Get deeper insights into your system with eBPF

Keen Observer

© Lead Image © Ioannis Kounadeas,

© Lead Image © Ioannis Kounadeas,

Article from Issue 225/2019

Use the eBPF in-kernel virtual machine to identify resource bottlenecks and optimize your installation.

eBPF [1] is a relatively new addition to the Linux kernel that takes over more monitoring, security, and networking duties from individual kernel modules. Originally called the Berkeley Packet Filter, BPF came to life in 1992 [2] in order to provide a better and optimized mechanism to filter packets.

BPF was first used as an HTTP packet filter in BSD. Several decades later, it was completely rehashed and took on new tasks. The new version of BPF is what is known as enhanced BPF or eBPF. In addition to various new features, eBPF also has a new mechanism to connect to the Linux kernel. Instead of just redirecting packets, eBPF can attach itself to any kernel event or any socket. eBPF is tightly integrated with the Linux kernel and can be used as an efficient mechanism for Linux tracing. You can also use eBPF behind the scenes on your Linux machines to discover performance issues and bottlenecks.

Get Started

eBPF requires a kernel newer than v4.4 and one that has been compiled with the CONFIG_BPF_SYSCALL option. Neither of these requirements should be a problem if you are using one of the mainstream distributions like Ubuntu and updating it regularly.

To experience the tracing benefits of ePBF, install the tools from BPF Compiler Collection commonly referred to as bcc [3]. Fire up a terminal in an Ubuntu installation and type:

$ sudo apt install bpfcc-tools linux-headers-$(uname -r)

This command will fetch the tools as well as the kernel headers for the kernel version that you are currently using. The bcc includes over 70 tools, and under Ubuntu, they are all installed in the /usr/sbin directory and will have a -bpfcc extension (Figure 1). The tools are, in fact, Python scripts that you can edit and modify as per your requirements, provided that you know what you're doing. If you are using another distro, refer to the bcc documentation [4] for distribution-specific installation instructions.

Figure 1: Remember that all the eBPF utilities need root privileges to run.

Keep a Close Watch

I'll start by showing how to keep an eye out for new processes using the execsnoop tool. This tool is especially useful for tracing processes that are short-lived, which is to say those processes that end before you can track them via the traditional process monitoring tools like top. Keeping an eye out for these processes that usually miss your attention might help you optimize your installation.

Fire up a terminal and type:

$ sudo execsnoop-bpfcc

The tool will now keep an eye out for new processes. To give it something to pick up, perform some action (Figure 2), such as firing up a new terminal. This action will produce the output shown in Listing 1.

Listing 1

execsnoop Output


Figure 2: The execsnoop utility catches an incoming SSH connection.

As you can see from this truncated output, execsnoop prints one line of output for each new process. The output shows the parent process or command name under the PCOMM column, the PID of the process along with its parent PID, and the return value of the exec() function under the RET column, as well as the command and any arguments under the ARGS column.

The -x option can be used to include failed execution (Listing 2).

Listing 2

the execsnoop -x Option


You can similarly use the -t option to include a timestamp column and the -n option to match on a name. For instance, sudo execsnoop-bpcc -n ssh will only catch processes where the command matches the specified name (in this case, ssh).

A related tool is opensnoop, which enables you to trace file opens. The open() system call brings up files for read and write operations, and keeping an eye on this process can reveal a lot of details about how a program works behind the scenes. It can, for instance, help you identify all the data files, config files, and log files that are associated with an app and the manner in which they are accessed by the app. If an application performs poorly, it could be because it is improperly configured and is wasting milliseconds trying to access files that don't exist.

The opensnoop tool traces the open() system calls and prints a line for each call that's found, as shown in Listing 3.

Listing 3

Tracing open() Calls


The first column is the process ID of the process that invoked the open() system call. The second column, COMM, displays the name of the process, and the third column displays the file descriptor as it was returned by open(). Then comes the error value if any error code is returned by open(), and the final column specifies the full path of the file used in the open() system call.

On an active Linux installation, the output of opensnoop will be very difficult to follow. It'll be helpful if you filter the output with grep to make sure you catch hold of what you're looking for. Another good idea is to filter using the process ID of the process that is of interest to you with the -p option or the -n option to filter on process name, such as:

sudo opensnoop-bpfcc -n gnome-shell

Network Inspector

Another useful eBPF tool is tcpconnect, which enables you to inspect active TCP connections by watching all connect() system calls. The tcpconnect tool prints all TCP connections, along with their source and destination addresses (Listing 4).

Listing 4



The first and second column as usual list the process ID and the name of the process that is called connect(). The third column specifies whether you're using IPv4 or IPv6. The fourth and fifth columns are the source IP and destination IP for the connections, and the last column is the port number at the destination address.

One common option is -U, which appends the UID of the processes to the output. You can then use it along with the -u option to filter the output using UID, such as:

sudo tcpconnect-bpfcc -Uu 1000

Another tool that is useful for debugging TCP processes is tcpaccept (Figure 3), which traces the accept() system call. The overhead of tcpaccept is negligible, and although netstat can do the job of both tcpconnect and tcpaccept, the two eBPF tools are more versatile. You can even use the -p option with both the tools to watch specific processes, such as:

sudo tcpaccept-bpfcc -p 14786
Figure 3: Note that tcpaccept will only trace successful TCP accept() calls and will not list attempts to connect to closed ports.

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