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use do { } while (!bottom) instead of again: ... if (!bottom) goto again;

/*	$OpenBSD: print-mpls.c,v 1.4 2018/07/06 07:00:49 dlg Exp $	*/

/*
 * Copyright (c) 2005 Jason L. Wright (jason@thought.net)
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/socket.h>
#include <sys/types.h>
#include <netmpls/mpls.h>

#include <stdio.h>

#include "interface.h"
#include "extract.h"		    /* must come after interface.h */

#define CW_SEQUENCE_MASK       (0x0000ffffU)

int controlword_tryprint(const u_char **, u_int *);

void
mpls_print(const u_char *bp, u_int len)
{
	u_int32_t tag, label, exp, bottom, ttl;
	int has_cw;

	do {
		if (bp + sizeof(tag) > snapend)
			goto trunc;

		tag = EXTRACT_32BITS(bp);
		bp += sizeof(tag);
		len -= sizeof(tag);

		label = (tag >> 12) & 0xfffff;
		exp = (tag >> 9) & 0x7;
		bottom = (tag >> 8) & 0x1;
		ttl = (tag >> 0) & 0xff;

		printf("MPLS(label %u, exp %u, ttl %u) ", label, exp, ttl);

		/* XXX decode "Router Alert Label" */
	} while (!bottom);

	/* Handle pseudowire control word. */
	has_cw = controlword_tryprint(&bp, &len);

	/*
	 * guessing the underlying protocol is about all we can do if
	 * it's not explicitly defined.
	 */

	switch (label) {
	case 0x00000:			/* IPv4 Explicit NULL */
		ip_print(bp, len);
		break;
	case 0x00001:			/* Router Alert */
		/* shouldn't happen at stack bottom */
		printf("Route-Alert");
		break;
	case 0x00002:			/* IPv6 Explicit NULL */
		ip6_print(bp, len);
		break;
	case 0x00003:			/* Implicit NULL */
		/* shouldn't happen in the tag stack */
		printf("Implicit-NULL");
		break;

	case 0x00004:			/* reserved labels */
	case 0x00005:
	case 0x00006:
	case 0x00007:
	case 0x00008:
	case 0x00009:
	case 0x0000a:
	case 0x0000b:
	case 0x0000c:
	case 0x0000d:
	case 0x0000e:
	case 0x0000f:
		break;

	default:			/* dunno, guess? */
		if (len == 0)
			break;
		if (bp >= snapend)
			goto trunc;

		switch (bp[0] & 0xf0) {
		case 0x40:
			ip_print(bp, len);
			break;
		case 0x60:
			ip6_print(bp, len);
			break;
		}
	}

	if (has_cw)
		ether_tryprint(bp, len, 0);

	return;
trunc:
	printf("[|mpls]");
}

/* Print control word if any and returns 1 on success. */
int
controlword_tryprint(const u_char **bp, u_int *lenp)
{
	uint32_t cw, frag, seq;

	if (*lenp < 4)
		return (0);

	cw = EXTRACT_32BITS(*bp);
	if (cw & CW_ZERO_MASK)
		return (0);

	*bp += sizeof(cw);
	*lenp += sizeof(cw);

	frag = (cw & CW_FRAG_MASK) >> 16;
	seq = cw & CW_SEQUENCE_MASK;

	printf("CW(frag %u, sequence %u) ", frag, seq);

	return (1);
}