xdp_prog_kern_03.c

/* SPDX-License-Identifier: GPL-2.0 */
#include <linux/bpf.h>
#include <linux/in.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>

// The parsing helper functions from the packet01 lesson have moved here
#include "../common/parsing_helpers.h"
#include "../common/rewrite_helpers.h"

/* Defines xdp_stats_map */
#include "../common/xdp_stats_kern_user.h"
#include "../common/xdp_stats_kern.h"

#ifndef memcpy
#define memcpy(dest, src, n) __builtin_memcpy((dest), (src), (n))
#endif

struct bpf_map_def SEC("maps") tx_port = {
	.type = BPF_MAP_TYPE_DEVMAP,
	.key_size = sizeof(int),
	.value_size = sizeof(int),
	.max_entries = 256,
};

struct bpf_map_def SEC("maps") redirect_params = {
	.type = BPF_MAP_TYPE_HASH,
	.key_size = ETH_ALEN,
	.value_size = ETH_ALEN,
	.max_entries = 1,
};

static __always_inline __u16 csum_fold_helper(__u32 csum)
{
	return ~((csum & 0xffff) + (csum >> 16));
}

/*
 * The icmp_checksum_diff function takes pointers to old and new structures and
 * the old checksum and returns the new checksum.  It uses the bpf_csum_diff
 * helper to compute the checksum difference. Note that the sizes passed to the
 * bpf_csum_diff helper should be multiples of 4, as it operates on 32-bit
 * words.
 */
static __always_inline __u16 icmp_checksum_diff(
		__u16 seed,
		struct icmphdr_common *icmphdr_new,
		struct icmphdr_common *icmphdr_old)
{
	__u32 csum, size = sizeof(struct icmphdr_common);

	csum = bpf_csum_diff((__be32 *)icmphdr_old, size, (__be32 *)icmphdr_new, size, seed);
	return csum_fold_helper(csum);
}

/* Solution to packet03/assignment-1 */
SEC("xdp_icmp_echo")
int xdp_icmp_echo_func(struct xdp_md *ctx)
{
	void *data_end = (void *)(long)ctx->data_end;
	void *data = (void *)(long)ctx->data;
	struct hdr_cursor nh;
	struct ethhdr *eth;
	int eth_type;
	int ip_type;
	int icmp_type;
	struct iphdr *iphdr;
	struct ipv6hdr *ipv6hdr;
	__u16 echo_reply, old_csum;
	struct icmphdr_common *icmphdr;
	struct icmphdr_common icmphdr_old;
	__u32 action = XDP_PASS;

	/* These keep track of the next header type and iterator pointer */
	nh.pos = data;

	/* Parse Ethernet and IP/IPv6 headers */
	eth_type = parse_ethhdr(&nh, data_end, &eth);
	if (eth_type == bpf_htons(ETH_P_IP)) {
		ip_type = parse_iphdr(&nh, data_end, &iphdr);
		if (ip_type != IPPROTO_ICMP)
			goto out;
	} else if (eth_type == bpf_htons(ETH_P_IPV6)) {
		ip_type = parse_ip6hdr(&nh, data_end, &ipv6hdr);
		if (ip_type != IPPROTO_ICMPV6)
			goto out;
	} else {
		goto out;
	}

	/*
	 * We are using a special parser here which returns a stucture
	 * containing the "protocol-independent" part of an ICMP or ICMPv6
	 * header.  For purposes of this Assignment we are not interested in
	 * the rest of the structure.
	 */
	icmp_type = parse_icmphdr_common(&nh, data_end, &icmphdr);
	if (eth_type == bpf_htons(ETH_P_IP) && icmp_type == ICMP_ECHO) {
		/* Swap IP source and destination */
		swap_src_dst_ipv4(iphdr);
		echo_reply = ICMP_ECHOREPLY;
	} else if (eth_type == bpf_htons(ETH_P_IPV6)
		   && icmp_type == ICMPV6_ECHO_REQUEST) {
		/* Swap IPv6 source and destination */
		swap_src_dst_ipv6(ipv6hdr);
		echo_reply = ICMPV6_ECHO_REPLY;
	} else {
		goto out;
	}

	/* Swap Ethernet source and destination */
	swap_src_dst_mac(eth);


	/* Patch the packet and update the checksum.*/
	old_csum = icmphdr->cksum;
	icmphdr->cksum = 0;
	icmphdr_old = *icmphdr;
	icmphdr->type = echo_reply;
	icmphdr->cksum = icmp_checksum_diff(~old_csum, icmphdr, &icmphdr_old);

