⁠Chapter 5. Useful SystemTap Scripts

This chapter enumerates several SystemTap scripts you can use to monitor and investigate different subsystems. All of these scripts are available at /usr/share/systemtap/testsuite/systemtap.examples/ once you install the systemtap-testsuite RPM.

⁠5.1. Network

The following sections showcase scripts that trace network-related functions and build a profile of network activity.

⁠5.1.1. Network Profiling

This section describes how to profile network activity. nettop.stp provides a glimpse into how much network traffic each process is generating on a machine.

nettop.stp

#! /usr/bin/env stap

global ifxmit, ifrecv
global ifmerged

probe netdev.transmit
{
  ifxmit[pid(), dev_name, execname(), uid()] <<< length
  ifmerged[pid(), dev_name, execname(), uid()] <<< 1
}

probe netdev.receive
{
  ifrecv[pid(), dev_name, execname(), uid()] <<< length
  ifmerged[pid(), dev_name, execname(), uid()] <<< 1
}

function print_activity()
{
  printf("%5s %5s %-12s %7s %7s %7s %7s %-15s\n",
         "PID", "UID", "DEV", "XMIT_PK", "RECV_PK",
         "XMIT_KB", "RECV_KB", "COMMAND")

  foreach ([pid, dev, exec, uid] in ifmerged-) {
    n_xmit = @count(ifxmit[pid, dev, exec, uid])
    n_recv = @count(ifrecv[pid, dev, exec, uid])
    printf("%5d %5d %-12s %7d %7d %7d %7d %-15s\n",
           pid, uid, dev, n_xmit, n_recv,
           @sum(ifxmit[pid, dev, exec, uid])/1024,
           @sum(ifrecv[pid, dev, exec, uid])/1024,
           exec)
  }

  print("\n")

  delete ifxmit
  delete ifrecv
  delete ifmerged
}

probe timer.ms(5000), end, error
{
  print_activity()
}

Note that function print_activity() uses the following expressions:

n_xmit ? @sum(ifxmit[pid, dev, exec, uid])/1024 : 0
n_recv ? @sum(ifrecv[pid, dev, exec, uid])/1024 : 0

These expressions are if/else conditionals. The first statement is a more concise way of writing the following psuedo code:

if n_recv != 0 then
  @sum(ifrecv[pid, dev, exec, uid])/1024
else
  0

nettop.stp tracks which processes are generating network traffic on the system, and provides the following information about each process:

PID — the ID of the listed process.
UID — user ID. A user ID of 0 refers to the root user.
DEV — which ethernet device the process used to send / receive data (for example, eth0, eth1)
XMIT_PK — number of packets transmitted by the process
RECV_PK — number of packets received by the process
XMIT_KB — amount of data sent by the process, in kilobytes
RECV_KB — amount of data received by the service, in kilobytes

nettop.stp provides network profile sampling every 5 seconds. You can change this setting by editing probe timer.ms(5000) accordingly. Example 5.1, “nettop.stp Sample Output” contains an excerpt of the output from nettop.stp over a 20-second period:

⁠

Example 5.1. nettop.stp Sample Output

[...]
  PID   UID DEV     XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND
    0     0 eth0          0       5       0       0 swapper
11178     0 eth0          2       0       0       0 synergyc

  PID   UID DEV     XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND
 2886     4 eth0         79       0       5       0 cups-polld
11362     0 eth0          0      61       0       5 firefox
    0     0 eth0          3      32       0       3 swapper
 2886     4 lo            4       4       0       0 cups-polld
11178     0 eth0          3       0       0       0 synergyc

  PID   UID DEV     XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND
    0     0 eth0          0       6       0       0 swapper
 2886     4 lo            2       2       0       0 cups-polld
11178     0 eth0          3       0       0       0 synergyc
 3611     0 eth0          0       1       0       0 Xorg

  PID   UID DEV     XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND
    0     0 eth0          3      42       0       2 swapper
11178     0 eth0         43       1       3       0 synergyc
11362     0 eth0          0       7       0       0 firefox
 3897     0 eth0          0       1       0       0 multiload-apple
[...]

