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/*
* Copyright 2013 Google Inc.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
* 02110-1301, USA.
*/
/*
* Author: ncardwell@google.com (Neal Cardwell)
*
* Implementation for module to allow iteration over TCP options in
* wire format.
*/
#include "tcp_options_iterator.h"
#include <stdlib.h>
#include <string.h>
#include "packet.h"
#include "tcp.h"
#include "tcp_options.h"
/* Return the length (in bytes) we expect to see for the TCP option of
* the given kind, or 0 if the option is variable-length. Returns
* STATUS_OK on success; on failure returns STATUS_ERR and sets
* error message.
*/
static int get_expected_tcp_option_length(u8 kind, u8 *expected_length,
char **error)
{
switch (kind) {
case TCPOPT_EOL:
case TCPOPT_NOP:
*expected_length = 1; /* no length byte or data */
break;
case TCPOPT_MAXSEG:
*expected_length = TCPOLEN_MAXSEG;
break;
case TCPOPT_WINDOW:
*expected_length = TCPOLEN_WINDOW;
break;
case TCPOPT_SACK_PERMITTED:
*expected_length = TCPOLEN_SACK_PERMITTED;
break;
case TCPOPT_TIMESTAMP:
*expected_length = TCPOLEN_TIMESTAMP;
break;
case TCPOPT_SACK:
case TCPOPT_MD5SIG:
case TCPOPT_FASTOPEN:
case TCPOPT_EXP:
*expected_length = 0; /* variable-length option */
break;
default:
asprintf(error, "unexpected TCP option kind: %u", kind);
return STATUS_ERR;
}
return STATUS_OK;
}
/* Calculate the length of the TCP option at 'opt', in a block of TCP
* options that ends at 'end'. If 'expected_length' is non-zero,
* verify that length matches the expectation. Return length of
* option in bytes in *length. Returns STATUS_OK on success; on
* failure returns STATUS_ERR and sets error message.
*/
static int get_tcp_option_length(const u8 *option, const u8 *end,
u8 expected_length, u8 *length, char **error)
{
int result = STATUS_ERR;
if (option + 1 >= end) {
asprintf(error, "TCP option length byte extends too far");
goto out;
}
*length = *(option + 1);
if (*length < 2) {
asprintf(error, "TCP option with length byte is too short");
goto out;
}
if (option + (*length) > end) {
asprintf(error, "TCP option data extends too far");
goto out;
}
if (expected_length && (*length != expected_length)) {
asprintf(error,
"bad TCP option length: was %u but expected %u",
*length, expected_length);
goto out;
}
result = STATUS_OK;
out:
return result;
}
static struct tcp_option *get_current_option(
struct tcp_options_iterator *iter)
{
assert(iter->current_option <= iter->options_end);
if (iter->current_option >= iter->options_end)
iter->current_option = NULL;
return (struct tcp_option *)iter->current_option;
}
struct tcp_option *tcp_options_begin(
struct packet *packet,
struct tcp_options_iterator *iter)
{
memset(iter, 0, sizeof(*iter));
iter->current_option = packet_tcp_options(packet);
iter->options_end = packet_payload(packet);
return get_current_option(iter);
}
struct tcp_option *tcp_options_next(
struct tcp_options_iterator *iter, char **error)
{
/* Ensure we haven't hit the end. */
assert(iter->current_option < iter->options_end);
assert(iter->current_option != NULL);
/* Find the length we expect for this kind of option. */
u8 length = 0; /* length of this option in bytes */
u8 expected_length = 0; /* expected length for this kind */
struct tcp_option *option = (struct tcp_option *)iter->current_option;
if (get_expected_tcp_option_length(
option->kind, &expected_length, error))
goto out;
/* Calculate and validate the actual length of the option. */
if (expected_length == 1) {
/* 1 byte length means no length byte, so real length is 1. */
length = 1;
} else {
/* Parse and validate length byte. */
if (get_tcp_option_length(iter->current_option,
iter->options_end,
expected_length, &length, error))
goto out;
}
/* Advance to the next TCP option. */
assert(length > 0);
iter->current_option += length;
assert(iter->current_option <= iter->options_end);
return get_current_option(iter);
out:
return NULL;
}
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