	/* Another, less generic, but a bit more efficient way to update the
	 * checksum is listed below.  As only one 16-bit word changed, the sum
	 * can be patched using this formula: sum' = ~(~sum + ~m0 + m1), where
	 * sum' is a new sum, sum is an old sum, m0 and m1 are the old and new
	 * 16-bit words, correspondingly. In the formula above the + operation
	 * is defined as the following function:
	 *
	 *     static __always_inline __u16 csum16_add(__u16 csum, __u16 addend)
	 *     {
	 *         csum += addend;
	 *         return csum + (csum < addend);
	 *     }
	 *
	 * So an alternative code to update the checksum might look like this:
	 *
	 *     __u16 m0 = * (__u16 *) icmphdr;
	 *     icmphdr->type = echo_reply;
	 *     __u16 m1 = * (__u16 *) icmphdr;
	 *     icmphdr->checksum = ~(csum16_add(csum16_add(~icmphdr->checksum, ~m0), m1));
	 */

	action = XDP_TX;

out:
	return xdp_stats_record_action(ctx, action);
}

/* Solution to packet03/assignment-2 */
SEC("xdp_redirect")
int xdp_redirect_func(struct xdp_md *ctx)
{
	void *data_end = (void *)(long)ctx->data_end;
	void *data = (void *)(long)ctx->data;
	struct hdr_cursor nh;
	struct ethhdr *eth;
	int eth_type;
	int action = XDP_PASS;
	unsigned char dst[ETH_ALEN] = { /* TODO: put your values here */ };
	unsigned ifindex = 0/* TODO: put your values here */;

	/* These keep track of the next header type and iterator pointer */
	nh.pos = data;

	/* Parse Ethernet and IP/IPv6 headers */
	eth_type = parse_ethhdr(&nh, data_end, &eth);
	if (eth_type == -1)
		goto out;

	/* Set a proper destination address */
	memcpy(eth->h_dest, dst, ETH_ALEN);
	action = bpf_redirect(ifindex, 0);

out:
	return xdp_stats_record_action(ctx, action);
}

/* Solution to packet03/assignment-3 */
SEC("xdp_redirect_map")
int xdp_redirect_map_func(struct xdp_md *ctx)
{
	void *data_end = (void *)(long)ctx->data_end;
	void *data = (void *)(long)ctx->data;
	struct hdr_cursor nh;
	struct ethhdr *eth;
	int eth_type;
	int action = XDP_PASS;
	unsigned char *dst;

	/* These keep track of the next header type and iterator pointer */
	nh.pos = data;

	/* Parse Ethernet and IP/IPv6 headers */
	eth_type = parse_ethhdr(&nh, data_end, &eth);
	if (eth_type == -1)
		goto out;

	/* Do we know where to redirect this packet? */
	dst = bpf_map_lookup_elem(&redirect_params, eth->h_source);
	if (!dst)
		goto out;

	/* Set a proper destination address */
	memcpy(eth->h_dest, dst, ETH_ALEN);
	action = bpf_redirect_map(&tx_port, 0, 0);

out:
	return xdp_stats_record_action(ctx, action);
}

#define AF_INET 2
#define AF_INET6 10
#define IPV6_FLOWINFO_MASK bpf_htonl(0x0FFFFFFF)

/* from include/net/ip.h */
static __always_inline int ip_decrease_ttl(struct iphdr *iph)
{
	__u32 check = iph->check;
	check += bpf_htons(0x0100);
	iph->check = (__u16)(check + (check >= 0xFFFF));
	return --iph->ttl;
}

/* Solution to packet03/assignment-4 */
/* xdp_router is the name of the xdp program */
SEC("xdp_router")
int xdp_router_func(struct xdp_md *ctx)
{
	/* this is the packet context*/
	void *data_end = (void *)(long)ctx->data_end;
	void *data = (void *)(long)ctx->data;
	struct bpf_fib_lookup fib_params = {};
	struct ethhdr *eth = data;
	struct ipv6hdr *ip6h;
	struct iphdr *iph;
	__u16 h_proto;
	__u64 nh_off;
	int rc;
	/* default action is to pass */
	int action = XDP_PASS;

	nh_off = sizeof(*eth);
	if (data + nh_off > data_end) {
		action = XDP_DROP;
		goto out;
	}

