eBPF hello world

hello_map.py

#!/usr/bin/python
from bcc import BPF
from time import sleep

# This outputs a count of how many times the clone and execve syscalls have been made
# showing the use of an eBPF map (called syscall). 

program = """
 BPF_HASH(syscall);

 int kprobe__sys_clone(void *ctx) {
     u64 counter = 0;
     u64 key = 56;
     u64 *p; 

     p = syscall.lookup(&key);

     // The verifier will reject access to a pointer if you don't check that it's non-null first
     // Try commenting out the if test (and its closing brace) if you want to see the verifier do its thing
     if (p != 0) {
         counter = *p;
     }

     counter++;
     syscall.update(&key, &counter);

     return 0;
 }

int kprobe__sys_execve(void *ctx) {
     u64 counter = 0;
     u64 key = 59;
     u64 *p; 

     p = syscall.lookup(&key);
     if (p != 0) {
         counter = *p;
     }

     counter++;
     syscall.update(&key, &counter);

     return 0;
 }
"""

b = BPF(text=program)
while True:
    sleep(2)
    line = ""
    for k, v in b["syscall"].items():
        line += "syscall {0}: {1}\t".format(k.value, v.value)
    print(line)

hello_world.py

#!/usr/bin/python
from bcc import BPF

prog = """
int hello(void *ctx) {
    bpf_trace_printk("Hello world\\n");
    return 0;
}
"""

b = BPF(text=prog)
clone = b.get_syscall_fnname("clone")
b.attach_kprobe(event=clone, fn_name="hello")
b.trace_print()

# This prints out a trace line every time the clone system call is called

# If you rename hello() to kprobe__sys_clone() you can delete the b.attach_kprobe() line, because bcc can work
# out what event to attach this to from the function name.

Seems BPF's class method attach_kprobe() is broken: the current hello_map.py throws the following exception in python2.7:
Exception: Failed to attach BPF program hello to kprobe__sys_clone

Workaround: as mentioned in the code, comment out b.attach_kprobe() and rename hello to kprobe__sys_clone

this is great, a bit from a different angle, can you list what ebpf programs are attached to a given syscall from user land?

this is great, a bit from a different angle, can you list what ebpf programs are attached to a given syscall from user land?

I think what you are aiming at is bpftool, as shown in this BPF overview talk by Brendan Gregg @brendangregg:
https://youtu.be/7pmXdG8-7WU?t=1714

These examples use bcc which makes it easy to load eBPF programs into the kernel. I would recommend starting with hello_world.py and then hello_map.py.

Try running them under strace to see the system calls that are happening! For example strace -e bpf ./hello_world.py will show you the bpf() system call that loads the program into the kernel (BPF_PROG_LOAD).

You can see the accompanying slides from DockerCon here

Thanks for this tutorial and all your awesome tutorials! Your link to bcc is broken - fixed in the quoted text above.

Thanks for the correction! I've fixed it

…

On Thu, Aug 19, 2021 at 10:10 PM Vikas Rangarajan ***@***.***> wrote: ***@***.**** commented on this gist. ------------------------------ These examples use bcc <https://github.com/iovisor/bcc> which makes it easy to load eBPF programs into the kernel. I would recommend starting with hello_world.py and then hello_map.py. Try running them under strace to see the system calls that are happening! For example strace -e bpf ./hello_world.py will show you the bpf() system call that loads the program into the kernel (BPF_PROG_LOAD). You can see the accompanying slides from DockerCon here <https://speakerdeck.com/lizrice/ebpf-superpowers> Thanks for this tutorial and all your awesome tutorials! Your link to bcc is broken - fixed in the quoted text above. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <https://gist.github.com/47ad44a15cce912502f8667a403f5649#gistcomment-3866346>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AADP66B7OTMNPDD2QO5UMBDT5VXLZANCNFSM4HTAO73A> .

Thank you for the example!
In my Ubuntu I had the error:

cannot attach kprobe, probe entry may not exist
Traceback (most recent call last):
  File "h.py", line 12, in <module>
    b.attach_kprobe(event="sys_clone", fn_name="hello")
  File "/usr/lib/python3/dist-packages/bcc/__init__.py", line 658, in attach_kprobe
    raise Exception("Failed to attach BPF program %s to kprobe %s" %
Exception: Failed to attach BPF program b'hello' to kprobe b'sys_clone'

I modified attach_probe first parameter to:

b.attach_kprobe(event="__x64_sys_clone", fn_name="hello")
Then it worked.

@rodolk a more portable way to do this is to use b.get_syscall_fnname("clone") - I've updated the gist to do that now

@lizrice Thank you for taking the time to explain me!