Linux探测工具BCC(网络)

承接上文，本节以ICMP和TCP为例介绍与网络相关的部分内容。

Linux探测工具BCC(网络)
- Icmp的探测
- TCP的探测

Icmp的探测

首先看下促使我学习bcc的这篇文章中的程序traceicmpsoftirq.py，使用该程序的本意是找出对ping响应的进程位于哪个cpu core上，然后使用perf扫描该core，找出造成网络延迟的原因。源码如下：

#!/usr/bin/python
bpf_text = """
#include <linux/ptrace.h>
#include <linux/sched.h>        /* For TASK_COMM_LEN */
#include <linux/icmp.h>
#include <linux/netdevice.h>
struct probe_icmp_data_t
{
        u64 timestamp_ns;
        u32 tgid;
        u32 pid;
        char comm[TASK_COMM_LEN];
        int v0;
};
BPF_PERF_OUTPUT(probe_icmp_events);
static inline unsigned char *my_skb_transport_header(const struct sk_buff *skb)
{
    return skb->head + skb->transport_header;
}
static inline struct icmphdr *my_icmp_hdr(const struct sk_buff *skb)
{
    return (struct icmphdr *)my_skb_transport_header(skb);
}
int probe_icmp(struct pt_regs *ctx,struct sk_buff *skb)
{
        u64 __pid_tgid = bpf_get_current_pid_tgid();
        u32 __tgid = __pid_tgid >> 32;
        u32 __pid = __pid_tgid; // implicit cast to u32 for bottom half
        
        struct probe_icmp_data_t __data = {0};
        __data.timestamp_ns = bpf_ktime_get_ns();
        __data.tgid = __tgid;
        __data.pid = __pid;
        bpf_get_current_comm(&__data.comm,sizeof(__data.comm));
        __be16 seq;
        bpf_probe_read_kernel(&seq,sizeof(seq),&my_icmp_hdr(skb)->un.echo.sequence);
        __data.v0 = (int)seq;
        probe_icmp_events.perf_submit(ctx,&__data,sizeof(__data));
        return 0;
}
"""

from bcc import BPF
import ctypes as ct

class Data_icmp(ct.Structure):
    _fields_ = [
        ("timestamp_ns",ct.c_ulonglong),("tgid",ct.c_uint),("pid",("comm",ct.c_char * 16),# TASK_COMM_LEN
        ('v0',]

b = BPF(text=bpf_text)

def print_icmp_event(cpu,data,size):
    #event = b["probe_icmp_events"].event(data)
    event = ct.cast(data,ct.POINTER(Data_icmp)).contents
    print("%-7d %-7d %-15s %s" %
                      (event.tgid,event.pid,event.comm.decode('utf-8','replace'),event.v0))

b.attach_kprobe(event="icmp_echo",fn_name="probe_icmp")

b["probe_icmp_events"].open_perf_buffer(print_icmp_event)
while 1:
    try:
        b.kprobe_poll()
    except KeyboardInterrupt:
        exit()

上面程序对icmp_echo内核函数进行打点探测，当内核运行该函数时会执行自定义的函数probe_icmp，并获取当前的tgid，pid以及icmp报文的序列号。

内容如下：

my_skb_transport_header：该函数通过偏移sk_buff指针获取传输层首部地址，用于后续获取icmp首部的序列号。此处的操作可以直接参考static bool icmp_echo(struct sk_buff *skb)的内核源码，其获取icmp首部的方式依次为：
```
static inline struct icmphdr *icmp_hdr(const struct sk_buff *skb)
{
	return (struct icmphdr *)skb_transport_header(skb);
}

static inline unsigned char *skb_transport_header(const struct sk_buff *skb)
{
	return skb->head + skb->transport_header;
}
```
可以看到skb_transport_header的处理与本程序的方式是一样的，将该函数的实现直接移植过去即可。需要注意的是，不能直接调用内核函数skb_transport_header获取transport_header的地址。
bpf_get_current_pid_tgid()：获取当前的PID。需要注意的是该函数获取的是当前cpu上运行的进程ID，而不是某一个特定的进程ID。其内核源码如下：
```
BPF_CALL_1(bpf_get_current_ancestor_cgroup_id,int,ancestor_level)
{
	struct cgroup *cgrp = task_dfl_cgroup(current);
	struct cgroup *ancestor;

	ancestor = cgroup_ancestor(cgrp,ancestor_level);
	if (!ancestor)
		return 0;
	return cgroup_id(ancestor);
}
```
而current定义如下，用于获得当前执行进程的task_struct指针。更多参见这篇文章。
```
#define current get_current()
```
因此以本程序为例，如果对icmp_echo的打点采集中如果发生了上下文切换，可能bpf_get_current_pid_tgid获取到的可能是切换后的程序。本文也是借助这种机制，发现在切换到cadvisor导致了网络延时。
bpf_probe_read_kernel:读取内核结构体的成员，原文中使用的是bpf_probe_read，更多参见issue。