	/* determine if this is IP4 or IPv6 by looking at the Ethernet protocol field */
	h_proto = eth->h_proto;
	if (h_proto == bpf_htons(ETH_P_IP)) {
		/* IPv4 part of the code */
		iph = data + nh_off;

		if (iph + 1 > data_end) {
			action = XDP_DROP;
			goto out;
		}
		/* as a real router, we need to check the TTL to prevent never ending loops*/
		if (iph->ttl <= 1)
			goto out;

		/* populate the fib_params fields to prepare for the lookup */
		fib_params.family	= AF_INET;
		fib_params.tos		= iph->tos;
		fib_params.l4_protocol	= iph->protocol;
		fib_params.sport	= 0;
		fib_params.dport	= 0;
		fib_params.tot_len	= bpf_ntohs(iph->tot_len);
		fib_params.ipv4_src	= iph->saddr;
		fib_params.ipv4_dst	= iph->daddr;
	} else if (h_proto == bpf_htons(ETH_P_IPV6)) {
		/* IPv6 part of the code */
		struct in6_addr *src = (struct in6_addr *) fib_params.ipv6_src;
		struct in6_addr *dst = (struct in6_addr *) fib_params.ipv6_dst;

		ip6h = data + nh_off;
		if (ip6h + 1 > data_end) {
			action = XDP_DROP;
			goto out;
		}
		/* as a real router, we need to check the TTL to prevent never ending loops*/
		if (ip6h->hop_limit <= 1)
			goto out;

		/* populate the fib_params fields to prepare for the lookup */
		fib_params.family	= AF_INET6;
		fib_params.flowinfo	= *(__be32 *) ip6h & IPV6_FLOWINFO_MASK;
		fib_params.l4_protocol	= ip6h->nexthdr;
		fib_params.sport	= 0;
		fib_params.dport	= 0;
		fib_params.tot_len	= bpf_ntohs(ip6h->payload_len);
		*src			= ip6h->saddr;
		*dst			= ip6h->daddr;
	} else {
		goto out;
	}

	fib_params.ifindex = ctx->ingress_ifindex;

	/* this is where the FIB lookup happens. If the lookup is successful */
	/* it will populate the fib_params.ifindex with the egress interface index */
	
	rc = bpf_fib_lookup(ctx, &fib_params, sizeof(fib_params), 0);
	switch (rc) {
	case BPF_FIB_LKUP_RET_SUCCESS:         /* lookup successful */
		   /* we are a router, so we need to decrease the ttl */
		if (h_proto == bpf_htons(ETH_P_IP))
			ip_decrease_ttl(iph);
		else if (h_proto == bpf_htons(ETH_P_IPV6))
			ip6h->hop_limit--;
		/* set the correct new source and destionation mac addresses */
		/* can be found in fib_params.dmac and fib_params.smac */
		memcpy(eth->h_dest, fib_params.dmac, ETH_ALEN);
		memcpy(eth->h_source, fib_params.smac, ETH_ALEN);
		/* and done, now we set the action to bpf_redirect_map with fib_params.ifindex which is the egress port as paramater */
		action = bpf_redirect_map(&tx_port, fib_params.ifindex, 0);
		break;
	case BPF_FIB_LKUP_RET_BLACKHOLE:    /* dest is blackholed; can be dropped */
	case BPF_FIB_LKUP_RET_UNREACHABLE:  /* dest is unreachable; can be dropped */
	case BPF_FIB_LKUP_RET_PROHIBIT:     /* dest not allowed; can be dropped */
		action = XDP_DROP;
		break;
	case BPF_FIB_LKUP_RET_NOT_FWDED:    /* packet is not forwarded */
	case BPF_FIB_LKUP_RET_FWD_DISABLED: /* fwding is not enabled on ingress */
	case BPF_FIB_LKUP_RET_UNSUPP_LWT:   /* fwd requires encapsulation */
	case BPF_FIB_LKUP_RET_NO_NEIGH:     /* no neighbor entry for nh */
	case BPF_FIB_LKUP_RET_FRAG_NEEDED:  /* fragmentation required to fwd */
		/* PASS */
		break;
	}

out:
	/* and done, update stats and return action */
	return xdp_stats_record_action(ctx, action);
}

SEC("xdp_pass")
int xdp_pass_func(struct xdp_md *ctx)
{
	return xdp_stats_record_action(ctx, XDP_PASS);
}

char _license[] SEC("license") = "GPL";