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/* Copyright (c) 2001 Matej Pfajfar.
* Copyright (c) 2001-2004, Roger Dingledine.
* Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
* Copyright (c) 2007-2020, The Tor Project, Inc. */
/* See LICENSE for licensing information */
/**
* @file proto_cell.c
* @brief Decodes Tor cells from buffers.
**/
/* Right now it only handles variable-length cells, but eventually
* we should refactor other cell-reading code into here. */
#include "core/or/or.h"
#include "lib/buf/buffers.h"
#include "core/proto/proto_cell.h"
#include "core/or/connection_or.h"
#include "core/or/var_cell_st.h"
/** True iff the cell command <b>command</b> is one that implies a
* variable-length cell in Tor link protocol <b>linkproto</b>. */
static inline int
cell_command_is_var_length(uint8_t command, int linkproto)
{
/* If linkproto is v2 (2), CELL_VERSIONS is the only variable-length cells
* work as implemented here. If it's 1, there are no variable-length cells.
* Tor does not support other versions right now, and so can't negotiate
* them.
*/
switch (linkproto) {
case 1:
/* Link protocol version 1 has no variable-length cells. */
return 0;
case 2:
/* In link protocol version 2, VERSIONS is the only variable-length cell */
return command == CELL_VERSIONS;
case 0:
case 3:
default:
/* In link protocol version 3 and later, and in version "unknown",
* commands 128 and higher indicate variable-length. VERSIONS is
* grandfathered in. */
return command == CELL_VERSIONS || command >= 128;
}
}
/** Check <b>buf</b> for a variable-length cell according to the rules of link
* protocol version <b>linkproto</b>. If one is found, pull it off the buffer
* and assign a newly allocated var_cell_t to *<b>out</b>, and return 1.
* Return 0 if whatever is on the start of buf_t is not a variable-length
* cell. Return 1 and set *<b>out</b> to NULL if there seems to be the start
* of a variable-length cell on <b>buf</b>, but the whole thing isn't there
* yet. */
int
fetch_var_cell_from_buf(buf_t *buf, var_cell_t **out, int linkproto)
{
char hdr[VAR_CELL_MAX_HEADER_SIZE];
var_cell_t *result;
uint8_t command;
uint16_t length;
const int wide_circ_ids = linkproto >= MIN_LINK_PROTO_FOR_WIDE_CIRC_IDS;
const int circ_id_len = get_circ_id_size(wide_circ_ids);
const unsigned header_len = get_var_cell_header_size(wide_circ_ids);
*out = NULL;
if (buf_datalen(buf) < header_len)
return 0;
buf_peek(buf, hdr, header_len);
command = get_uint8(hdr + circ_id_len);
if (!(cell_command_is_var_length(command, linkproto)))
return 0;
length = ntohs(get_uint16(hdr + circ_id_len + 1));
if (buf_datalen(buf) < (size_t)(header_len+length))
return 1;
result = var_cell_new(length);
result->command = command;
if (wide_circ_ids)
result->circ_id = ntohl(get_uint32(hdr));
else
result->circ_id = ntohs(get_uint16(hdr));
buf_drain(buf, header_len);
buf_peek(buf, (char*) result->payload, length);
buf_drain(buf, length);
*out = result;
return 1;
}
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