packet_length == SSL3_RT_HEADER_LENGTH */
  i=rr->length;
  n=ssl3_read_n(s,i,i,1);
  if (n <= 0) return(n); /* error or non-blocking io */
  /* now n == rr->length,
   * and s->packet_length == SSL3_RT_HEADER_LENGTH + rr->length */
  }
//
// 状态转为已经接收到SSL头信息
 s->rstate=SSL_ST_READ_HEADER; /* set state for later operations */

 /* At this point, s->packet_length == SSL3_RT_HEADER_LNGTH + rr->length,
  * and we have that many bytes in s->packet
  */
// input指向SSL原始数据头后的数据
 rr->input= &(s->packet[SSL3_RT_HEADER_LENGTH]);

 /* ok, we can now read from 's->packet' data into 'rr'
  * rr->input points at rr->length bytes, which
  * need to be copied into rr->data by either
  * the decryption or by the decompression
  * When the data is 'copied' into the rr->data buffer,
  * rr->input will be pointed at the new buffer */

 /* We now have - encrypted [ MAC [ compressed [ plain ] ] ]
  * rr->length bytes of encrypted compressed stuff. */

 /* check is not needed I believe */
 if (rr->length > SSL3_RT_MAX_ENCRYPTED_LENGTH+extra)
  {
// 数据又读多了
  al=SSL_AD_RECORD_OVERFLOW;
  SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_ENCRYPTED_LENGTH_TOO_LONG);
  goto f_err;
  }

 /* decrypt in place in 'rr->input' */
// 记录数据
 rr->data=rr->input;

// 对数据进行解密
 enc_err = s->method->ssl3_enc->enc(s,0);
 if (enc_err <= 0)
  {
  if (enc_err == 0)
   /* SSLerr() and ssl3_send_alert() have been called */
   goto err;

  /* Otherwise enc_err == -1, which indicates bad padding
   * (rec->length has not been changed in this case).
   * To minimize information leaked via timing, we will perform
   * the MAC computation anyway. */
// 注意 enc_err<0 时并不立即转到错误处理,只是作个标志
  decryption_failed_or_bad_record_mac = 1;
  }

 /* r->length is now the compressed data plus mac */
 if ( (sess == NULL) ||
  (s->enc_read_ctx == NULL) ||
  (s->read_hash == NULL))
// 会话,加密上下文,读认证码操作都为空的情况，设置标志clear
  clear=1;

 if (!clear)
  {
// HASH算法的认证码长度
  mac_size=EVP_MD_size(s->read_hash);

  if (rr->length > SSL3_RT_MAX_COMPRESSED_LENGTH+extra+mac_size)
   {
#if 0 /* OK only for stream ciphers (then rr->length is visible from ciphertext anyway) */
   al=SSL_AD_RECORD_OVERFLOW;
   SSLerr(SSL_F_SSL3_GET_RECORD,
    SSL_R_PRE_MAC_LENGTH_TOO_LONG);
   goto f_err;
#else
// 长度过长, 设置出错标志
   decryption_failed_or_bad_record_mac = 1;
#endif   
   }
  /* check the MAC for rr->input (it's in mac_size bytes at the tail) */
  if (rr->length >= mac_size)
   {
// 记录长度中减去认证码长度
   rr->length -= mac_size;
// 认证码数据指针
   mac = &rr->data[rr->length];
   }
  else
   {
   /* record (minus padding) is too short to contain a MAC */
#if 0 /* OK only for stream ciphers */
   al=SSL_AD_DECODE_ERROR;
   SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_LENGTH_TOO_SHORT);
   goto f_err;
#else
// 记录长度还没认证码长度长, 设置出错标志
   decryption_failed_or_bad_record_mac = 1;
   rr->length = 0;
#endif
   }
// 用认证码对数据进行认证
  i=s->method->ssl3_enc->mac(s,md,0);
  if (mac == NULL || memcmp(md, mac, mac_size) != 0)
   {
   decryption_failed_or_bad_record_mac = 1;
   }
  }

 if (decryption_failed_or_bad_record_mac)
  {
// 解密错误, 跳出
  /* A separate 'decryption_failed' alert was introduced with TLS 1.0,
   * SSL 3.0 only has 'bad_record_mac'.  But unless a decryption
   * failure is directly visible from the ciphertext anyway,
   * we should not reveal which kind of error occured -- this
   * might become visible to an attacker (e.g. via a logfile) */
  al=SSL_AD_BAD_RECORD_MAC;
  SSLerr(SSL_F_SSL3_GET_RECORD,
   SSL_R_DECRYPTION_FAILED_OR_BAD_RECORD_MAC);
  goto f_err;
  }