其余部分与检测可观测性相同。

TCP的探测

下面看一下TCP的探测，用于跟踪内核代码tcp_v4_connect或tcp_v6_connect，代码源自官方库tools/tcpconnect

#!/usr/bin/python

from __future__ import print_function
from bcc import BPF
from bcc.containers import filter_by_containers
from bcc.utils import printb
import argparse
from socket import inet_ntop,ntohs,AF_INET,AF_INET6
from struct import pack
from time import sleep

# arguments
examples = """examples:
    ./tcpconnect           # trace all TCP connect()s
    ./tcpconnect -t        # include timestamps
    ./tcpconnect -p 181    # only trace PID 181
    ./tcpconnect -P 80     # only trace port 80
    ./tcpconnect -P 80,81  # only trace port 80 and 81
    ./tcpconnect -U        # include UID
    ./tcpconnect -u 1000   # only trace UID 1000
    ./tcpconnect -c        # count connects per src ip and dest ip/port
    ./tcpconnect --cgroupmap mappath  # only trace cgroups in this BPF map
    ./tcpconnect --mntnsmap mappath   # only trace mount namespaces in the map
"""
parser = argparse.ArgumentParser(
    description="Trace TCP connects",formatter_class=argparse.RawDescriptionHelpFormatter,epilog=examples)
parser.add_argument("-t","--timestamp",action="store_true",help="include timestamp on output")
parser.add_argument("-p","--pid",help="trace this PID only")
parser.add_argument("-P","--port",help="comma-separated list of destination ports to trace.")
parser.add_argument("-U","--print-uid",help="include UID on output")
parser.add_argument("-u","--uid",help="trace this UID only")
parser.add_argument("-c","--count",help="count connects per src ip and dest ip/port")
parser.add_argument("--cgroupmap",help="trace cgroups in this BPF map only")
parser.add_argument("--mntnsmap",help="trace mount namespaces in this BPF map only")
parser.add_argument("--ebpf",help=argparse.SUPPRESS)
args = parser.parse_args() #解析入参
debug = 0

# define BPF program
bpf_text = """
#include <uapi/linux/ptrace.h>
#include <net/sock.h>
#include <bcc/proto.h>

BPF_HASH(currsock,u32,struct sock *); #创建保存socket指针的哈希

// separate data structs for ipv4 and ipv6
struct ipv4_data_t {
    u64 ts_us;
    u32 pid;
    u32 uid;
    u32 saddr;
    u32 daddr;
    u64 ip;
    u16 dport;
    char task[TASK_COMM_LEN];
};
BPF_PERF_OUTPUT(ipv4_events); //创建ipv4的输出

struct ipv6_data_t {
    u64 ts_us;
    u32 pid;
    u32 uid;
    unsigned __int128 saddr;
    unsigned __int128 daddr;
    u64 ip;
    u16 dport;
    char task[TASK_COMM_LEN];
};
BPF_PERF_OUTPUT(ipv6_events); //创建ipv6的输出

// separate flow keys per address family
struct ipv4_flow_key_t { //用于根据地址统计执行tcp_v4_connect的次数，即指定了"-c"或"--count"选项
    u32 saddr;
    u32 daddr;
    u16 dport;
};
BPF_HASH(ipv4_count,struct ipv4_flow_key_t); //统计执行tcp_v4_connect的次数

struct ipv6_flow_key_t { //用于根据地址统计执行tcp_v6_connect的次数，即指定了"-c"或"--count"选项
    unsigned __int128 saddr;
    unsigned __int128 daddr;
    u16 dport;
};
BPF_HASH(ipv6_count,struct ipv6_flow_key_t); //统计执行tcp_v6_connect的次数

int trace_connect_entry(struct pt_regs *ctx,struct sock *sk) //在进入tcp_v4_connect时调用
{
    if (container_should_be_filtered()) {
        return 0;
    }

