网络排障工具 PWRU

pwru是 cilium 推出的基于 eBPF 开发的网络数据包排查工具，它提供了更细粒度的网络数据包排查方案。本文将介绍pwru的使用方法和经典场景，并介绍其实现原理。

一、安装部署

1.部署要求

pwru 要求内核代码在 5.5 版本之上，--output-skb 要求内核版本在 5.9 之上，并且要求内核开启以下配置：

2.使用方法

Usage of ./pwru:
      --filter-dst-ip string        filter destination IP addr
      --filter-dst-port uint16      filter destination port
      --filter-func string          filter kernel functions to be probed by name (exact match, supports RE2 regular expression)
      --filter-mark uint32          filter skb mark
      --filter-netns uint32         filter netns inode
      --filter-proto string         filter L4 protocol (tcp, udp, icmp)
      --filter-src-ip string        filter source IP addr
      --filter-src-port uint16      filter source port
      --output-limit-lines uint     exit the program after the number of events has been received/printed
      --output-meta                 print skb metadata
      --output-relative-timestamp   print relative timestamp per skb
      --output-skb                  print skb
      --output-stack                print stack
      --output-tuple                print L4 tuple

二、案例演示

下图案例演示了pwru展现出快速定位出数据包被 iptables 规则 drop 掉的原因：

在不设置 iptables 规则之前：

添加了 iptables 规则之后

iptables -t filter -I OUTPUT 1 -m tcp --proto tcp --dst 1.1.1.1/32 -j DROP

可以看到在 nf_hook_slow 函数后发生了变化：

我们可以看到数据包在&n版权声明：本文遵循 CC 4.0 BY-SA 版权协议，若要转载请务必附上原文出处链接及本声明，谢谢合作！bsp;nf_hook_slow 判决为 NF_DROP，调用了 kfree_skb

int nf_hook_slow(struct sk_buff *skb, struct nf_hook_state *state,
     const struct nf_hook_entries *e, unsigned int s)
{
  unsigned int verdict;
  int ret;

  for (; s < e->num_hook_entries; s++) {
    verdict = nf_hook_entry_hookfn(&e->hooks[s], skb, state);
    switch (verdict & NF_VERDICT_MASK) {
    case NF_ACCEPT:
      break;
    case NF_DROP:
      kfree_skb(skb);
      ret = NF_DROP_GETERR(verdict);
      if (ret == 0)
        ret = -EPERM;
      return ret;
    case NF_QUEUE:
      ret = nf_queue(skb, state, s, verdict);
      if (ret == 1)
        continue;
      return ret;
    default:
      /* Implicit handling for NF_STOLEN, as well as any other
       * non conventional verdicts.
       */
      return 0;
    }
  }

  return 1;
}

三、原理实现

pwru 本质上是向 kprobe 注册了一些 eBPF code，根据 pwru版权声明：本文遵循 CC 4.0 BY-SA 版权协议，若要转载请务必附上原文出处链接及本声明，谢谢合作！ 传入的参数可以更新 eBPF Map，改变限制条件，从而更新输出。

比如在 FilterCfg 里面制定了过滤的 IP 地址和协议等条件

type FilterCfg struct {
	FilterMark uint32

	//Filter l3
	FilterIPv6  uint8
	FilterSrcIP [16]byte
	FilterDstIP [16]byte

	//Filter l4
	FilterProto   uint8
	FilterSrcPort uint16
	FilterDstPort uint16

	//TODO: if there are more options later, then you can consider using a bit map
	OutputRelativeTS uint8
	OutputMeta       uint8
	OutputTuple      uint8
	OutputSkb        uint8
	OutputStack      uint8

	Pad byte
}

会根据 pwru 传入的参数更新这个 eBPF Map

func ConfigBPFMap(flags *Flags, cfgMap *ebpf.Map) {
	cfg := FilterCfg{
		FilterMark: flags.FilterMark,
	}

	if flags.FilterSrcPort > 0 {
		cfg.FilterSrcPort = byteorder.HostToNetwork16(flags.FilterSrcPort)
	}
	if flags.FilterDstPort > 0 {
		cfg.FilterDstPort = byteorder.HostToNetwork16(flags.FilterDstPort)
	}


	switch strings.ToLower(flags.FilterProto) {
	case "tcp":
		cfg.FilterProto = syscall.IPPROTO_TCP
	case "udp":
		cfg.FilterProto = syscall.IPPROTO_UDP
	case "icmp":
		cfg.FilterProto = syscall.IPPROTO_ICMP
	case "icmp6":
		cfg.FilterProto = syscall.IPPROTO_ICMPV6
	}

  // ... 
  
	if err := cfgMap.Update(uint32(0), cfg, 0); err != nil {
		log.Fatalf("Failed to set filter map: %v", err)
	}
}

在 eBPF code 中，可以看到会读取配置 bpf_map_lookup_elem，然后进而执行真正的 filter：

struct config {
	u32 mark;
	u8 ipv6;
	union addr saddr;
	union addr daddr;
	u8 l4_proto;
	u16 sport;
	u16 dport;
	u8 output_timestamp;
	u8 output_meta;
	u8 output_tuple;
	u8 output_skb;
	u8 output_stack;
	u8 pad;
} __attribute__((packed));

static __always_inline int
handle_everything(struct sk_buff *skb, struct pt_regs *ctx) {
	struct event_t event = {};

	u32 index = 0;
	struct config *cfg = bpf_map_lookup_elem(&cfg_map, &index);

	if (cfg) {
		if (!filter(skb, cfg))
			return 0;

		set_output(ctx, skb, &event, cfg);
	}

	event.pid = bpf_get_current_pid_tgid();
	event.addr = PT_REGS_IP(ctx);
	event.skb_addr = (u64) skb;
	event.ts = bpf_ktime_get_ns();
	bpf_perf_event_output(ctx, &events, BPF_F_CURRENT_CPU, &event, sizeof(event));

	return 0;
}

可以看到，这里通过 bpf_perf_event_output 将过滤结果以 Perf ev版权声明：本文遵循 CC 4.0 BY-SA 版权协议，若要转载请务必附上原文出处链接及本声明，谢谢合作！ent 传递上来。

rd, err := perf.NewReader(events, os.Getpagesize())
if err != nil {
	log.Fatalf("Creating perf event reader: %s", err)
}
defer rd.Close()

 // ...
 var event pwru.Event
for {
	record, err := rd.Read()
	if err != nil {
		if perf.IsClosed(err) {
			return
		}
		log.Printf("Reading from perf event reader: %s", err)
	}

	if record.LostSamples != 0 {
		log.Printf("Perf event ring buffer full, dropped %d samples", record.LostSamples)
		continue
	}

	if err := binary.Read(bytes.NewBuffer(record.RawSample), binary.LittleEndian, &event); err != nil {
		log.Printf("Parsing perf event: %s", err)
		continue
	}

	output.Print(&event)

	select {
	case <-ctx.Done():
		break
	default:
		continue
	}
}