 /* r->length is now just compressed */
 if (s->expand != NULL)
// 现在数据是经过压缩处理的
  {
  if (rr->length > SSL3_RT_MAX_COMPRESSED_LENGTH+extra)
   {
// 压缩数据长度又太长了
   al=SSL_AD_RECORD_OVERFLOW;
   SSLerr(SSL_F_SSL3_GET_RECORD,
    SSL_R_COMPRESSED_LENGTH_TOO_LONG);
   goto f_err;
   }
// 对数据解压
  if (!do_uncompress(s))
   {
// 解压失败, 跳出
   al=SSL_AD_DECOMPRESSION_FAILURE;
   SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_BAD_DECOMPRESSION);
   goto f_err;
   }
  }

 if (rr->length > SSL3_RT_MAX_PLAIN_LENGTH+extra)
  {
// 解压缩后的明文数据长度过长, 跳出
  al=SSL_AD_RECORD_OVERFLOW;
  SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_DATA_LENGTH_TOO_LONG);
  goto f_err;
  }

 rr->off=0;
 /* So at this point the following is true
  * ssl->s3->rrec.type  is the type of record
  * ssl->s3->rrec.length == number of bytes in record
  * ssl->s3->rrec.off == offset to first valid byte
  * ssl->s3->rrec.data == where to take bytes from, increment
  *      after use :-).
  */

 /* we have pulled in a full packet so zero things */
 s->packet_length=0;

 /* just read a 0 length packet */
// 如果记录长度为0, 转到继续接收数据
 if (rr->length == 0) goto again;

 return(1);

f_err:
// 错误发送告警信息
 ssl3_send_alert(s,SSL3_AL_FATAL,al);
err:
 return(ret);
 }

最后看一下ssl3_read_n()函数，接收SSL原始数据, 该函数只被ssl3_get_record()函数调用，是static的：

/* ssl/s3_pkt.c */
static int ssl3_read_n(SSL *s, int n, int max, int extend)
 {
// extend为0时表示要读全新数据，非0表示可以用现在缓冲中的数据
 /* If extend == 0, obtain new n-byte packet; if extend == 1, increase
  * packet by another n bytes.
  * The packet will be in the sub-array of s->s3->rbuf.buf specified
  * by s->packet and s->packet_length.
  * (If s->read_ahead is set, 'max' bytes may be stored in rbuf
  * [plus s->packet_length bytes if extend == 1].)
  */
 int i,off,newb;

 if (!extend)
  {
  /* start with empty packet ... */
// 确定包缓冲地址, 置当前的包长为0
  if (s->s3->rbuf.left == 0)
   s->s3->rbuf.offset = 0;
  s->packet = s->s3->rbuf.buf + s->s3->rbuf.offset;
  s->packet_length = 0;
  /* ... now we can act as if 'extend' was set */
  }

 /* if there is enough in the buffer from a previous read, take some */
 if (s->s3->rbuf.left >= (int)n)
  {
// 如果已读缓冲区中剩余数据超过要读的数据,直接移动指针即可
  s->packet_length+=n;
  s->s3->rbuf.left-=n;
  s->s3->rbuf.offset+=n;
  return(n);
  }

 /* else we need to read more data */
// 预读数据长度为0
 if (!s->read_ahead)
  max=n;

 {
  /* avoid buffer overflow */
// 当前缓冲区最大空闲值
  int max_max = s->s3->rbuf.len - s->packet_length;
  if (max > max_max)
   max = max_max;
 }
 if (n > max) /* does not happen */
  {
// 要读的数据量超过了缓冲区量,出错
  SSLerr(SSL_F_SSL3_READ_N,ERR_R_INTERNAL_ERROR);
  return -1;
  }

 off = s->packet_length;
// newb为读缓冲区中剩下的有效数据长度
 newb = s->s3->rbuf.left;
 /* Move any available bytes to front of buffer:
  * 'off' bytes already pointed to by 'packet',
  * 'newb' extra ones at the end */
 if (s->packet != s->s3->rbuf.buf)
  {
  /*  off > 0 */
  memmove(s->s3->rbuf.buf, s->packet, off+newb);
  s->packet = s->s3->rbuf.buf;
  }

 while (newb < n)
  {
 /* Now we have off+newb bytes at the front of s->s3->rbuf.buf and need
  * to read in more until we have off+n (up to off+max if possible) */

  clear_sys_error();
  if (s->rbio != NULL)
   {
   s->rwstate=SSL_READING;
// 通过SSL的读BIO读取数据
   i=BIO_read(s->rbio, &(s->s3->rbuf.buf[off+newb]), max-newb);
   }
  else
   {
   SSLerr(SSL_F_SSL3_READ_N,SSL_R_READ_BIO_NOT_SET);
   i = -1;
   }

  if (i <= 0)
   {
// 读完或出错, newb为缓冲区剩余有效数据长度
   s->s3->rbuf.left = newb;
   return(i);
   }
  newb+=i;
  }

 /* done reading, now the book-keeping */
// 读数据完成, 修改偏移量和剩余数据长度
 s->s3->rbuf.offset = off + n;
 s->s3->rbuf.left = newb - n;
 s->packet_length += n;
 s->rwstate=SSL_NOTHING;
 return(n);
 }

...... 待续 ......