KFunc bpf_dynptr_from_skb

v6.4

Get dynptrs whose underlying pointer points to a skb.

Definition

For bpf program types that don't support writes on skb data, the dynptr is read-only (bpf_dynptr_write() will return an error)

For reads and writes through the bpf_dynptr_read() and bpf_dynptr_write() interfaces, reading and writing from/to data in the head as well as from/to non-linear paged buffers is supported. Data slices through the bpf_dynptr_data API are not supported; instead bpf_dynptr_slice() and bpf_dynptr_slice_rdwr() should be used.

int bpf_dynptr_from_skb(struct __sk_buff *s, u64 flags, struct bpf_dynptr *ptr__uninit)

Usage

The dynptr acts on skb data. skb dynptrs have two main benefits. One is that they allow operations on sizes that are not statically known at compile-time (for example variable-sized accesses). Another is that parsing the packet data through dynptrs (instead of through direct access of skb->data and skb->data_end) can be more ergonomic and less brittle (does not need manual if checking for being within bounds of data_end).

Program types

The following program types can make use of this kfunc:

• BPF_PROG_TYPE_CGROUP_SKB
• BPF_PROG_TYPE_LWT_IN
• BPF_PROG_TYPE_LWT_OUT
• BPF_PROG_TYPE_LWT_SEG6LOCAL
• BPF_PROG_TYPE_LWT_XMIT
• BPF_PROG_TYPE_NETFILTER
• BPF_PROG_TYPE_SCHED_ACT
• BPF_PROG_TYPE_SCHED_CLS
• BPF_PROG_TYPE_SK_SKB
• BPF_PROG_TYPE_SOCKET_FILTER

Example

L4LB no inline dynptr

// SPDX-License-Identifier: GPL-2.0
// Copyright (c) 2017 Facebook
#include <stddef.h>
#include <stdbool.h>
#include <string.h>
#include <linux/pkt_cls.h>
#include <linux/bpf.h>
#include <linux/in.h>
#include <linux/if_ether.h>
#include <linux/ip.h>
#include <linux/ipv6.h>
#include <linux/icmp.h>
#include <linux/icmpv6.h>
#include <linux/tcp.h>
#include <linux/udp.h>
#include <bpf/bpf_helpers.h>
#include "test_iptunnel_common.h"
#include <bpf/bpf_endian.h>

#include "bpf_kfuncs.h"

static __always_inline __u32 rol32(__u32 word, unsigned int shift)
{
    return (word << shift) | (word >> ((-shift) & 31));
}

/* copy paste of jhash from kernel sources to make sure llvm
* can compile it into valid sequence of bpf instructions
*/
#define __jhash_mix(a, b, c)            \
{                       \
    a -= c;  a ^= rol32(c, 4);  c += b; \
    b -= a;  b ^= rol32(a, 6);  a += c; \
    c -= b;  c ^= rol32(b, 8);  b += a; \
    a -= c;  a ^= rol32(c, 16); c += b; \
    b -= a;  b ^= rol32(a, 19); a += c; \
    c -= b;  c ^= rol32(b, 4);  b += a; \
}

#define __jhash_final(a, b, c)          \
{                       \
    c ^= b; c -= rol32(b, 14);      \
    a ^= c; a -= rol32(c, 11);      \
    b ^= a; b -= rol32(a, 25);      \
    c ^= b; c -= rol32(b, 16);      \
    a ^= c; a -= rol32(c, 4);       \
    b ^= a; b -= rol32(a, 14);      \
    c ^= b; c -= rol32(b, 24);      \
}

#define JHASH_INITVAL       0xdeadbeef

typedef unsigned int u32;

static __noinline u32 jhash(const void *key, u32 length, u32 initval)
{
    u32 a, b, c;
    const unsigned char *k = key;

    a = b = c = JHASH_INITVAL + length + initval;

    while (length > 12) {
        a += *(u32 *)(k);
        b += *(u32 *)(k + 4);
        c += *(u32 *)(k + 8);
        __jhash_mix(a, b, c);
        length -= 12;
        k += 12;
    }
    switch (length) {
    case 12: c += (u32)k[11]<<24;
    case 11: c += (u32)k[10]<<16;
    case 10: c += (u32)k[9]<<8;
    case 9:  c += k[8];
    case 8:  b += (u32)k[7]<<24;
    case 7:  b += (u32)k[6]<<16;
    case 6:  b += (u32)k[5]<<8;
    case 5:  b += k[4];
    case 4:  a += (u32)k[3]<<24;
    case 3:  a += (u32)k[2]<<16;
    case 2:  a += (u32)k[1]<<8;
    case 1:  a += k[0];
        __jhash_final(a, b, c);
    case 0: /* Nothing left to add */
        break;
    }

    return c;
}

static __noinline u32 __jhash_nwords(u32 a, u32 b, u32 c, u32 initval)
{
    a += initval;
    b += initval;
    c += initval;
    __jhash_final(a, b, c);
    return c;
}

static __noinline u32 jhash_2words(u32 a, u32 b, u32 initval)
{
    return __jhash_nwords(a, b, 0, initval + JHASH_INITVAL + (2 << 2));
}