⁠5.1.2. Tracing Functions Called in Network Socket Code

This section describes how to trace functions called from the kernel's net/socket.c file. This task helps you identify, in finer detail, how each process interacts with the network at the kernel level.

socket-trace.stp

#! /usr/bin/env stap

probe kernel.function("*@net/socket.c").call {
  printf ("%s -> %s\n", thread_indent(1), ppfunc())
}
probe kernel.function("*@net/socket.c").return {
  printf ("%s <- %s\n", thread_indent(-1), ppfunc())
}

socket-trace.stp is identical to Example 3.6, “thread_indent.stp”, which was earlier used in SystemTap Functions to illustrate how thread_indent() works.

⁠

Example 5.2. socket-trace.stp Sample Output

[...]
0 Xorg(3611): -> sock_poll
3 Xorg(3611): <- sock_poll
0 Xorg(3611): -> sock_poll
3 Xorg(3611): <- sock_poll
0 gnome-terminal(11106): -> sock_poll
5 gnome-terminal(11106): <- sock_poll
0 scim-bridge(3883): -> sock_poll
3 scim-bridge(3883): <- sock_poll
0 scim-bridge(3883): -> sys_socketcall
4 scim-bridge(3883):  -> sys_recv
8 scim-bridge(3883):   -> sys_recvfrom
12 scim-bridge(3883):-> sock_from_file
16 scim-bridge(3883):<- sock_from_file
20 scim-bridge(3883):-> sock_recvmsg
24 scim-bridge(3883):<- sock_recvmsg
28 scim-bridge(3883):   <- sys_recvfrom
31 scim-bridge(3883):  <- sys_recv
35 scim-bridge(3883): <- sys_socketcall
[...]

Example 5.2, “socket-trace.stp Sample Output” contains a 3-second excerpt of the output for socket-trace.stp. For more information about the output of this script as provided by thread_indent(), refer to SystemTap Functions Example 3.6, “thread_indent.stp”.

⁠5.1.3. Monitoring Incoming TCP Connections

This section illustrates how to monitor incoming TCP connections. This task is useful in identifying any unauthorized, suspicious, or otherwise unwanted network access requests in real time.

tcp_connections.stp

#! /usr/bin/env stap

probe begin {
  printf("%6s %16s %6s %6s %16s\n",
         "UID", "CMD", "PID", "PORT", "IP_SOURCE")
}

probe kernel.{function("tcp_accept"),function("inet_csk_accept")}.return? {
  sock = $return
  if (sock != 0)
    printf("%6d %16s %6d %6d %16s\n", uid(), execname(), pid(),
           inet_get_local_port(sock), inet_get_ip_source(sock))
}

While tcp_connections.stp is running, it will print out the following information about any incoming TCP connections accepted by the system in real time:

Current UID
CMD - the command accepting the connection
PID of the command
Port used by the connection
IP address from which the TCP connection originated

⁠

Example 5.3. tcp_connections.stp Sample Output

UID            CMD    PID   PORT        IP_SOURCE
0             sshd   3165     22      10.64.0.227
0             sshd   3165     22      10.64.0.227

⁠5.1.4. Monitoring TCP Packets

This section illustrates how to monitor TCP packets received by the system. This is useful in analyzing network traffic generated by applications running on the system.

tcpdumplike.stp

	#! /usr/bin/env stap

// A TCP dump like example

probe begin, timer.s(1) {
  printf("-----------------------------------------------------------------\n")
  printf("       Source IP         Dest IP  SPort  DPort  U  A  P  R  S  F \n")
  printf("-----------------------------------------------------------------\n")
}

probe udp.recvmsg /* ,udp.sendmsg */ {
  printf(" %15s %15s  %5d  %5d  UDP\n",
         saddr, daddr, sport, dport)
}

probe tcp.receive {
  printf(" %15s %15s  %5d  %5d  %d  %d  %d  %d  %d  %d\n",
         saddr, daddr, sport, dport, urg, ack, psh, rst, syn, fin)
}

While tcpdumplike.stp is running, it will print out the following information about any received TCP packets in real time:

Source and destination IP address (saddr, daddr, respectively)
Source and destination ports (sport, dport, respectively)
Packet flags

To determine the flags used by the packet, tcpdumplike.stp uses the following functions:

urg - urgent
ack - acknowledgement
psh - push
rst - reset
syn - synchronize
fin - finished

The aforementioned functions return 1 or 0 to specify whether the packet uses the corresponding flag.

