目录

1. Loading via iproute2 ip

2 bpf程序绑定

3 bpf程序触发

---

经过上一节的内容，bpf程序和map已经加载到内核当中了。什么时候bpf程序才能发挥它的作用呢？
这就需要bpf的应用系统把其挂载到适当的钩子上，当钩子所在点的路径被执行，钩子被触发，BPF程序得以执行。

目前应用bpf的子系统分为两大类：

• tracing：kprobe、tracepoint、perf_event
• filter：sk_filter、sched_cls、sched_act、xdp、cg_skb

 接下来分析XDP类型的bpf程序的绑定和触发过程

1. Loading via iproute2 ip

It does seem overkill to write a C program to simply load and attach a specific BPF-program. However, we still include this in the tutorial since it will help you integrate BPF into other Open Source projects.

As an alternative to writing a new loader, the standard iproute2 tool also contains a BPF ELF loader. However, this loader is not based on libbpf, which unfortunately makes it incompatible when starting to use BPF maps.

The iproute2 loader can be used with the standard ip tool; so in this case you can actually load our ELF-file xdp_pass_kern.o (where we named our ELF section “xdp”) like this:

ip link set dev lo xdpgeneric obj xdp_pass_kern.o sec xdp

Listing the device via ip link show also shows the XDP info:

$ ip link show dev lo
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 xdpgeneric qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    prog/xdp id 220 tag 3b185187f1855c4c jited

Removing the XDP program again from the device:

ip link set dev lo xdpgeneric off

2 bpf程序绑定

使用的是netlink消息跟内核通信，把fd和dev信息以及flag发送到内核，netlink使用的是NETLINK_ROUTE消息ID

int bpf_set_link_xdp_fd(int ifindex, int fd, __u32 flags)
{
    int sock, seq = 0, ret;
    struct nlattr *nla, *nla_xdp;
    struct {
        struct nlmsghdr  nh;
        struct ifinfomsg ifinfo;
        char             attrbuf[64];
    } req;
    __u32 nl_pid;
    sock = libbpf_netlink_open(&nl_pid);
    if (sock < 0)
        return sock;
    memset(&req, 0, sizeof(req));
    req.nh.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg));
    req.nh.nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
    req.nh.nlmsg_type = RTM_SETLINK;
    req.nh.nlmsg_pid = 0;
    req.nh.nlmsg_seq = ++seq;
    req.ifinfo.ifi_family = AF_UNSPEC;
    req.ifinfo.ifi_index = ifindex;
    /* started nested attribute for XDP */
    nla = (struct nlattr *)(((char *)&req)
                + NLMSG_ALIGN(req.nh.nlmsg_len));
    nla->nla_type = NLA_F_NESTED | IFLA_XDP;
    nla->nla_len = NLA_HDRLEN;
    /* add XDP fd */
    nla_xdp = (struct nlattr *)((char *)nla + nla->nla_len);
    nla_xdp->nla_type = IFLA_XDP_FD;
    nla_xdp->nla_len = NLA_HDRLEN + sizeof(int);
    memcpy((char *)nla_xdp + NLA_HDRLEN, &fd, sizeof(fd));
    nla->nla_len += nla_xdp->nla_len;
    /* if user passed in any flags, add those too */
    if (flags) {
        nla_xdp = (struct nlattr *)((char *)nla + nla->nla_len);
        nla_xdp->nla_type = IFLA_XDP_FLAGS;
        nla_xdp->nla_len = NLA_HDRLEN + sizeof(flags);
        memcpy((char *)nla_xdp + NLA_HDRLEN, &flags, sizeof(flags));
        nla->nla_len += nla_xdp->nla_len;
    }
    req.nh.nlmsg_len += NLA_ALIGN(nla->nla_len);
    if (send(sock, &req, req.nh.nlmsg_len, 0) < 0) {
        ret = -errno;
        goto cleanup;
    }
    ret = bpf_netlink_recv(sock, nl_pid, seq, NULL, NULL, NULL);
cleanup:
    close(sock);
    return ret;
}
 
 
 
int libbpf_netlink_open(__u32 *nl_pid)
{
    struct sockaddr_nl sa;
    socklen_t addrlen;
    int one = 1, ret;
    int sock;
    memset(&sa, 0, sizeof(sa));
    sa.nl_family = AF_NETLINK;
    sock = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (sock < 0)
        return -errno;
    if (setsockopt(sock, SOL_NETLINK, NETLINK_EXT_ACK,
               &one, sizeof(one)) < 0) {
        fprintf(stderr, "Netlink error reporting not supported\n");
    }
    if (bind(sock, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
        ret = -errno;
        goto cleanup;
    }
    addrlen = sizeof(sa);
    if (getsockname(sock, (struct sockaddr *)&sa, &addrlen) < 0) {
        ret = -errno;
        goto cleanup;
    }
    if (addrlen != sizeof(sa)) {
        ret = -LIBBPF_ERRNO__INTERNAL;
        goto cleanup;
    }
    *nl_pid = sa.nl_pid;
    return sock;
cleanup:
    close(sock);
    return ret;
}

