Re: Error with printk and bpf_trace_printk

[Jesper Dangaard Brouer via iovisor-dev <iovisor-dev-9jONkmmOlFHEE9lA1F8Ukti2O/JbrIOy@public.gmane.org>]

Notice, there are two mailing lists (Cc'ed) that you should likely ask
these kind of questions on (instead of netdev), depending on if this is
mostly related to bpf (iovisor-dev-9jONkmmOlFHEE9lA1F8Ukti2O/JbrIOy@public.gmane.org) or somehow
related to XDP (xdp-newbies-u79uwXL29TY76Z2rM5mHXA@public.gmane.org).

See my answer inlined below:

On Sun, 28 May 2017 17:48:20 +0300 Adel Fuchs <adelfuchs-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> I have a working eBPF program, and I'm trying to add outputs to it.
> I'm not able to use both printk and bpf_trace_printk functions. I get
> this error:
> 
> ELF contains non-map related relo data in entry 0 pointing to section
> 8! Compiler bug?!
> 
> Prog section 'ingress' rejected: Invalid argument (22)!
>  - Type:         3
>  - Instructions: 16 (0 over limit)
>  - License:      GPL
> 
> Verifier analysis:
> 
> 0: (bf) r6 = r1
> 1: (18) r1 = 0x0
> 3: (85) call bpf_unspec#0
> unknown func bpf_unspec#0
> 
> Error fetching program/map!
> Failed to retrieve (e)BPF data!
> 
> Are there certain "includes" that I need to add?
> In addition, I'm not sure I'm using the function correctly. I just
> wrote: printk("hi")

You obviously cannot call printk directly from and eBPF program.
I wonder how you got this compiling...

As you hinted yourself, you should be using: bpf_trace_printk().
But it is actually tricky to use... and not much help is around to
figure this out.

First of all the output end-up in this file: /sys/kernel/debug/tracing/trace_pipe
Remember to read the output use 'cat' like:

 sudo cat /sys/kernel/debug/tracing/trace_pipe

And only the first process to read the output gets the output...


I deduct you are using the TC/iproute2 examples:
 https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/tree/examples/bpf

Next gotcha is that, you need to provide the char* string in a very
special way to make this compile correctly.  The iproute2 provide a
helper define called "printt()" in include/bpf_api.h for this:

#ifndef printt
# define printt(fmt, ...)                                               \
        ({                                                              \
                char ____fmt[] = fmt;                                   \
                trace_printk(____fmt, sizeof(____fmt), ##__VA_ARGS__);  \
        })
#endif

Or see my solution here:
[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/samples/bpf/xdp_ddos01_blacklist_kern.c#L86:L99


Another gotcha I've experienced is that if you format the string
incorrectly, or use a modifier like %X, which bpf_trace_printk() does
not seem to understand, then you "hear-nothing"...  Also experienced if
using more than 3 arguments, then it fails or also go silent. Be
careful when using this somewhat "flaky" debug facility.

Do remember these bpf_trace_printk() should only be used for debugging,
as it is very slow...
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer