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explicitly use BPF_FILDROP_CAPTURE when configuring BIOCSFILDROP

BIOCSFILDROP used to just be a flag, ie, any non-zero value was
treated the same, but i'm changing it so different values do different
things. this way the programs should keep working even if i decide
to change the values for these macros.

/*	$OpenBSD: bpf.c,v 1.3 2019/03/18 00:00:59 dlg Exp $ 	*/

/* BPF socket interface code, originally contributed by Archie Cobbs. */

/*
 * Copyright (c) 1995, 1996, 1998, 1999
 * The Internet Software Consortium.    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.
 * 3. Neither the name of The Internet Software Consortium nor the names
 *    of its contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND
 * CONTRIBUTORS ``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 INTERNET SOFTWARE CONSORTIUM OR
 * CONTRIBUTORS 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.
 *
 * This software has been written for the Internet Software Consortium
 * by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
 * Enterprises.  To learn more about the Internet Software Consortium,
 * see ``http://www.vix.com/isc''.  To learn more about Vixie
 * Enterprises, see ``http://www.vix.com''.
 */

#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/socket.h>

#include <net/bpf.h>
#include <net/if.h>

#include <netinet/in.h>
#include <netinet/if_ether.h>

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "dhcp.h"
#include "dhcpd.h"
#include "log.h"

ssize_t send_packet_layer3(struct interface_info *,
    void *, size_t, struct packet_ctx *);

/*
 * Called by get_interface_list for each interface that's discovered.
 * Opens a packet filter for each interface and adds it to the select
 * mask.
 */
int
if_register_bpf(struct interface_info *info)
{
	int sock;

	/* Open the BPF device */
	if ((sock = open("/dev/bpf", O_RDWR)) == -1)
		fatal("Can't open bpf device");

	/* Set the BPF device to point at this interface. */
	if (ioctl(sock, BIOCSETIF, &info->ifr) == -1)
		fatal("Can't attach interface %s to bpf device", info->name);

	return (sock);
}

void
if_register_send(struct interface_info *info)
{
	/*
	 * If we're using the bpf API for sending and receiving, we
	 * don't need to register this interface twice.
	 */
	info->wfdesc = info->rfdesc;
}

/* DHCPv6 BPF filters. */

/*
 * Packet filter program: 'ip6 and udp and dst port DHCP6_SERVER_PORT'
 */
struct bpf_insn dhcp6_bpf_sfilter[] = {
	/* Make sure this is an IP packet... */
	BPF_STMT(BPF_LD + BPF_H + BPF_ABS, 12),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ETHERTYPE_IPV6, 0, 5),

	/* Make sure this is an UDP packet. */
	BPF_STMT(BPF_LD + BPF_B + BPF_ABS, 20),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 0, 3),

	/* Make sure it is the right port. */
	BPF_STMT(BPF_LD + BPF_H + BPF_ABS, 56),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, DHCP6_SERVER_PORT, 0, 1),

	/* If we passed all the tests, ask for the whole packet. */
	BPF_STMT(BPF_RET + BPF_K, (u_int)-1),

	/* Otherwise, drop it. */
	BPF_STMT(BPF_RET + BPF_K, 0),
};

int dhcp6_bpf_sfilter_len = sizeof(dhcp6_bpf_sfilter) / sizeof(struct bpf_insn);

/*
 * Packet filter program: 'ip6 and udp'
 */
struct bpf_insn dhcp6_bpf_wfilter[] = {
	/* Make sure this is an IP packet... */
	BPF_STMT(BPF_LD + BPF_H + BPF_ABS, 12),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ETHERTYPE_IPV6, 0, 3),

	/* Make sure this is an UDP packet. */
	BPF_STMT(BPF_LD + BPF_B + BPF_ABS, 20),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 0, 1),

	/* If we passed all the tests, ask for the whole packet. */
	BPF_STMT(BPF_RET + BPF_K, (u_int)-1),

	/* Otherwise, drop it. */
	BPF_STMT(BPF_RET + BPF_K, 0),
};

int dhcp6_bpf_wfilter_len = sizeof(dhcp6_bpf_wfilter) / sizeof(struct bpf_insn);


void
if_register_receive(struct interface_info *info)
{
	struct bpf_version v;
	struct bpf_program p;
	int flag = 1, sz, cmplt = 0;

	/* Open a BPF device and hang it on this interface... */
	info->rfdesc = if_register_bpf(info);