⁠

Example 5.4. tcpdumplike.stp Sample Output

-----------------------------------------------------------------
       Source IP         Dest IP  SPort  DPort  U  A  P  R  S  F
-----------------------------------------------------------------
  209.85.229.147       10.0.2.15     80  20373  0  1  1  0  0  0
  92.122.126.240       10.0.2.15     80  53214  0  1  0  0  1  0
  92.122.126.240       10.0.2.15     80  53214  0  1  0  0  0  0
  209.85.229.118       10.0.2.15     80  63433  0  1  0  0  1  0
  209.85.229.118       10.0.2.15     80  63433  0  1  0  0  0  0
  209.85.229.147       10.0.2.15     80  21141  0  1  1  0  0  0
  209.85.229.147       10.0.2.15     80  21141  0  1  1  0  0  0
  209.85.229.147       10.0.2.15     80  21141  0  1  1  0  0  0
  209.85.229.147       10.0.2.15     80  21141  0  1  1  0  0  0
  209.85.229.147       10.0.2.15     80  21141  0  1  1  0  0  0
  209.85.229.118       10.0.2.15     80  63433  0  1  1  0  0  0
[...]

⁠5.1.5. Monitoring Network Packets Drops in Kernel

The network stack in Linux can discard packets for various reasons. Some Linux kernels include a tracepoint, kernel.trace("kfree_skb"), which easily tracks where packets are discarded. dropwatch.stp uses kernel.trace("kfree_skb") to trace packet discards; the script summarizes which locations discard packets every five-second interval.

dropwatch.stp

#! /usr/bin/env stap

############################################################
# Dropwatch.stp
# Author: Neil Horman <nhorman@redhat.com>
# An example script to mimic the behavior of the dropwatch utility
# http://fedorahosted.org/dropwatch
############################################################

# Array to hold the list of drop points we find
global locations

# Note when we turn the monitor on and off
probe begin { printf("Monitoring for dropped packets\n") }
probe end { printf("Stopping dropped packet monitor\n") }

# increment a drop counter for every location we drop at
probe kernel.trace("kfree_skb") { locations[$location] <<< 1 }

# Every 5 seconds report our drop locations
probe timer.sec(5)
{
  printf("\n%s\n", ctime(gettimeofday_s()))
  foreach (l in locations-) {
    printf("%d packets dropped at %s\n",
           @count(locations[l]), symdata(l))
  }
  delete locations
}

The kernel.trace("kfree_skb") traces which places in the kernel drop network packets. The kernel.trace("kfree_skb") has two arguments: a pointer to the buffer being freed ($skb) and the location in kernel code the buffer is being freed ($location). The dropwatch.stp script provides the function containing $location where possible. The information to map $location back to the function is not in the instrumentation by default. On SystemTap 1.4 the --all-modules option will include the required mapping information and the following command can be used to run the script:

stap --all-modules dropwatch.stp

On older versions of SystemTap you can use the following command to emulate the --all-modules option:

stap -dkernel \
`cat /proc/modules | awk 'BEGIN { ORS = " " } {print "-d"$1}'` \
dropwatch.stp

Running the dropwatch.stp script 15 seconds would result in output similar in Example 5.5, “dropwatch.stp Sample Output”. The output lists the number of misses for each tracepoint location with either the function name or the address.

⁠

Example 5.5. dropwatch.stp Sample Output

Monitoring for dropped packets

Tue Nov 17 00:26:51 2020
1762 packets dropped at unix_stream_recvmsg
4 packets dropped at tun_do_read
2 packets dropped at nf_hook_slow

Tue Nov 17 00:26:56 2020
467 packets dropped at unix_stream_recvmsg
20 packets dropped at nf_hook_slow
6 packets dropped at tun_do_read

Tue Nov 17 00:27:01 2020
446 packets dropped at unix_stream_recvmsg
4 packets dropped at tun_do_read
4 packets dropped at nf_hook_slow
Stopping dropped packet monitor

When the script is being compiled on one machine and run on another the --all-modules and /proc/modules directory are not available; the symname function will just print out the raw address. To make the raw address of packet drops more meaningful, refer to the /boot/System.map-`uname -r` file. This file lists the starting addresses for each function, allowing you to map the addresses in the output of Example 5.5, “dropwatch.stp Sample Output” to a specific function name. Given the following snippet of the /boot/System.map-`uname -r` file, the address 0xffffffff8149a8ed maps to the function unix_stream_recvmsg:

[...]
ffffffff8149a420 t unix_dgram_poll
ffffffff8149a5e0 t unix_stream_recvmsg
ffffffff8149ad00 t unix_find_other
[...]