GCC Code Coverage Report
Directory: ./ Exec Total Coverage
File: usr.sbin/tcpdump/in_cksum.c Lines: 0 5 0.0 %
Date: 2017-11-13 Branches: 0 0 0.0 %

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/*	$OpenBSD: in_cksum.c,v 1.1 2014/06/20 04:01:42 lteo Exp $	*/
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/*
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 * Copyright (c) 1988, 1992, 1993
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 *	The Regents of the University of California.  All rights reserved.
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 *
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 * Redistribution and use in source and binary forms, with or without
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 * modification, are permitted provided that the following conditions
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 * are met:
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 * 1. Redistributions of source code must retain the above copyright
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 *    notice, this list of conditions and the following disclaimer.
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 * 2. Redistributions in binary form must reproduce the above copyright
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 *    notice, this list of conditions and the following disclaimer in the
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 *    documentation and/or other materials provided with the distribution.
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 * 3. Neither the name of the University nor the names of its contributors
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 *    may be used to endorse or promote products derived from this software
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 *    without specific prior written permission.
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 *
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 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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 * SUCH DAMAGE.
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 *
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 *	@(#)in_cksum.c	8.1 (Berkeley) 6/10/93
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 */
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#include <sys/types.h>
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#include "interface.h"
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/*
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 * Given the host-byte-order value of the checksum field in a packet
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 * header, and the network-byte-order computed checksum of the data
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 * that the checksum covers (including the checksum itself), compute
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 * what the checksum field *should* have been.
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 */
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u_int16_t
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in_cksum_shouldbe(u_int16_t sum, u_int16_t computed_sum)
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{
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	u_int32_t shouldbe;
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	/*
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	 * The value that should have gone into the checksum field
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	 * is the negative of the value gotten by summing up everything
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	 * *but* the checksum field.
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	 *
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	 * We can compute that by subtracting the value of the checksum
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	 * field from the sum of all the data in the packet, and then
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	 * computing the negative of that value.
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	 *
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	 * "sum" is the value of the checksum field, and "computed_sum"
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	 * is the negative of the sum of all the data in the packets,
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	 * so that's -(-computed_sum - sum), or (sum + computed_sum).
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	 *
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	 * All the arithmetic in question is one's complement, so the
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	 * addition must include an end-around carry; we do this by
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	 * doing the arithmetic in 32 bits (with no sign-extension),
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	 * and then adding the upper 16 bits of the sum, which contain
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	 * the carry, to the lower 16 bits of the sum, and then do it
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	 * again in case *that* sum produced a carry.
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	 *
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	 * As RFC 1071 notes, the checksum can be computed without
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	 * byte-swapping the 16-bit words; summing 16-bit words
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	 * on a big-endian machine gives a big-endian checksum, which
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	 * can be directly stuffed into the big-endian checksum fields
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	 * in protocol headers, and summing words on a little-endian
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	 * machine gives a little-endian checksum, which must be
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	 * byte-swapped before being stuffed into a big-endian checksum
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	 * field.
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	 *
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	 * "computed_sum" is a network-byte-order value, so we must put
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	 * it in host byte order before subtracting it from the
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	 * host-byte-order value from the header; the adjusted checksum
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	 * will be in host byte order, which is what we'll return.
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	 */
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	shouldbe = sum;
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	shouldbe += ntohs(computed_sum);
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	shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
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	shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
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	return shouldbe;
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}