	/* Make sure the BPF version is in range... */
	if (ioctl(info->rfdesc, BIOCVERSION, &v) == -1)
		fatal("Can't get BPF version");

	if (v.bv_major != BPF_MAJOR_VERSION ||
	    v.bv_minor < BPF_MINOR_VERSION)
		fatalx("Kernel BPF version out of range - recompile dhcpd!");

	/*
	 * Set immediate mode so that reads return as soon as a packet
	 * comes in, rather than waiting for the input buffer to fill
	 * with packets.
	 */
	if (ioctl(info->rfdesc, BIOCIMMEDIATE, &flag) == -1)
		fatal("Can't set immediate mode on bpf device");

	/* make sure kernel fills in the source ethernet address */
	if (ioctl(info->rfdesc, BIOCSHDRCMPLT, &cmplt) == -1)
		fatal("Can't set header complete flag on bpf device");

	/* Get the required BPF buffer length from the kernel. */
	if (ioctl(info->rfdesc, BIOCGBLEN, &sz) == -1)
		fatal("Can't get bpf buffer length");
	info->rbuf_max = sz;
	info->rbuf = malloc(info->rbuf_max);
	if (!info->rbuf)
		fatalx("Can't allocate %lu bytes for bpf input buffer.",
		    (unsigned long)info->rbuf_max);
	info->rbuf_offset = 0;
	info->rbuf_len = 0;

	/* Set up the bpf filter program structure. */
	p.bf_len = dhcp6_bpf_sfilter_len;
	p.bf_insns = dhcp6_bpf_sfilter;
	if (ioctl(info->rfdesc, BIOCSETF, &p) == -1)
		fatal("Can't install packet filter program");

	/* Set up the bpf write filter program structure. */
	p.bf_len = dhcp6_bpf_wfilter_len;
	p.bf_insns = dhcp6_bpf_wfilter;
	if (ioctl(info->rfdesc, BIOCSETWF, &p) == -1)
		fatal("Can't install write filter program");

	/* Only get input packets. */
	flag = BPF_DIRECTION_OUT;
	if (ioctl(info->rfdesc, BIOCSDIRFILT , &flag) == -1)
		fatal("Can't set BPF direction capture");

	/* Drop them so they don't go up in the network stack. */
	flag = BPF_FILDROP_CAPTURE;
	if (ioctl(info->rfdesc, BIOCSFILDROP, &flag) == -1)
		fatal("Can't set BPF filter drop");

	/* make sure these settings cannot be changed after dropping privs */
	if (ioctl(info->rfdesc, BIOCLOCK) == -1)
		fatal("Failed to lock bpf descriptor");
}

ssize_t
send_packet_layer3(struct interface_info *intf, void *raw, size_t len,
    struct packet_ctx *pc)
{
	struct cmsghdr		*cmsg;
	ssize_t			 sendlen;
	struct msghdr		 msg;
	struct in6_pktinfo	*ipi6;
	struct sockaddr_storage	 ss;
	struct iovec		 iov[2];
	uint8_t			 cmsgbuf[
	    CMSG_SPACE(sizeof(struct in6_pktinfo))
	];

	log_debug("  sending %ld bytes to %s:%d via %s",
	    len, v6addr2str(&ss2sin6(&pc->pc_dst)->sin6_addr),
	    ntohs(ss2sin6(&pc->pc_dst)->sin6_port), intf->name);

	memset(&msg, 0, sizeof(msg));
	iov[0].iov_base = raw;
	iov[0].iov_len = len;
	msg.msg_iov = iov;
	msg.msg_iovlen = 1;

	ss = pc->pc_dst;
	ss2sin6(&ss)->sin6_scope_id = intf->index;
	msg.msg_name = &ss;
	msg.msg_namelen = ss.ss_len;

	/* If binded to multicast we should select an interface. */
	if (IN6_IS_ADDR_MULTICAST(&ss2sin6(&ss)->sin6_addr)) {
		memset(cmsgbuf, 0, sizeof(cmsgbuf));
		msg.msg_control = cmsgbuf;
		msg.msg_controllen = sizeof(cmsgbuf);

		/* Use the IPV6_PKTINFO to select the interface. */
		cmsg = (struct cmsghdr *)CMSG_FIRSTHDR(&msg);
		cmsg->cmsg_len = CMSG_LEN(sizeof(*ipi6));
		cmsg->cmsg_level = IPPROTO_IPV6;
		cmsg->cmsg_type = IPV6_PKTINFO;

		ipi6 = (struct in6_pktinfo *)CMSG_DATA(cmsg);
		ipi6->ipi6_ifindex = intf->index;
	}

	if ((sendlen = sendmsg(pc->pc_sd, &msg, 0)) == -1) {
		log_warn("  failed to send message");
		return -1;
	}
	if (sendlen < (ssize_t)len)
		log_warnx("  sent less bytes than expected (%ld < %ld)",
		    sendlen, len);

	return sendlen;
}

ssize_t
send_packet(struct interface_info *interface,
    void *raw, size_t len, struct packet_ctx *pc)
{
	unsigned char buf[256];
	struct iovec iov[2];
	int result, bufp = 0;

	if (pc->pc_sd != 0)
		return send_packet_layer3(interface, raw, len, pc);

