samples/bpf/xdp_adjust_tail_kern.c

Source file repositories/reference/linux-study-clean/samples/bpf/xdp_adjust_tail_kern.c

File Facts

System
Linux kernel
Corpus path
samples/bpf/xdp_adjust_tail_kern.c
Extension
.c
Size
3959 bytes
Lines
157
Domain
Support Tooling And Documentation
Bucket
samples
Inferred role
Support Tooling And Documentation: implementation source
Status
source implementation candidate

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Dependency Surface

Detected Declarations

Annotated Snippet

#define KBUILD_MODNAME "foo"
#include <uapi/linux/bpf.h>
#include <linux/in.h>
#include <linux/if_ether.h>
#include <linux/if_packet.h>
#include <linux/if_vlan.h>
#include <linux/ip.h>
#include <linux/icmp.h>
#include <bpf/bpf_helpers.h>

#define DEFAULT_TTL 64
#define MAX_PCKT_SIZE 600
#define ICMP_TOOBIG_SIZE 98
#define ICMP_TOOBIG_PAYLOAD_SIZE 92

/* volatile to prevent compiler optimizations */
static volatile __u32 max_pcktsz = MAX_PCKT_SIZE;

struct {
	__uint(type, BPF_MAP_TYPE_ARRAY);
	__type(key, __u32);
	__type(value, __u64);
	__uint(max_entries, 1);
} icmpcnt SEC(".maps");

static __always_inline void count_icmp(void)
{
	u64 key = 0;
	u64 *icmp_count;

	icmp_count = bpf_map_lookup_elem(&icmpcnt, &key);
	if (icmp_count)
		*icmp_count += 1;
}

static __always_inline void swap_mac(void *data, struct ethhdr *orig_eth)
{
	struct ethhdr *eth;

	eth = data;
	memcpy(eth->h_source, orig_eth->h_dest, ETH_ALEN);
	memcpy(eth->h_dest, orig_eth->h_source, ETH_ALEN);
	eth->h_proto = orig_eth->h_proto;
}

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

static __always_inline void ipv4_csum(void *data_start, int data_size,
				      __u32 *csum)
{
	*csum = bpf_csum_diff(0, 0, data_start, data_size, *csum);
	*csum = csum_fold_helper(*csum);
}

static __always_inline int send_icmp4_too_big(struct xdp_md *xdp)
{
	int headroom = (int)sizeof(struct iphdr) + (int)sizeof(struct icmphdr);

	if (bpf_xdp_adjust_head(xdp, 0 - headroom))
		return XDP_DROP;
	void *data = (void *)(long)xdp->data;
	void *data_end = (void *)(long)xdp->data_end;

	if (data + (ICMP_TOOBIG_SIZE + headroom) > data_end)
		return XDP_DROP;

	struct iphdr *iph, *orig_iph;
	struct icmphdr *icmp_hdr;
	struct ethhdr *orig_eth;
	__u32 csum = 0;
	__u64 off = 0;

	orig_eth = data + headroom;
	swap_mac(data, orig_eth);
	off += sizeof(struct ethhdr);
	iph = data + off;
	off += sizeof(struct iphdr);
	icmp_hdr = data + off;
	off += sizeof(struct icmphdr);
	orig_iph = data + off;
	icmp_hdr->type = ICMP_DEST_UNREACH;
	icmp_hdr->code = ICMP_FRAG_NEEDED;
	icmp_hdr->un.frag.mtu = htons(max_pcktsz - sizeof(struct ethhdr));
	icmp_hdr->checksum = 0;
	ipv4_csum(icmp_hdr, ICMP_TOOBIG_PAYLOAD_SIZE, &csum);
	icmp_hdr->checksum = csum;

Annotation

Implementation Notes