#define PCKT_FRAGMENTED 65343
#define IPV4_HDR_LEN_NO_OPT 20
#define IPV4_PLUS_ICMP_HDR 28
#define IPV6_PLUS_ICMP_HDR 48
#define RING_SIZE 2
#define MAX_VIPS 12
#define MAX_REALS 5
#define CTL_MAP_SIZE 16
#define CH_RINGS_SIZE (MAX_VIPS * RING_SIZE)
#define F_IPV6 (1 << 0)
#define F_HASH_NO_SRC_PORT (1 << 0)
#define F_ICMP (1 << 0)
#define F_SYN_SET (1 << 1)

struct packet_description {
    union {
        __be32 src;
        __be32 srcv6[4];
    };
    union {
        __be32 dst;
        __be32 dstv6[4];
    };
    union {
        __u32 ports;
        __u16 port16[2];
    };
    __u8 proto;
    __u8 flags;
};

struct ctl_value {
    union {
        __u64 value;
        __u32 ifindex;
        __u8 mac[6];
    };
};

struct vip_meta {
    __u32 flags;
    __u32 vip_num;
};

struct real_definition {
    union {
        __be32 dst;
        __be32 dstv6[4];
    };
    __u8 flags;
};

struct vip_stats {
    __u64 bytes;
    __u64 pkts;
};

struct eth_hdr {
    unsigned char eth_dest[ETH_ALEN];
    unsigned char eth_source[ETH_ALEN];
    unsigned short eth_proto;
};

struct {
    __uint(type, BPF_MAP_TYPE_HASH);
    __uint(max_entries, MAX_VIPS);
    __type(key, struct vip);
    __type(value, struct vip_meta);
} vip_map SEC(".maps");

struct {
    __uint(type, BPF_MAP_TYPE_ARRAY);
    __uint(max_entries, CH_RINGS_SIZE);
    __type(key, __u32);
    __type(value, __u32);
} ch_rings SEC(".maps");

struct {
    __uint(type, BPF_MAP_TYPE_ARRAY);
    __uint(max_entries, MAX_REALS);
    __type(key, __u32);
    __type(value, struct real_definition);
} reals SEC(".maps");

struct {
    __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
    __uint(max_entries, MAX_VIPS);
    __type(key, __u32);
    __type(value, struct vip_stats);
} stats SEC(".maps");

struct {
    __uint(type, BPF_MAP_TYPE_ARRAY);
    __uint(max_entries, CTL_MAP_SIZE);
    __type(key, __u32);
    __type(value, struct ctl_value);
} ctl_array SEC(".maps");

static __noinline __u32 get_packet_hash(struct packet_description *pckt, bool ipv6)
{
    if (ipv6)
        return jhash_2words(jhash(pckt->srcv6, 16, MAX_VIPS),
                    pckt->ports, CH_RINGS_SIZE);
    else
        return jhash_2words(pckt->src, pckt->ports, CH_RINGS_SIZE);
}

static __noinline bool get_packet_dst(struct real_definition **real,
                    struct packet_description *pckt,
                    struct vip_meta *vip_info,
                    bool is_ipv6)
{
    __u32 hash = get_packet_hash(pckt, is_ipv6);
    __u32 key = RING_SIZE * vip_info->vip_num + hash % RING_SIZE;
    __u32 *real_pos;

    if (hash != 0x358459b7 /* jhash of ipv4 packet */  &&
        hash != 0x2f4bc6bb /* jhash of ipv6 packet */)
        return false;

    real_pos = bpf_map_lookup_elem(&ch_rings, &key);
    if (!real_pos)
        return false;
    key = *real_pos;
    *real = bpf_map_lookup_elem(&reals, &key);
    if (!(*real))
        return false;
    return true;
}

static __noinline int parse_icmpv6(struct bpf_dynptr *skb_ptr, __u64 off,
                struct packet_description *pckt)
{
    __u8 buffer[sizeof(struct ipv6hdr)] = {};
    struct icmp6hdr *icmp_hdr;
    struct ipv6hdr *ip6h;

    icmp_hdr = bpf_dynptr_slice(skb_ptr, off, buffer, sizeof(buffer));
    if (!icmp_hdr)
        return TC_ACT_SHOT;