	/* Assemble the headers... */
	assemble_hw_header(buf, &bufp, pc);
	assemble_udp_ip6_header(buf, &bufp, pc, raw, len);

	/* Fire it off */
	iov[0].iov_base = (char *)buf;
	iov[0].iov_len = bufp;
	iov[1].iov_base = (char *)raw;
	iov[1].iov_len = len;

	result = writev(interface->wfdesc, iov, 2);
	if (result == -1)
		log_warn("send_packet");

	return (result);
}

ssize_t
receive_packet(struct interface_info *interface, unsigned char *buf,
    size_t len, struct packet_ctx *pc)
{
	int length = 0, offset = 0;
	struct bpf_hdr hdr;

	/*
	 * All this complexity is because BPF doesn't guarantee that
	 * only one packet will be returned at a time.  We're getting
	 * what we deserve, though - this is a terrible abuse of the BPF
	 * interface.  Sigh.
	 */

	/* Process packets until we get one we can return or until we've
	 * done a read and gotten nothing we can return...
	 */
	do {
		/* If the buffer is empty, fill it. */
		if (interface->rbuf_offset == interface->rbuf_len) {
			length = read(interface->rfdesc, interface->rbuf,
			    interface->rbuf_max);
			if (length <= 0)
				return (length);
			interface->rbuf_offset = 0;
			interface->rbuf_len = length;
		}

		/*
		 * If there isn't room for a whole bpf header, something
		 * went wrong, but we'll ignore it and hope it goes
		 * away... XXX
		 */
		if (interface->rbuf_len - interface->rbuf_offset <
		    sizeof(hdr)) {
			interface->rbuf_offset = interface->rbuf_len;
			continue;
		}

		/* Copy out a bpf header... */
		memcpy(&hdr, &interface->rbuf[interface->rbuf_offset],
		    sizeof(hdr));

		/*
		 * If the bpf header plus data doesn't fit in what's
		 * left of the buffer, stick head in sand yet again...
		 */
		if (interface->rbuf_offset + hdr.bh_hdrlen + hdr.bh_caplen >
		    interface->rbuf_len) {
			interface->rbuf_offset = interface->rbuf_len;
			continue;
		}

		/*
		 * If the captured data wasn't the whole packet, or if
		 * the packet won't fit in the input buffer, all we can
		 * do is drop it.
		 */
		if (hdr.bh_caplen != hdr.bh_datalen) {
			interface->rbuf_offset += hdr.bh_hdrlen =
			    hdr.bh_caplen;
			continue;
		}

		/* Skip over the BPF header... */
		interface->rbuf_offset += hdr.bh_hdrlen;

		/* Decode the physical header... */
		offset = decode_hw_header(interface->rbuf,
		    interface->rbuf_offset, pc);

		/*
		 * If a physical layer checksum failed (dunno of any
		 * physical layer that supports this, but WTH), skip
		 * this packet.
		 */
		if (offset < 0) {
			interface->rbuf_offset += hdr.bh_caplen;
			continue;
		}
		interface->rbuf_offset += offset;
		hdr.bh_caplen -= offset;

		/* Decode the IP and UDP headers... */
		offset = decode_udp_ip6_header(interface->rbuf,
		    interface->rbuf_offset, pc, hdr.bh_caplen);

		/* If the IP or UDP checksum was bad, skip the packet... */
		if (offset < 0) {
			interface->rbuf_offset += hdr.bh_caplen;
			continue;
		}
		interface->rbuf_offset += offset;
		hdr.bh_caplen -= offset;

		/*
		 * If there's not enough room to stash the packet data,
		 * we have to skip it (this shouldn't happen in real
		 * life, though).
		 */
		if (hdr.bh_caplen > len) {
			interface->rbuf_offset += hdr.bh_caplen;
			continue;
		}

		/* Copy out the data in the packet... */
		memcpy(buf, interface->rbuf + interface->rbuf_offset,
		    hdr.bh_caplen);
		interface->rbuf_offset += hdr.bh_caplen;
		return (hdr.bh_caplen);
	} while (!length);
	return (0);
}