NETLINK_ROUTE消息在内核注册的处理函数：

rtnl_register(PF_UNSPEC, RTM_SETLINK, rtnl_setlink, NULL, 0);

rtnl_setlink--->do_setlink

static int do_setlink(const struct sk_buff *skb,
		      struct net_device *dev, struct ifinfomsg *ifm,
		      struct netlink_ext_ack *extack,
		      struct nlattr **tb, char *ifname, int status)
{
	const struct net_device_ops *ops = dev->netdev_ops;
	int err;

	err = validate_linkmsg(dev, tb);
	if (err < 0)
		return err;

	if (tb[IFLA_NET_NS_PID] || tb[IFLA_NET_NS_FD] || tb[IFLA_IF_NETNSID]) {
		struct net *net = rtnl_link_get_net_capable(skb, dev_net(dev),
							    tb, CAP_NET_ADMIN);
		if (IS_ERR(net)) {
			err = PTR_ERR(net);
			goto errout;
		}

		err = dev_change_net_namespace(dev, net, ifname);
		put_net(net);
		if (err)
			goto errout;
		status |= DO_SETLINK_MODIFIED;
	}

	if (tb[IFLA_MAP]) {
		struct rtnl_link_ifmap *u_map;
		struct ifmap k_map;

		if (!ops->ndo_set_config) {
			err = -EOPNOTSUPP;
			goto errout;
		}

		if (!netif_device_present(dev)) {
			err = -ENODEV;
			goto errout;
		}

		u_map = nla_data(tb[IFLA_MAP]);
		k_map.mem_start = (unsigned long) u_map->mem_start;
		k_map.mem_end = (unsigned long) u_map->mem_end;
		k_map.base_addr = (unsigned short) u_map->base_addr;
		k_map.irq = (unsigned char) u_map->irq;
		k_map.dma = (unsigned char) u_map->dma;
		k_map.port = (unsigned char) u_map->port;

		err = ops->ndo_set_config(dev, &k_map);
		if (err < 0)
			goto errout;

		status |= DO_SETLINK_NOTIFY;
	}

	if (tb[IFLA_ADDRESS]) {
		struct sockaddr *sa;
		int len;

		len = sizeof(sa_family_t) + max_t(size_t, dev->addr_len,
						  sizeof(*sa));
		sa = kmalloc(len, GFP_KERNEL);
		if (!sa) {
			err = -ENOMEM;
			goto errout;
		}
		sa->sa_family = dev->type;
		memcpy(sa->sa_data, nla_data(tb[IFLA_ADDRESS]),
		       dev->addr_len);
		err = dev_set_mac_address(dev, sa);
		kfree(sa);
		if (err)
			goto errout;
		status |= DO_SETLINK_MODIFIED;
	}

    .........................

	if (tb[IFLA_XDP]) {
		struct nlattr *xdp[IFLA_XDP_MAX + 1];
		u32 xdp_flags = 0;

		err = nla_parse_nested(xdp, IFLA_XDP_MAX, tb[IFLA_XDP],
				       ifla_xdp_policy, NULL);
		if (err < 0)
			goto errout;

		if (xdp[IFLA_XDP_ATTACHED] || xdp[IFLA_XDP_PROG_ID]) {
			err = -EINVAL;
			goto errout;
		}

		if (xdp[IFLA_XDP_FLAGS]) {
			xdp_flags = nla_get_u32(xdp[IFLA_XDP_FLAGS]);
			if (xdp_flags & ~XDP_FLAGS_MASK) {
				err = -EINVAL;
				goto errout;
			}
			if (hweight32(xdp_flags & XDP_FLAGS_MODES) > 1) {
				err = -EINVAL;
				goto errout;
			}
		}

		if (xdp[IFLA_XDP_FD]) {
			err = dev_change_xdp_fd(dev, extack,
						nla_get_s32(xdp[IFLA_XDP_FD]),
						xdp_flags);
			if (err)
				goto errout;
			status |= DO_SETLINK_NOTIFY;
		}
	}

