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`bpftrace` demo


In this post, we will use bpftrace to showcase the capabilities of eBPF.

Prerequisites

When We All Fall Asleep…

Let’s start with a basic bpftrace command:

bpftrace -e 'kprobe:do_nanosleep { printf("%d is falling asleep\n", pid); }'

When you run this command, you will see the following output:

Attaching 1 probe...
209 is falling asleep
209 is falling asleep
209 is falling asleep
451 is falling asleep
209 is falling asleep
451 is falling asleep
^C

What is happening here? The bpftrace script follows the syntax probe { action }. In this case, the command triggers a printf action (similar to C) whenever the kprobe:do_nanosleep event occurs.

There are several kinds of probes available. For example, kprobe:do_sys_open can be used to trace when the system call open is executed. A complete list of probes can be found in the reference guide.

Tracing User Probes

In addition to kernel functions, bpftrace can also trace user functions. Let’s write a simple C program and compile it to /root/a.out.

#include <stdlib.h>
void increment(int *p) { (*p)++; }
int main() {
	int v = 0;
	for (int i = 0; i < 10; i++)
		increment(&v);
	return 0;
}

Next, let’s set up a bpftrace command to trace the increment function:

bpftrace -e 'uprobe:/root/a.out:increment { printf("increment %p\n", arg0); }'

Now, run the program in another terminal window. You should see output similar to the following in the terminal running bpftrace:

Attaching 1 probe...
increment 0x7ffc6c4b3658
increment 0x7ffc6c4b3658
increment 0x7ffc6c4b3658
increment 0x7ffc6c4b3658
increment 0x7ffc6c4b3658
increment 0x7ffc6c4b3658
increment 0x7ffc6c4b3658
increment 0x7ffc6c4b3658
increment 0x7ffc6c4b3658
increment 0x7ffc6c4b3658
^C

Conclusion

In this post, we have shown some basic usage of bpftrace to trace eBPF events. It is worth noting that tracing is just a subset of all the functions that eBPF can perform.