    if (icmp_hdr->icmp6_type != ICMPV6_PKT_TOOBIG)
        return TC_ACT_OK;
    off += sizeof(struct icmp6hdr);
    ip6h = bpf_dynptr_slice(skb_ptr, off, buffer, sizeof(buffer));
    if (!ip6h)
        return TC_ACT_SHOT;
    pckt->proto = ip6h->nexthdr;
    pckt->flags |= F_ICMP;
    memcpy(pckt->srcv6, ip6h->daddr.s6_addr32, 16);
    memcpy(pckt->dstv6, ip6h->saddr.s6_addr32, 16);
    return TC_ACT_UNSPEC;
}

static __noinline int parse_icmp(struct bpf_dynptr *skb_ptr, __u64 off,
                struct packet_description *pckt)
{
    __u8 buffer_icmp[sizeof(struct iphdr)] = {};
    __u8 buffer_ip[sizeof(struct iphdr)] = {};
    struct icmphdr *icmp_hdr;
    struct iphdr *iph;

    icmp_hdr = bpf_dynptr_slice(skb_ptr, off, buffer_icmp, sizeof(buffer_icmp));
    if (!icmp_hdr)
        return TC_ACT_SHOT;
    if (icmp_hdr->type != ICMP_DEST_UNREACH ||
        icmp_hdr->code != ICMP_FRAG_NEEDED)
        return TC_ACT_OK;
    off += sizeof(struct icmphdr);
    iph = bpf_dynptr_slice(skb_ptr, off, buffer_ip, sizeof(buffer_ip));
    if (!iph || iph->ihl != 5)
        return TC_ACT_SHOT;
    pckt->proto = iph->protocol;
    pckt->flags |= F_ICMP;
    pckt->src = iph->daddr;
    pckt->dst = iph->saddr;
    return TC_ACT_UNSPEC;
}

static __noinline bool parse_udp(struct bpf_dynptr *skb_ptr, __u64 off,
                struct packet_description *pckt)
{
    __u8 buffer[sizeof(struct udphdr)] = {};
    struct udphdr *udp;

    udp = bpf_dynptr_slice(skb_ptr, off, buffer, sizeof(buffer));
    if (!udp)
        return false;

    if (!(pckt->flags & F_ICMP)) {
        pckt->port16[0] = udp->source;
        pckt->port16[1] = udp->dest;
    } else {
        pckt->port16[0] = udp->dest;
        pckt->port16[1] = udp->source;
    }
    return true;
}

static __noinline bool parse_tcp(struct bpf_dynptr *skb_ptr, __u64 off,
                struct packet_description *pckt)
{
    __u8 buffer[sizeof(struct tcphdr)] = {};
    struct tcphdr *tcp;

    tcp = bpf_dynptr_slice(skb_ptr, off, buffer, sizeof(buffer));
    if (!tcp)
        return false;

    if (tcp->syn)
        pckt->flags |= F_SYN_SET;

    if (!(pckt->flags & F_ICMP)) {
        pckt->port16[0] = tcp->source;
        pckt->port16[1] = tcp->dest;
    } else {
        pckt->port16[0] = tcp->dest;
        pckt->port16[1] = tcp->source;
    }
    return true;
}

static __noinline int process_packet(struct bpf_dynptr *skb_ptr,
                    struct eth_hdr *eth, __u64 off,
                    bool is_ipv6, struct __sk_buff *skb)
{
    struct packet_description pckt = {};
    struct bpf_tunnel_key tkey = {};
    struct vip_stats *data_stats;
    struct real_definition *dst;
    struct vip_meta *vip_info;
    struct ctl_value *cval;
    __u32 v4_intf_pos = 1;
    __u32 v6_intf_pos = 2;
    struct ipv6hdr *ip6h;
    struct vip vip = {};
    struct iphdr *iph;
    int tun_flag = 0;
    __u16 pkt_bytes;
    __u64 iph_len;
    __u32 ifindex;
    __u8 protocol;
    __u32 vip_num;
    int action;

    tkey.tunnel_ttl = 64;
    if (is_ipv6) {
        __u8 buffer[sizeof(struct ipv6hdr)] = {};

        ip6h = bpf_dynptr_slice(skb_ptr, off, buffer, sizeof(buffer));
        if (!ip6h)
            return TC_ACT_SHOT;