    ...............
	}

	return err;
}

根据IFLA_XDP_FD 调用dev_change_xdp_fd

int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
              int fd, u32 flags)
{
    const struct net_device_ops *ops = dev->netdev_ops;
    enum bpf_netdev_command query;
    struct bpf_prog *prog = NULL;
    bpf_op_t bpf_op, bpf_chk;
    int err;
    ASSERT_RTNL();
    query = flags & XDP_FLAGS_HW_MODE ? XDP_QUERY_PROG_HW : XDP_QUERY_PROG;
    bpf_op = bpf_chk = ops->ndo_bpf;
 
    /* xdp 有三中模式 1.generic XDP 2.native XDP 3.xdpoffload 后面两种需要驱动或者硬件支持 */
    if (!bpf_op && (flags & (XDP_FLAGS_DRV_MODE | XDP_FLAGS_HW_MODE)))
        return -EOPNOTSUPP;
    if (!bpf_op || (flags & XDP_FLAGS_SKB_MODE))
        bpf_op = generic_xdp_install;
    if (bpf_op == bpf_chk)
        bpf_chk = generic_xdp_install;
    if (fd >= 0) {
        if (__dev_xdp_query(dev, bpf_chk, XDP_QUERY_PROG) ||
            __dev_xdp_query(dev, bpf_chk, XDP_QUERY_PROG_HW))
            return -EEXIST;
        if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) &&
            __dev_xdp_query(dev, bpf_op, query))
            return -EBUSY;
        prog = bpf_prog_get_type_dev(fd, BPF_PROG_TYPE_XDP,
                         bpf_op == ops->ndo_bpf);
        if (IS_ERR(prog))
            return PTR_ERR(prog);
        if (!(flags & XDP_FLAGS_HW_MODE) &&
            bpf_prog_is_dev_bound(prog->aux)) {
            NL_SET_ERR_MSG(extack, "using device-bound program without HW_MODE flag is not supported");
            bpf_prog_put(prog);
            return -EINVAL;
        }
    }
    err = dev_xdp_install(dev, bpf_op, extack, flags, prog);
    if (err < 0 && prog)
        bpf_prog_put(prog);
    return err;
}
 
 
static int dev_xdp_install(struct net_device *dev, bpf_op_t bpf_op,
               struct netlink_ext_ack *extack, u32 flags,
               struct bpf_prog *prog)
{
    struct netdev_bpf xdp;
    memset(&xdp, 0, sizeof(xdp));
    if (flags & XDP_FLAGS_HW_MODE)
        xdp.command = XDP_SETUP_PROG_HW;
    else
        xdp.command = XDP_SETUP_PROG;
    xdp.extack = extack;
    xdp.flags = flags;
    xdp.prog = prog;
    return bpf_op(dev, &xdp);
}
 
 
static int generic_xdp_install(struct net_device *dev, struct netdev_bpf *xdp)
{
    struct bpf_prog *old = rtnl_dereference(dev->xdp_prog);
    struct bpf_prog *new = xdp->prog;
    int ret = 0;
    switch (xdp->command) {
    case XDP_SETUP_PROG:
        /* 修改dev的xdp_prog指针，指向新的prog */
        rcu_assign_pointer(dev->xdp_prog, new);
        if (old)
            bpf_prog_put(old);
        if (old && !new) {
            static_branch_dec(&generic_xdp_needed_key);
        } else if (new && !old) {
            static_branch_inc(&generic_xdp_needed_key);
            dev_disable_lro(dev);
            dev_disable_gro_hw(dev);
        }
        break;
    case XDP_QUERY_PROG:
        xdp->prog_id = old ? old->aux->id : 0;
        break;
    default:
        ret = -EINVAL;
        break;
    }
    return ret;
}

3 bpf程序触发

xdp 有三种工作模式，具体描述和应用限制如下：

 接下来只分析通用XDP的触发流程，__netif_receive_skb 中会调用会判断dev是否挂载了xdp_prog，如果挂载了xdp则会调用do_xdp_generic

static int __netif_receive_skb_core(struct sk_buff **pskb, bool pfmemalloc,
				    struct packet_type **ppt_prev)

{
    。。。。。。。。。。。。。。。。。。。。

    	if (static_branch_unlikely(&generic_xdp_needed_key)) {
		int ret2;

		preempt_disable();
		ret2 = do_xdp_generic(rcu_dereference(skb->dev->xdp_prog), skb);
		preempt_enable();

		if (ret2 != XDP_PASS) {
			ret = NET_RX_DROP;
			goto out;
		}
		skb_reset_mac_len(skb);
	}