    u64 pid_tgid = bpf_get_current_pid_tgid(); //获取64位的pid_tgid
    u32 pid = pid_tgid >> 32; //tgid位于高32位，右移32位获取
    u32 tid = pid_tgid;       //tid线程唯一
    FILTER_PID //bpf程序对python来说就是一段字符串，此处可以看作是一个标记符，后续使用python的string.replace进行替换。此处表示过滤特定的PID

    u32 uid = bpf_get_current_uid_gid();
    FILTER_UID //过滤特定的UID

    // stash the sock ptr for lookup on return
    currsock.update(&tid,&sk); //使用tid作为key，保存sk指针指向的地址

    return 0;
};

static int trace_connect_return(struct pt_regs *ctx,short ipver) //在从tcp_v4_connect返回时调用
{
    int ret = PT_REGS_RC(ctx); //获取tcp_v4_connect函数的返回值
    u64 pid_tgid = bpf_get_current_pid_tgid();
    u32 pid = pid_tgid >> 32;
    u32 tid = pid_tgid;

    struct sock **skpp;
    skpp = currsock.lookup(&tid); //判断当前线程在进入tcp_v4_connect时是否打点采集，即是否执行了上面的trace_connect_entry
    if (skpp == 0) {
        return 0;   // missed entry
    }

    if (ret != 0) { //如果tcp_v4_connect的返回值非0，表示无法发送SYNC报文
        // Failed to send SYNC packet,may not have populated
        // socket __sk_common.{skc_rcv_saddr,...}
        currsock.delete(&tid); //本次采集失败，删除哈希
        return 0;
    }

    // pull in details
    struct sock *skp = *skpp;
    u16 dport = skp->__sk_common.skc_dport;

    FILTER_PORT //过滤特定的端口

    if (ipver == 4) {
        IPV4_CODE //根据入参替换为IPV4的处理
    } else /* 6 */ {
        IPV6_CODE //根据入参替换为位IPV6的处理
    }

    currsock.delete(&tid);

    return 0;
}

int trace_connect_v4_return(struct pt_regs *ctx)
{
    return trace_connect_return(ctx,4);
}

int trace_connect_v6_return(struct pt_regs *ctx)
{
    return trace_connect_return(ctx,6);
}
"""

struct_init = { 'ipv4':
        { 'count' : #统计执行tcp_v4_connect的次数
               """
               struct ipv4_flow_key_t flow_key = {};
               flow_key.saddr = skp->__sk_common.skc_rcv_saddr;
               flow_key.daddr = skp->__sk_common.skc_daddr;
               flow_key.dport = ntohs(dport);
               ipv4_count.increment(flow_key);""",'trace' : #默认执行tcp_v4_connect的跟踪，记录地址，端口等信息
               """
               struct ipv4_data_t data4 = {.pid = pid,.ip = ipver};
               data4.uid = bpf_get_current_uid_gid();
               data4.ts_us = bpf_ktime_get_ns() / 1000;
               data4.saddr = skp->__sk_common.skc_rcv_saddr;
               data4.daddr = skp->__sk_common.skc_daddr;
               data4.dport = ntohs(dport);
               bpf_get_current_comm(&data4.task,sizeof(data4.task));
               ipv4_events.perf_submit(ctx,&data4,sizeof(data4));"""
               },'ipv6':
        { 'count' :#统计执行tcp_v6_connect的次数
               """
               struct ipv6_flow_key_t flow_key = {};
               bpf_probe_read_kernel(&flow_key.saddr,sizeof(flow_key.saddr),skp->__sk_common.skc_v6_rcv_saddr.in6_u.u6_addr32);
               bpf_probe_read_kernel(&flow_key.daddr,sizeof(flow_key.daddr),skp->__sk_common.skc_v6_daddr.in6_u.u6_addr32);
               flow_key.dport = ntohs(dport);
               ipv6_count.increment(flow_key);""",'trace' : #默认执行tcp_v6_connect的跟踪，记录地址，端口等信息
               """
               struct ipv6_data_t data6 = {.pid = pid,.ip = ipver};
               data6.uid = bpf_get_current_uid_gid();
               data6.ts_us = bpf_ktime_get_ns() / 1000;
               bpf_probe_read_kernel(&data6.saddr,sizeof(data6.saddr),skp->__sk_common.skc_v6_rcv_saddr.in6_u.u6_addr32);
               bpf_probe_read_kernel(&data6.daddr,sizeof(data6.daddr),skp->__sk_common.skc_v6_daddr.in6_u.u6_addr32);
               data6.dport = ntohs(dport);
               bpf_get_current_comm(&data6.task,sizeof(data6.task));
               ipv6_events.perf_submit(ctx,&data6,sizeof(data6));"""
               }
        }

