Re: Error with printk and bpf_trace_printk

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Thu, 1 Jun 2017 14:10:01 +0300
Adel Fuchs <adelfuchs@gmail.com> wrote:

> Hi Jesper,
> I tried adding your solution, bpf_debug,  and I'm now able to run the
> program with no errors but the trace_pipe file stays empty.
> I just added this to my program:
> 
> #ifdef DEBUG
> /* Only use this for debug output. Notice output from bpf_trace_printk()
> * end-up in /sys/kernel/debug/tracing/trace_pipe
> */
> #define bpf_debug(fmt, ...) \
> ({ \
> char ____fmt[] = fmt; \
> bpf_trace_printk(____fmt, sizeof(____fmt), \
> ##__VA_ARGS__); \
> })
> #else
> #define bpf_debug(fmt, ...) { } while (0)
> #endif
> 
> 
> And added a printing command:
> bpf_debug("hi");
> 
> Do you know what's the problem?

I assume you are doing:
 sudo cat /sys/kernel/debug/tracing/trace_pipe

The problem could be that the kernel need to be compiled with the right
trace config options... does anybody on the list(s) know? 

(p.s. removed netdev list cc)
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer



> On Tue, May 30, 2017 at 3:24 PM, Jesper Dangaard Brouer
> <brouer@redhat.com> wrote:
> >
> > 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@lists.iovisor.org) or somehow
> > related to XDP (xdp-newbies@vger.kernel.org).
> >
> > See my answer inlined below:
> >
> > On Sun, 28 May 2017 17:48:20 +0300 Adel Fuchs <adelfuchs@gmail.com> 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