Helper function bpf_sk_assign
This helper function is used to direct incoming traffic to a specific socket, overruling the normal socket selection logic.
Definition
Copyright (c) 2015 The Libbpf Authors. All rights reserved.
static long (* const bpf_sk_assign)(void *ctx, void *sk, __u64 flags) = (void *) 124;
Return value in BPF_PROG_TYPE_SCHED_CLS and BPF_PROG_TYPE_SCHED_ACT
0 on success, or a negative error in case of failure:
-EINVALif specifiedflagsare not supported.-ENOENTif the socket is unavailable for assignment.-ENETUNREACHif the socket is unreachable (wrong netns).-EOPNOTSUPPif the operation is not supported, for example a call from outside of TC ingress.-ESOCKTNOSUPPORTif the socket type is not supported (reuseport).
Return value in BPF_PROG_TYPE_SK_LOOKUP returns
0 on success, or a negative errno in case of failure.
-EAFNOSUPPORTif socket family (sk->family) is not compatible with packet family (ctx->family).-EEXISTif socket has been already selected, potentially by another program, andBPF_SK_LOOKUP_F_REPLACEflag was not specified.-EINVALif unsupported flags were specified.-EPROTOTYPEif socket L4 protocol (sk->protocol) doesn't match packet protocol (ctx->protocol).-ESOCKTNOSUPPORTif socket is not in allowed state (TCP listening or UDP unconnected).
Usage
This helper is overloaded and has slightly different semantics in different program types.
In BPF_PROG_TYPE_SCHED_CLS and BPF_PROG_TYPE_SCHED_ACT programs
This helper assign the sk to the skb. When combined with appropriate routing configuration to receive the packet towards the socket, will cause skb to be delivered to the specified socket. Subsequent redirection of skb via bpf_redirect, bpf_clone_redirect or other methods outside of BPF may interfere with successful delivery to the socket.
This operation is only valid from TC ingress path.
The flags argument must be zero.
In BPF_PROG_TYPE_SK_LOOKUP programs
Select the sk as a result of a socket lookup.
For the operation to succeed passed socket must be compatible with the packet description provided by the ctx object.
L4 protocol (IPPROTO_TCP or IPPROTO_UDP) must be an exact match. While IP family (AF_INET or AF_INET6) must be compatible, that is IPv6 sockets that are not v6-only can be selected for IPv4 packets.
Only TCP listeners and UDP unconnected sockets can be selected. sk can also be NULL to reset any previous selection.
flags argument can combination of following values:
BPF_SK_LOOKUP_F_REPLACE- to override the previous socket selection, potentially done by a BPF program that ran before us.BPF_SK_LOOKUP_F_NO_REUSEPORT- to skip load-balancing within reuseport group for the socket being selected.
On success ctx->sk will point to the selected socket.
Program types
This helper call can be used in the following program types:
Examples
Socket lookup program example usage
// Copyright (c) 2020 Cloudflare
struct {
__uint(type, BPF_MAP_TYPE_SOCKMAP);
__uint(max_entries, 32);
__type(key, __u32);
__type(value, __u64);
} redir_map SEC(".maps");
static const __u16 DST_PORT = 7007; /* Host byte order */
static const __u32 DST_IP4 = IP4(127, 0, 0, 1);
static const __u32 KEY_SERVER_A = 0;
/* Redirect packets destined for DST_IP4 address to socket at redir_map[0]. */
SEC("sk_lookup")
int redir_ip4(struct bpf_sk_lookup *ctx)
{
struct bpf_sock *sk;
int err;
if (ctx->family != AF_INET)
return SK_PASS;
if (ctx->local_port != DST_PORT)
return SK_PASS;
if (ctx->local_ip4 != DST_IP4)
return SK_PASS;
sk = bpf_map_lookup_elem(&redir_map, &KEY_SERVER_A);
if (!sk)
return SK_PASS;
err = bpf_sk_assign(ctx, sk, 0);
bpf_sk_release(sk);
return err ? SK_DROP : SK_PASS;
}
Traffic control classifier example usage
// SPDX-License-Identifier: GPL-2.0
// Copyright (c) 2019 Cloudflare Ltd.
// Copyright (c) 2020 Isovalent, Inc.
