tor

The Tor anonymity network
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proto_cell.c (3067B)

      1 /* Copyright (c) 2001 Matej Pfajfar.
      2 * Copyright (c) 2001-2004, Roger Dingledine.
      3 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
      4 * Copyright (c) 2007-2021, The Tor Project, Inc. */
      5 /* See LICENSE for licensing information */
      6 
      7 /**
      8 * @file proto_cell.c
      9 * @brief Decodes Tor cells from buffers.
     10 **/
     11 /* Right now it only handles variable-length cells, but eventually
     12 * we should refactor other cell-reading code into here. */
     13 
     14 #include "core/or/or.h"
     15 #include "lib/buf/buffers.h"
     16 #include "core/proto/proto_cell.h"
     17 
     18 #include "core/or/connection_or.h"
     19 
     20 #include "core/or/var_cell_st.h"
     21 
     22 /** True iff the cell command <b>command</b> is one that implies a
     23 * variable-length cell in Tor link protocol <b>linkproto</b>. */
     24 static inline int
     25 cell_command_is_var_length(uint8_t command, int linkproto)
     26 {
     27  /* If linkproto is v2 (2), CELL_VERSIONS is the only variable-length cells
     28   * work as implemented here. If it's 1, there are no variable-length cells.
     29   * Tor does not support other versions right now, and so can't negotiate
     30   * them.
     31   */
     32  switch (linkproto) {
     33  case 1:
     34    /* Link protocol version 1 has no variable-length cells. */
     35    return 0;
     36  case 2:
     37    /* In link protocol version 2, VERSIONS is the only variable-length cell */
     38    return command == CELL_VERSIONS;
     39  case 0:
     40  case 3:
     41  default:
     42    /* In link protocol version 3 and later, and in version "unknown",
     43     * commands 128 and higher indicate variable-length. VERSIONS is
     44     * grandfathered in. */
     45    return command == CELL_VERSIONS || command >= 128;
     46  }
     47 }
     48 
     49 /** Check <b>buf</b> for a variable-length cell according to the rules of link
     50 * protocol version <b>linkproto</b>.  If one is found, pull it off the buffer
     51 * and assign a newly allocated var_cell_t to *<b>out</b>, and return 1.
     52 * Return 0 if whatever is on the start of buf_t is not a variable-length
     53 * cell.  Return 1 and set *<b>out</b> to NULL if there seems to be the start
     54 * of a variable-length cell on <b>buf</b>, but the whole thing isn't there
     55 * yet. */
     56 int
     57 fetch_var_cell_from_buf(buf_t *buf, var_cell_t **out, int linkproto)
     58 {
     59  char hdr[VAR_CELL_MAX_HEADER_SIZE];
     60  var_cell_t *result;
     61  uint8_t command;
     62  uint16_t length;
     63  const int wide_circ_ids = linkproto >= MIN_LINK_PROTO_FOR_WIDE_CIRC_IDS;
     64  const int circ_id_len = get_circ_id_size(wide_circ_ids);
     65  const unsigned header_len = get_var_cell_header_size(wide_circ_ids);
     66  *out = NULL;
     67  if (buf_datalen(buf) < header_len)
     68    return 0;
     69  buf_peek(buf, hdr, header_len);
     70 
     71  command = get_uint8(hdr + circ_id_len);
     72  if (!(cell_command_is_var_length(command, linkproto)))
     73    return 0;
     74 
     75  length = ntohs(get_uint16(hdr + circ_id_len + 1));
     76  if (buf_datalen(buf) < (size_t)(header_len+length))
     77    return 1;
     78 
     79  result = var_cell_new(length);
     80  result->command = command;
     81  if (wide_circ_ids)
     82    result->circ_id = ntohl(get_uint32(hdr));
     83  else
     84    result->circ_id = ntohs(get_uint16(hdr));
     85 
     86  buf_drain(buf, header_len);
     87  buf_peek(buf, (char*) result->payload, length);
     88  buf_drain(buf, length);
     89 
     90  *out = result;
     91  return 1;
     92 }