Debugging using bpf_printk / bpf_trace_printk?

[XieGuochao]

Hello, I want to do some simple observations first before developing tools.

However, it seems that the bpf_printk or bpf_trace_printk does not work and nothing is printed on the terminal. Is it common? Is there any simple alternative to print something in the BPF program? Is there any guide for debugging/observation?

[lmb]

bpf_printk doesn't write to the console, it writes to the kernel's trace pipe. Take a look at https://man7.org/linux/man-pages/man7/bpf-helpers.7.html for entry bpf_trace_printk.

[liel-almog]

I've seen the @lizrice talk where she used bpf_printl and it printed to the screen using libbpfgo and the bpf.TracePrint().
Do have the same functionality?

[ti-mo]

@liel-almog Assuming you mean bpf_printk, and given it doesn't look like bpf.TracePrint() is part of libbpfgo anymore, a sudo cat /sys/kernel/debug/tracing/trace_pipe will get you to the same place.

[lizrice]

If you have bpftool installed you can also use bpftool prog tracelog to show the same trace output. (It's doing the same thing, I just find it easier to remember than the location of the trace_pipe pseudofile!)

[liel-almog]

Thank you so much!
I do have it installed.
By the way @lizrice , I'm a big fan of your talk and work!
Helped me so much.