# code substitutions
if args.count: #如果入参指定了"-c"或"-count",则执行count
    bpf_text = bpf_text.replace("IPV4_CODE",struct_init['ipv4']['count'])
    bpf_text = bpf_text.replace("IPV6_CODE",struct_init['ipv6']['count'])
else: #如果入参没有指定"-c"或"-count",则执行trace
    bpf_text = bpf_text.replace("IPV4_CODE",struct_init['ipv4']['trace'])
    bpf_text = bpf_text.replace("IPV6_CODE",struct_init['ipv6']['trace'])

if args.pid: #如果入参指定了"-p"或"--pid",则对PID进行过滤
    bpf_text = bpf_text.replace('FILTER_PID','if (pid != %s) { return 0; }' % args.pid)
if args.port:#如果入参指定了"-P"或"--port",则对端口进行过滤
    dports = [int(dport) for dport in args.port.split(',')]
    dports_if = ' && '.join(['dport != %d' % ntohs(dport) for dport in dports])
    bpf_text = bpf_text.replace('FILTER_PORT','if (%s) { currsock.delete(&pid); return 0; }' % dports_if)
if args.uid:#如果入参指定了"-u"或"--uid",则对UID进行过滤
    bpf_text = bpf_text.replace('FILTER_UID','if (uid != %s) { return 0; }' % args.uid)
bpf_text = filter_by_containers(args) + bpf_text

#下面的处理在没有指定特定的过滤时去除标记符
bpf_text = bpf_text.replace('FILTER_PID','')
bpf_text = bpf_text.replace('FILTER_PORT','')
bpf_text = bpf_text.replace('FILTER_UID','')

if debug or args.ebpf:
    print(bpf_text)
    if args.ebpf:
        exit()

# process event
def print_ipv4_event(cpu,size): #TCP4跟踪的打印函数
    event = b["ipv4_events"].event(data)
    global start_ts
    if args.timestamp:
        if start_ts == 0:
            start_ts = event.ts_us
        printb(b"%-9.3f" % ((float(event.ts_us) - start_ts) / 1000000),nl="")
    if args.print_uid:
        printb(b"%-6d" % event.uid,nl="")
    printb(b"%-6d %-12.12s %-2d %-16s %-16s %-4d" % (event.pid,event.task,event.ip,inet_ntop(AF_INET,pack("I",event.saddr)).encode(),#转换为主机序地址
        inet_ntop(AF_INET,event.daddr)).encode(),event.dport)) #转换为主机序地址和端口

def print_ipv6_event(cpu,size): #TCP6跟踪的打印函数
    event = b["ipv6_events"].event(data)
    global start_ts
    if args.timestamp:
        if start_ts == 0:
            start_ts = event.ts_us
        printb(b"%-9.3f" % ((float(event.ts_us) - start_ts) / 1000000),inet_ntop(AF_INET6,event.saddr).encode(),event.daddr).encode(),event.dport))

def depict_cnt(counts_tab,l3prot='ipv4'): #
    for k,v in sorted(counts_tab.items(),key=lambda counts: counts[1].value,reverse=True):
        depict_key = ""
        if l3prot == 'ipv4':
            depict_key = "%-25s %-25s %-20s" %  ((inet_ntop(AF_INET,pack('I',k.saddr))),k.daddr)),k.dport)
        else:
            depict_key = "%-25s %-25s %-20s" % ((inet_ntop(AF_INET6,k.saddr)),k.daddr),k.dport)

        print ("%s %-10d" % (depict_key,v.value))

# initialize BPF
b = BPF(text=bpf_text)
b.attach_kprobe(event="tcp_v4_connect",fn_name="trace_connect_entry")
b.attach_kprobe(event="tcp_v6_connect",fn_name="trace_connect_entry")
b.attach_kretprobe(event="tcp_v4_connect",fn_name="trace_connect_v4_return")
b.attach_kretprobe(event="tcp_v6_connect",fn_name="trace_connect_v6_return")

print("Tracing connect ... Hit Ctrl-C to end")
if args.count:
    try:
        while 1:
            sleep(99999999)
    except KeyboardInterrupt:
        pass