    。。。。。。。。。。。。。。。。。。。。
}

int do_xdp_generic(struct bpf_prog *xdp_prog, struct sk_buff *skb)
{
	if (xdp_prog) {
		struct xdp_buff xdp;
		u32 act;
		int err;

		act = netif_receive_generic_xdp(skb, &xdp, xdp_prog);
		if (act != XDP_PASS) {
			switch (act) {
			case XDP_REDIRECT:
				err = xdp_do_generic_redirect(skb->dev, skb,
							      &xdp, xdp_prog);
				if (err)
					goto out_redir;
				break;
			case XDP_TX:
				generic_xdp_tx(skb, xdp_prog);
				break;
			}
			return XDP_DROP;
		}
	}
	return XDP_PASS;
out_redir:
	kfree_skb(skb);
	return XDP_DROP;
}

static u32 netif_receive_generic_xdp(struct sk_buff *skb,
				     struct xdp_buff *xdp,
				     struct bpf_prog *xdp_prog)
{
	struct netdev_rx_queue *rxqueue;
	void *orig_data, *orig_data_end;
	u32 metalen, act = XDP_DROP;
	__be16 orig_eth_type;
	struct ethhdr *eth;
	bool orig_bcast;
	int hlen, off;
	u32 mac_len;

	/* Reinjected packets coming from act_mirred or similar should
	 * not get XDP generic processing.
	 */
	if (skb_is_tc_redirected(skb))
		return XDP_PASS;

	/* XDP packets must be linear and must have sufficient headroom
	 * of XDP_PACKET_HEADROOM bytes. This is the guarantee that also
	 * native XDP provides, thus we need to do it here as well.
	 */
	if (skb_cloned(skb) || skb_is_nonlinear(skb) ||
	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
		int hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);
		int troom = skb->tail + skb->data_len - skb->end;

		/* In case we have to go down the path and also linearize,
		 * then lets do the pskb_expand_head() work just once here.
		 */
		if (pskb_expand_head(skb,
				     hroom > 0 ? ALIGN(hroom, NET_SKB_PAD) : 0,
				     troom > 0 ? troom + 128 : 0, GFP_ATOMIC))
			goto do_drop;
		if (skb_linearize(skb))
			goto do_drop;
	}

	/* The XDP program wants to see the packet starting at the MAC
	 * header.
	 */
	mac_len = skb->data - skb_mac_header(skb);
	hlen = skb_headlen(skb) + mac_len;
	xdp->data = skb->data - mac_len;
	xdp->data_meta = xdp->data;
	xdp->data_end = xdp->data + hlen;
	xdp->data_hard_start = skb->data - skb_headroom(skb);
	orig_data_end = xdp->data_end;
	orig_data = xdp->data;
	eth = (struct ethhdr *)xdp->data;
	orig_bcast = is_multicast_ether_addr_64bits(eth->h_dest);
	orig_eth_type = eth->h_proto;

	rxqueue = netif_get_rxqueue(skb);
	xdp->rxq = &rxqueue->xdp_rxq;

	act = bpf_prog_run_xdp(xdp_prog, xdp);

	/* check if bpf_xdp_adjust_head was used */
	off = xdp->data - orig_data;
	if (off) {
		if (off > 0)
			__skb_pull(skb, off);
		else if (off < 0)
			__skb_push(skb, -off);

		skb->mac_header += off;
		skb_reset_network_header(skb);
	}

	/* check if bpf_xdp_adjust_tail was used. it can only "shrink"
	 * pckt.
	 */
	off = orig_data_end - xdp->data_end;
	if (off != 0) {
		skb_set_tail_pointer(skb, xdp->data_end - xdp->data);
		skb->len -= off;

	}

	/* check if XDP changed eth hdr such SKB needs update */
	eth = (struct ethhdr *)xdp->data;
	if ((orig_eth_type != eth->h_proto) ||
	    (orig_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) {
		__skb_push(skb, ETH_HLEN);
		skb->protocol = eth_type_trans(skb, skb->dev);
	}

	switch (act) {
	case XDP_REDIRECT:
	case XDP_TX:
		__skb_push(skb, mac_len);
		break;
	case XDP_PASS:
		metalen = xdp->data - xdp->data_meta;
		if (metalen)
			skb_metadata_set(skb, metalen);
		break;
	default:
		bpf_warn_invalid_xdp_action(act);
		/* fall through */
	case XDP_ABORTED:
		trace_xdp_exception(skb->dev, xdp_prog, act);
		/* fall through */
	case XDP_DROP:
	do_drop:
		kfree_skb(skb);
		break;
	}

	return act;
}