#include <stddef.h>
#include <stdbool.h>
#include <string.h>
#include <linux/bpf.h>
#include <linux/if_ether.h>
#include <linux/in.h>
#include <linux/ip.h>
#include <linux/ipv6.h>
#include <linux/pkt_cls.h>
#include <linux/tcp.h>
#include <sys/socket.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>
/* Pin map under /sys/fs/bpf/tc/globals/<map name> */
#define PIN_GLOBAL_NS 2
/* Must match struct bpf_elf_map layout from iproute2 */
struct {
__u32 type;
__u32 size_key;
__u32 size_value;
__u32 max_elem;
__u32 flags;
__u32 id;
__u32 pinning;
} server_map SEC("maps") = {
.type = BPF_MAP_TYPE_SOCKMAP,
.size_key = sizeof(int),
.size_value = sizeof(__u64),
.max_elem = 1,
.pinning = PIN_GLOBAL_NS,
};
char _license[] SEC("license") = "GPL";
/* Fill 'tuple' with L3 info, and attempt to find L4. On fail, return NULL. */
static inline struct bpf_sock_tuple *
get_tuple(struct __sk_buff *skb, bool *ipv4, bool *tcp)
{
void *data_end = (void *)(long)skb->data_end;
void *data = (void *)(long)skb->data;
struct bpf_sock_tuple *result;
struct ethhdr *eth;
__u64 tuple_len;
__u8 proto = 0;
__u64 ihl_len;
eth = (struct ethhdr *)(data);
if (eth + 1 > data_end)
return NULL;
if (eth->h_proto == bpf_htons(ETH_P_IP)) {
struct iphdr *iph = (struct iphdr *)(data + sizeof(*eth));
if (iph + 1 > data_end)
return NULL;
if (iph->ihl != 5)
/* Options are not supported */
return NULL;
ihl_len = iph->ihl * 4;
proto = iph->protocol;
*ipv4 = true;
result = (struct bpf_sock_tuple *)&iph->saddr;
} else if (eth->h_proto == bpf_htons(ETH_P_IPV6)) {
struct ipv6hdr *ip6h = (struct ipv6hdr *)(data + sizeof(*eth));
if (ip6h + 1 > data_end)
return NULL;
ihl_len = sizeof(*ip6h);
proto = ip6h->nexthdr;
*ipv4 = false;
result = (struct bpf_sock_tuple *)&ip6h->saddr;
} else {
return (struct bpf_sock_tuple *)data;
}
if (proto != IPPROTO_TCP && proto != IPPROTO_UDP)
return NULL;
*tcp = (proto == IPPROTO_TCP);
return result;
}
static inline int
handle_udp(struct __sk_buff *skb, struct bpf_sock_tuple *tuple, bool ipv4)
{
struct bpf_sock *sk;
const int zero = 0;
size_t tuple_len;
__be16 dport;
int ret;
tuple_len = ipv4 ? sizeof(tuple->ipv4) : sizeof(tuple->ipv6);
if ((void *)tuple + tuple_len > (void *)(long)skb->data_end)
return TC_ACT_SHOT;
sk = bpf_sk_lookup_udp(skb, tuple, tuple_len, BPF_F_CURRENT_NETNS, 0);
if (sk)
goto assign;
dport = ipv4 ? tuple->ipv4.dport : tuple->ipv6.dport;
if (dport != bpf_htons(4321))
return TC_ACT_OK;
sk = bpf_map_lookup_elem(&server_map, &zero);
if (!sk)
return TC_ACT_SHOT;
assign:
ret = bpf_sk_assign(skb, sk, 0);
bpf_sk_release(sk);
return ret;
}
static inline int
handle_tcp(struct __sk_buff *skb, struct bpf_sock_tuple *tuple, bool ipv4)
{
struct bpf_sock *sk;
const int zero = 0;
size_t tuple_len;
__be16 dport;
int ret;
tuple_len = ipv4 ? sizeof(tuple->ipv4) : sizeof(tuple->ipv6);
if ((void *)tuple + tuple_len > (void *)(long)skb->data_end)
return TC_ACT_SHOT;
sk = bpf_skc_lookup_tcp(skb, tuple, tuple_len, BPF_F_CURRENT_NETNS, 0);
if (sk) {
if (sk->state != BPF_TCP_LISTEN)
goto assign;
bpf_sk_release(sk);
}
dport = ipv4 ? tuple->ipv4.dport : tuple->ipv6.dport;
if (dport != bpf_htons(4321))
return TC_ACT_OK;
sk = bpf_map_lookup_elem(&server_map, &zero);
if (!sk)
return TC_ACT_SHOT;
if (sk->state != BPF_TCP_LISTEN) {
bpf_sk_release(sk);
return TC_ACT_SHOT;
}
assign:
ret = bpf_sk_assign(skb, sk, 0);
bpf_sk_release(sk);
return ret;
}
SEC("tc")
int bpf_sk_assign_test(struct __sk_buff *skb)
{
struct bpf_sock_tuple *tuple;
bool ipv4 = false;
bool tcp = false;
int tuple_len;
int ret = 0;
tuple = get_tuple(skb, &ipv4, &tcp);
if (!tuple)
return TC_ACT_SHOT;
/* Note that the verifier socket return type for bpf_skc_lookup_tcp()
* differs from bpf_sk_lookup_udp(), so even though the C-level type is
* the same here, if we try to share the implementations they will
* fail to verify because we're crossing pointer types.
*/
if (tcp)
ret = handle_tcp(skb, tuple, ipv4);
else
ret = handle_udp(skb, tuple, ipv4);
return ret == 0 ? TC_ACT_OK : TC_ACT_SHOT;
}