    # header
    print("\n%-25s %-25s %-20s %-10s" % (
        "LADDR","RADDR","RPORT","CONNECTS"))
    depict_cnt(b["ipv4_count"])
    depict_cnt(b["ipv6_count"],l3prot='ipv6')
# read events
else:
    # header
    if args.timestamp:
        print("%-9s" % ("TIME(s)"),end="")
    if args.print_uid:
        print("%-6s" % ("UID"),end="")
    print("%-6s %-12s %-2s %-16s %-16s %-4s" % ("PID","COMM","IP","SADDR","DADDR","DPORT"))

    start_ts = 0

    # read events
    b["ipv4_events"].open_perf_buffer(print_ipv4_event)
    b["ipv6_events"].open_perf_buffer(print_ipv6_event)
    while 1:
        try:
            b.perf_buffer_poll()
        except KeyboardInterrupt:
            exit()

上面C程序采集了内核数据skp->sk_common.skc_dport，skp->sk_common.skc_rcv_saddr和skp->__sk_common.skc_daddr。与第一个例子类似，这类数据可以直接参考tcp_v4_connect内核源码的实现，源码中通过struct inet_sock *inet = inet_sk(sk);来获取源目的地址和端口，inet_sock的结构体定义如下，可以明显看到inet_daddr，inet_rcv_saddr和inet_dport与上述代码获取的内容相同，进而可以了解到获取这些成员的方式。

struct inet_sock {
	/* sk and pinet6 has to be the first two members of inet_sock */
	struct sock		sk;
#if IS_ENABLED(CONFIG_IPV6)
	struct ipv6_pinfo	*pinet6;
#endif
	/* Socket demultiplex comparisons on incoming packets. */
#define inet_daddr		sk.__sk_common.skc_daddr
#define inet_rcv_saddr		sk.__sk_common.skc_rcv_saddr
#define inet_dport		sk.__sk_common.skc_dport
#define inet_num		sk.__sk_common.skc_num
...

此外在inet_sock结构体的注释中给出详细的说明，非常明了：

 * @inet_daddr - Foreign IPv4 addr
 * @inet_rcv_saddr - Bound local IPv4 addr
 * @inet_dport - Destination port
 * @inet_num - Local port

因此可以直接参考tcp_v4_connect的源码修改ipv4中获取地址和端口的实现，效果是一样的：

struct_init = { 'ipv4':
        { 'count' :
               """
               struct ipv4_flow_key_t flow_key = {};
               struct inet_sock *inet  = inet_sk(skp);
               flow_key.saddr = inet->inet_rcv_saddr;
               flow_key.daddr = inet->inet_daddr;
               u16 dport = inet->inet_dport;
               flow_key.dport = ntohs(dport);
               ipv4_count.increment(flow_key);""",'trace' :
               """
               struct ipv4_data_t data4 = {.pid = pid,.ip = ipver};
               data4.uid = bpf_get_current_uid_gid();
               data4.ts_us = bpf_ktime_get_ns() / 1000;
               struct inet_sock *inet  = inet_sk(skp);
               data4.saddr = inet->inet_rcv_saddr;
               data4.daddr = inet->inet_daddr;
               u16 dport = inet->inet_dport;
               data4.dport = ntohs(dport);
               bpf_get_current_comm(&data4.task,'ipv6':
        { 'count' :
               """
               struct ipv6_flow_key_t flow_key = {};
               bpf_probe_read_kernel(&flow_key.saddr,'trace' :
               """
               struct ipv6_data_t data6 = {.pid = pid,sizeof(data6));"""
               }
        }

此外注意到读取TCP4的数据时没有用到bpf_probe_read_kernel，但读取TCP6的数据时用到了bpf_probe_read_kernel，这是因为TCP4的地址是一个u32类型的数据，直接赋值即可；而TCP6的地址结构如下，无法通过直接赋值获取，因此需要调用bpf_probe_read_kernel拷贝内存。

struct in6_addr {
	union {
		__u8		u6_addr8[16];
#if __UAPI_DEF_IN6_ADDR_ALT
		__be16		u6_addr16[8];
		__be32		u6_addr32[4];
#endif
	} in6_u;
#define s6_addr			in6_u.u6_addr8
#if __UAPI_DEF_IN6_ADDR_ALT
#define s6_addr16		in6_u.u6_addr16
#define s6_addr32		in6_u.u6_addr32
#endif
};

整体看，上面代码使用了python处理了一些C程序的替换和拼接，大部分跟可观测性并没有什么不同，当然，最主要的还是需要了解内核处理流程，选择正确的内核函数进行打点。

上面给出的方式无法修改报文内容以及对报文进行重定向等操作。ebpf提供了XDP和tc两种管理网络的方式，更多可以参见下一篇博客。

Linux探测工具BCC(网络)