        iph_len = sizeof(struct ipv6hdr);
        protocol = ip6h->nexthdr;
        pckt.proto = protocol;
        pkt_bytes = bpf_ntohs(ip6h->payload_len);
        off += iph_len;
        if (protocol == IPPROTO_FRAGMENT) {
            return TC_ACT_SHOT;
        } else if (protocol == IPPROTO_ICMPV6) {
            action = parse_icmpv6(skb_ptr, off, &pckt);
            if (action >= 0)
                return action;
            off += IPV6_PLUS_ICMP_HDR;
        } else {
            memcpy(pckt.srcv6, ip6h->saddr.s6_addr32, 16);
            memcpy(pckt.dstv6, ip6h->daddr.s6_addr32, 16);
        }
    } else {
        __u8 buffer[sizeof(struct iphdr)] = {};

        iph = bpf_dynptr_slice(skb_ptr, off, buffer, sizeof(buffer));
        if (!iph || iph->ihl != 5)
            return TC_ACT_SHOT;

        protocol = iph->protocol;
        pckt.proto = protocol;
        pkt_bytes = bpf_ntohs(iph->tot_len);
        off += IPV4_HDR_LEN_NO_OPT;

        if (iph->frag_off & PCKT_FRAGMENTED)
            return TC_ACT_SHOT;
        if (protocol == IPPROTO_ICMP) {
            action = parse_icmp(skb_ptr, off, &pckt);
            if (action >= 0)
                return action;
            off += IPV4_PLUS_ICMP_HDR;
        } else {
            pckt.src = iph->saddr;
            pckt.dst = iph->daddr;
        }
    }
    protocol = pckt.proto;

    if (protocol == IPPROTO_TCP) {
        if (!parse_tcp(skb_ptr, off, &pckt))
            return TC_ACT_SHOT;
    } else if (protocol == IPPROTO_UDP) {
        if (!parse_udp(skb_ptr, off, &pckt))
            return TC_ACT_SHOT;
    } else {
        return TC_ACT_SHOT;
    }

    if (is_ipv6)
        memcpy(vip.daddr.v6, pckt.dstv6, 16);
    else
        vip.daddr.v4 = pckt.dst;

    vip.dport = pckt.port16[1];
    vip.protocol = pckt.proto;
    vip_info = bpf_map_lookup_elem(&vip_map, &vip);
    if (!vip_info) {
        vip.dport = 0;
        vip_info = bpf_map_lookup_elem(&vip_map, &vip);
        if (!vip_info)
            return TC_ACT_SHOT;
        pckt.port16[1] = 0;
    }

    if (vip_info->flags & F_HASH_NO_SRC_PORT)
        pckt.port16[0] = 0;

    if (!get_packet_dst(&dst, &pckt, vip_info, is_ipv6))
        return TC_ACT_SHOT;

    if (dst->flags & F_IPV6) {
        cval = bpf_map_lookup_elem(&ctl_array, &v6_intf_pos);
        if (!cval)
            return TC_ACT_SHOT;
        ifindex = cval->ifindex;
        memcpy(tkey.remote_ipv6, dst->dstv6, 16);
        tun_flag = BPF_F_TUNINFO_IPV6;
    } else {
        cval = bpf_map_lookup_elem(&ctl_array, &v4_intf_pos);
        if (!cval)
            return TC_ACT_SHOT;
        ifindex = cval->ifindex;
        tkey.remote_ipv4 = dst->dst;
    }
    vip_num = vip_info->vip_num;
    data_stats = bpf_map_lookup_elem(&stats, &vip_num);
    if (!data_stats)
        return TC_ACT_SHOT;
    data_stats->pkts++;
    data_stats->bytes += pkt_bytes;
    bpf_skb_set_tunnel_key(skb, &tkey, sizeof(tkey), tun_flag);
    *(u32 *)eth->eth_dest = tkey.remote_ipv4;
    return bpf_redirect(ifindex, 0);
}

SEC("tc")
int balancer_ingress(struct __sk_buff *ctx)
{
    __u8 buffer[sizeof(struct eth_hdr)] = {};
    struct bpf_dynptr ptr;
    struct eth_hdr *eth;
    __u32 eth_proto;
    __u32 nh_off;
    int err;

    nh_off = sizeof(struct eth_hdr);

    bpf_dynptr_from_skb(ctx, 0, &ptr);
    eth = bpf_dynptr_slice_rdwr(&ptr, 0, buffer, sizeof(buffer));
    if (!eth)
        return TC_ACT_SHOT;
    eth_proto = eth->eth_proto;
    if (eth_proto == bpf_htons(ETH_P_IP))
        err = process_packet(&ptr, eth, nh_off, false, ctx);
    else if (eth_proto == bpf_htons(ETH_P_IPV6))
        err = process_packet(&ptr, eth, nh_off, true, ctx);
    else
        return TC_ACT_SHOT;

    if (eth == buffer)
        bpf_dynptr_write(&ptr, 0, buffer, sizeof(buffer), 0);

    return err;
}

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